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Evolução tectônica de um fragmento do Cráton São Francisco
Meridional com base em aspectos estruturais, geoquímicos (rocha
total) e geocronológicos (Rb-Sr, Sm-Nd, Ar-Ar, U-Pb).
i
ii
FUNDAÇÃO UNIVERSIDADE FEDERAL DE OURO PRETO
Reitor
Dirceu do Nascimento
Vice-Reitor
Marco Antônio Tourinho Furtado
Pró-Reitor de Pesquisa e Pós-Graduação
Newton Souza Gomes
ESCOLA DE MINAS
Diretor
Antônio Gomes de Araújo
Vice-Diretor
Antenor Barbosa
DEPARTAMENTO DE GEOLOGIA
Chefe
César Augusto Chicarino Varajão
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EVOLUÇÃO CRUSTAL E RECURSOS NATURAIS
iv
CONTRIBUIÇÕES ÀS CIÊNCIAS DA TERRA – VOL. 7
TESE DE DOUTORAMENTO Nº 009
Evolução tectônica de um fragmento do Cráton São Francisco
Meridional com base em aspectos estruturais, geoquímicos (rocha
total) e geocronológicos (Rb-Sr, Sm-Nd, Ar-Ar, U-Pb).
Arildo Henrique de Oliveira
Orientador Dr. Maurício Antônio Carneiro
Co-orientadores (University of Queensland) Dr. Keneth D. Collerson
Dr. Balz S. Kamber
Tese apresentada ao Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais do
Departamento de Geologia da Escola de Minas da Universidade Federal de Ouro Preto como requisito
parcial à obtenção do Título de Doutor Ciência Naturais, Área de Concentração: Geologia Estrutural e
Recursos Naturais.
OURO PRETO
2004
v
Universidade Federal de Ouro Preto – http://www.ufop.br
Escola de Minas - http://www.em.ufop.br Departamento de Geologia - http://www.degeo.ufop.br/ Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais Campus Morro do Cruzeiro s/n - Bauxita 35.400-000 Ouro Preto, Minas Gerais Tel. (31) 3559-1600, Fax: (31) 3559-1606 e-mail: pgrad@degeo.ufop.br Os direitos de tradução e reprodução reservados. Nenhuma parte desta publicação poderá ser gravada, armazenada em sistemas eletrônicos, fotocopiada ou reproduzida por meios mecânicos ou eletrônicos ou utilizada sem a observância das normas de direito autoral.
ISSN 85.230.0108-6
Depósito Legal na Biblioteca Nacional
Edição 1ª Evolução tectônica de um fragmento do Cráton São Francisco Meridional com base em aspectos estruturais,
geoquímicos (rocha total) e geocronológicos (Rb-Sr, Sm-Nd, Ar-Ar, U-Pb). Catalogação elaborada pela Biblioteca Prof. Luciano Jacques de Moraes do
Sistema de Bibliotecas e Informação - SISBIN - Universidade Federal de Ouro Preto
vi
Catalogação: sisbin@sisbin.ufop.br
O48e Oliveira, Arildo Henrique de. Evolução Tectônica de um Fragmento do Cráton São Francisco Meridional com base em aspectos Estruturais, Geoquímicos (rocha total) e geocronológicos (Rb- Sr, Sm-Nd, Ar-Ar, U-Pb)[manuscrito]. / Arildo Henrique de Oliveira.– 2004. xxii, 134f.: il. Color., grafs. , tabs; mapas – (Contribuições as Ciencias da Terra. Série D; v.7) ISSN: 85-230-0108-6 Orientador: Prof. Dr. Maurício Antônio Carneiro. Co-Orientadores: Prof. Dr. Keneth D. Collerson e Dr. Balz Kamber Área de concentração: Geologia Estrutural. Tectônica. Tese (Doutorado) – Universidade Federal de Ouro Preto. Escola de Minas. Departamento de Geologia. Programa de pós-graduação em Evolução Crustal e Recursos Naturais. 1.Geologia Estrutural - Teses. 2.Geoquímica - Teses. 3.Tempo geológico - Teses. Campo Belo (MG). I. Universidade Federal de Ouro Preto. Escola de Minas. Departamento de Geologia. Programa de pós-graduação em Evolução Crustal e Recursos Naturais. II. Título. CDU: 551(815.1)
Com todo carinho
dedico essa Tese
a minha e sempre querida Mãe Delcia,
à família
e a minha eterna Andreia
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Agradecimentos
Ao final desse trabalho que, perdurou por alguns anos, inúmera são as pessoas que fizeram parte dessa
caminhada e, de forma direta ou indireta contribuíram e ajudaram na superação dos obstáculos. Torna-
se difícil em citar nomes, pois são tantos que tenho receio de esquecer de alguns. Sendo assim, fica
aqui meu sincero agradecimento aos colegas, amigos, professores e funcionários tanto do
Departamento de Geologia da Universidade Federal de Ouro Preto, quanto do Department of Earth
Sciences da Universidade de Queensland/Austrália.
O meu agradecimento especial ao meu amigo e orientador Dr. Maurício Antônio Carneiro pelo apoio,
amizade e ajuda em várias etapas do desenvolvimento desse projeto.
Ao Co-orientador Professor Ken Collerson da Universidade de Queensland por abrir as portas de outra
Universidade e de outro país.
Ao Co-orientador Dr. Balz Kamber que foi fundamental na obtenção dos dados U-Pb e, em algumas
etapas de meu aprendizado na Universidade de Queensland.
Ao pessoal do ACQUIRE Centre (Alan Greig e Irina).
Especial agradecimento ao CNPq (Processo 200714-1/05) pela concessão da bolsa Sandwish por um
período de 18 meses e, pela assistência de seus funcionários nos momentos em que precisei.
Agradeço também a CAPES pela concessão de bolsa de Doutorado pelo período que estive no Brasil.
Como não seria justo continuar a citar nomes de uma grande lista e, incorrer no erro e deixando muitos
de fora, fica aqui meus sinceros agradecimetos a todos os amigos do Brasil e de Brisbane que direta ou
indiretamente deram sua contribuição.
Um grande abraço a todos e meu muitíssimo obrigado
Valeu....
OM SHIVA
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Sumário
AGRADECIMENTOS..........................................................................................................................ix
LISTA DE FIGURAS ..........................................................................................................................xv
LISTA DE TABELAS.........................................................................................................................xix
RESUMO .............................................................................................................................................xix
ABSTRACT .........................................................................................................................................xxi
CAPÍTULO 1. CONSIDERAÇÕES GERAIS.....................................................................................1
1.1. Introdução .........................................................................................................................................1
1.2. Objetivos da Tese ..............................................................................................................................3
1.3. Metodologia ......................................................................................................................................3
1.3.1. Revisão Bibliográfica 1........................................................................................................4
1.3.2 Revisão Bibliográfica 2.........................................................................................................4
1.3.3 Trabalho de Campo ..............................................................................................................4
1.3.4 Análise Petrográfica..............................................................................................................4
1.3.5 Preparação de Amostras........................................................................................................4
1.3.6 Geoquímica ...........................................................................................................................5
1.3.6.1 Metodologia ICP-OES.............................................................................................5
1.3.6.2 Metodologia ICP-MS...............................................................................................6
1.3.7 Geocronologia.......................................................................................................................7
1.3.7.1 Rb-Sr........................................................................................................................7
1.3.7.2 Sm-Nd......................................................................................................................8
1.3.7.3 Ar-Ar .......................................................................................................................9
1.3.7.4 U-Pb.........................................................................................................................9
1.3.8 Tratamento dos Dados ........................................................................................................10
1.3.9 Elaboração da Tese .............................................................................................................11
CAPÍTULO 2. CAMPO BELO METAMORPHIC COMPLEX: TECTONIC EVOLUTION OF AN ARCHAEAN SIALIC CRUST OF THE SOUTHERN SÃO FRANCISCO CRATON IN MINAS GERAIS (BRAZIL) ...............................................................................................................13 2.1. Abstract ...........................................................................................................................................13
2.2 Introduction ......................................................................................................................................13
2.3. Geologic Context ............................................................................................................................15
2.4. Geology of the area .........................................................................................................................16
2.4.1. Gneissic Units ....................................................................................................................16
2.4.1.1. Cláudio Gneissic Unit...........................................................................................18
2.4.1.2 Itapecerica Gneissic Unit ......................................................................................18
2.4.1.3 Candeias Gneissic Unit .........................................................................................19
2.4.2. Amphibolitic Unit ..............................................................................................................19
xi
2.4.3 Supracrustal Unit................................................................................................................ 19
2.4.4 Fissure Mafic Unit.............................................................................................................. 21
2.5 Metamorphism................................................................................................................................. 22
2.6 Structural Analysis .......................................................................................................................... 23
2.6.1 Foliations............................................................................................................................ 24
2.6.2 Shear bands ........................................................................................................................ 24
2.6.3 S/C Structures..................................................................................................................... 26
2.6.4 Folds................................................................................................................................... 27
2.6.5 Foliation Sigmoids ............................................................................................................. 27
2.6.6 Pegmatites .......................................................................................................................... 28
2.6.7 Quartz Veins....................................................................................................................... 28
2.6.8 Amphibolite Boudins ......................................................................................................... 28
2.6.9 Faults .................................................................................................................................. 28
2.7 Tectonic Evolution .......................................................................................................................... 29
2.7.1 Event 1 ............................................................................................................................... 29
2.7.2 Event 2 ............................................................................................................................... 29
2.7.3 Event 3 ............................................................................................................................... 30
2.7.4 Event 4 ............................................................................................................................... 31
2.7.5 Event 5 ............................................................................................................................... 31
2.8 Conclusions ..................................................................................................................................... 31
CAPÍTULO 3. ORIGIN OF CHARNOCKITES AND ENDERBITES INFERRED FROM INCOMPATIBLE ELEMENT LOSS DURING PROGRADE DEHYDRATION....................... 33 3.1 Abstract ........................................................................................................................................... 33
3.2 Introduction ..................................................................................................................................... 33
3.3 Brief Review of relevant geochemical research .............................................................................. 35
3.4 São Francisco Craton: Relevant Geologic Record .......................................................................... 36
3.5 Samples Selection............................................................................................................................ 38
3.6 Petrography Features ....................................................................................................................... 38
3.7 Analytical Methods ......................................................................................................................... 40
3.8 Results ............................................................................................................................................. 46
3.8.1 Similarities and differences from our rocks compared with typical Archaean
granitois from Barberton normalized by N-MOR....................................................................... 47
3.8.2 Deviations of expected trace element abundances ............................................................. 48
3.9 Discussion ....................................................................................................................................... 50
3.10 Summary ....................................................................................................................................... 55
CAPÍTULO 4 RECENT ADVANCES AMONG Sr AND Nd CONSTRAINTS CONCERNING TO THE TECTONIC EVOLUTION OF CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN PORTION OF THE SÃO FRANCISCO CRATON, BRAZIL................................ 59 4.1 Introduction ..................................................................................................................................... 59
4.2 Geochronological overview............................................................................................................. 60
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4.3.Sample selection and analytical procedures.....................................................................................63
4.4. Results .............................................................................................................................................63
4.4.1 Outline of petrography features ..........................................................................................63
4.4.2 Rb/Sr sytem.........................................................................................................................69
4.4.3 Sm/Nd sytem.......................................................................................................................73
4.5 Discussion ........................................................................................................................................73
4.6 Summary ..........................................................................................................................................79
CAPÍTULO 5 IMPLICATIONS FOR TRANSAMAZONIAN AGE RELATED EFFECT 40Ar/39Ar FOR THE CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN SAO FRANCISCO CRATON, BRAZIL ....................................................................................................81
5.1 Abstract ............................................................................................................................................81
5.2 Introduction ......................................................................................................................................81
5.3 Problems to be addressed .................................................................................................................83
5.4 Geological setting.............................................................................................................................84
5.5. Sample selection and analytical procedures....................................................................................85
5.6 Results ..............................................................................................................................................85
5.6.1 Petrograph features .............................................................................................................85
5.6.2 40Ar/39Ar System .................................................................................................................87
5.7 Discussion ........................................................................................................................................97
5.8 Summary ........................................................................................................................................104
CAPÍTULO 6 GEOCRONOLOGIA U-Pb......................................................................................107
6.1 Introdução ......................................................................................................................................107
6.2 Resultados U-Pb.............................................................................................................................108
6.2.1 Amostra AH 11 .................................................................................................................110
6.2.2 Amostra AH 14 .................................................................................................................112
6.2.3 Amostra AH 15 .................................................................................................................114
6.2.4 Amostra AH 07 .................................................................................................................117
6.2.5 Amostra AH 08 .................................................................................................................118
6.3 Sumário ..........................................................................................................................................120
CAPÍTULO 7 CONSIDERAÇÕES FINAIS ...................................................................................121
7.1 GENERALIDADES.......................................................................................................................121
7.2 QUESTÕES ACERCA DA EVOLUÇÃO TECTÔNICA DO CRÁTON SÃO FRANCISCO MERIDIONAL ....................................................................................................................................122
7.3 Discussão........................................................................................................................................123
7.4 Sumário ..........................................................................................................................................126
REFERÊNCIAS BIBLIOGRÁFICAS .............................................................................................127
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Lista de Figuras
CAPÍTULO 1
Figura 1.1 - (a) Mapa geológico da porção meridional do Cráton São Francisco (modificado de Machado Filho et al. 1983; (b) Mapa geológico simplificado da area estudada (modificado de Oliveira 1999, Oliveira & Carneiro 2001)............................................................................................................. 2
Figura 1.2 - Mapa esquemático dos zircões das 5 amostras selecionadas (a numeração das amostras selecionadas está em negrito) ................................................................................................................ 10
CAPÍTULO 2
Figura 2.1 - Geologic map of southern São Francisco Craton (modified from Machado Filho et al. 1983)...................................................................................................................................................... 14
Figure 2.2 – Geologic map of the studied area (modified from Oliveira 1999); ................................. 15
Figure 2.3 - Stereographic diagrams representing the polar projections of the foliations of the study area ........................................................................................................................................................ 25
Figure 2.4 - Kinematic indicators (A-G) and schematic models explaining one of the event active in the study area (H). ................................................................................................................................. 26
CAPÍTULO 3
Figure 3.1 - Panel (a) Photograph of massive charnockite with incipient foliation taken in Alemão dimension stone quarry (Candeias Unit). Panel (b) Photograph taken at Marilan dimension stone quarry (Itapecerica Unit) illustrating the quality of rocks collected from unweathered core.. ................................................................ 37
Figure 3.2 - Panel (a) Geological map of the southern São Francisco Craton (modified from Machado Filho et al. 1983); Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001)................................................................................................................................... 39
Figure 3.3 - (a) Chemical classification diagram in Na2O x CaO x K2O space (Glikson 1979) used to divide the studied rocks into: 1 - thondhjemites, 2 - tonalites, 3 - granodiorites, 4 – adamellites, and 5 – granites. (b) (MgO + Fe2O3 + MnO) vs SiO2 showing the expected trend from fractionation of ferromagnesian silicates and Fe-oxides. (c) (Na2O + K2O – CaO) vs K2O diagram shows compositional relationship for Archaean gneisses and granitoids, in which studied samples plot along the calc-alkaline trend. (d) Plot of La/Yb vs Yb comparing rocks from Candeias, Claudio and Itapecerica Units.. .................................................................................................................................. 46
Figure 3.4 - N-MORB-normalized trace element patterns of average gneisses from A – Candeias, B – Itapecerica, and C – Claudio Units. Elements arranged in order of decreasing incompatibility in MORB-melting (after Sun & McDonough 1989). Normalizing values were taken from Sun & MacDough (1989) except W where the average of Indian, Pacific and Atlantic MORB was calculated from data presented by Newson et al. (1996). ....................................................................................... 48
Figure 3.5 – Upper Continental Crust normalization of same data (average of Candeias Unit) as shows in Fig. 3.4. Normalizing values were taken from McLennan (2001). ................................................... 50
Figure 3.6 – Detail of Upper Continental Crust normalized average patterns of Candeias, Itapecerica, and Claudio Units. Panel (a) highlights depletion of Cs, Rb, U, Nb, Ta, W and Sr by comparison with average of typical Archaean granitoid rocks from Barberton (Kleinshanns et al. in press). ................. 51
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Figure 3.7 - Selected trace element rations of gneiss from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) unit: (a) Th/U vs La/W, (b) Th/U vs Hf/W, (c) Th/U vs Th/Cs, (d) Cs/Rb vs Cs/Th, and (e) Nb/Ta vs Hf/W. .............................................................................56
Figure 3.8 - Selected relationship between major and trace element from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) samples: (a) Eu/Eu* vs Sr/Nd, (b) Eu/Eu* vs Al2O3, (c) Pb vs (Na2O + K2O – CaO), and (d) Ce/Pb vs (MgO + Fe2O3 + MnO)........................................57
CAPÍTULO 4
Figure 4.1 - Panel (a) Geological map of the southern São Francisco Craton (modified from Machado Filho et al. 1983); Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) ...................................................................................................................................61
Figure 4.2 – Panel of pictures from gneisses quarry .............................................................................64
Figure 4.3 - Panel of pictures of amphibolites rocks.............................................................................65
Figure 4.4 -Rb-Sr isochron diagram showing the nearby correlation between Marilan quarry and Fazenda Corumba quarry. The three point isochron are considering be of uncertain realiability .........70
Figure 4.5 - Rb-Sr isochron diagram showing in close proximity correlation between the three gneisses units..........................................................................................................................................71
Figure 4.6 - Rb-Sr isochron diagram showing amphibolites from Kinawa and Corumbá quarry.........72
Figure 4.7 - Sm-Nd errochron diagram showing nearby correlation between the three gneisses units as demonstrated for Rb-Sr ..........................................................................................................................74
Figure 4.8 - Positive trend correlation between Cs;Rb vs Cs;Th from Cláudio (open triangle), Itapecerica (solid square) and Candeias (solid diamond) Units. ............................................................77
Figure 4.9 - Nd versus time (DePaolo 1981) for the studied samples (see table 4.2 for reference) ............78
CAPÍTULO 5
Figure 5.1 - Panel (a) Geological map of the southern São Francisco Craton (modified from Machado Filho et al. 1983); Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) ...................................................................................................................................82
Figure 5.2 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1436 (Fig. 5.1b-point A) ...............................................................................................................91
Figure 5.3 Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1197 (Fig. 5.1b-point B)................................................................................................................92
Figure 5.4 - Plot 40Ar/39Ar apparent age vs cumulative argon released for biotites of amphibolites from Corumbá quarry (Fig 5.1b, point A) ......................................................................................................94
Figure 5.5 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Corumbá quarry (Fig 5.1b, point A)..............................................................................................95
Figure 5.6 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Supracrustal Unit (Fig 1b, point C). ..............................................................................................96
Figure 5.7 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbronorite from fissural mafic Unit (Fig 5.1b – point D). .......................................................................................98
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Figure 5.8 - Plot 40Ar/39Ar apparent age vs cumulative argon released for biotites of gabbronorite from fissural mafic Unit (Fig 5.1b – point D). ............................................................................................... 99
Figure 5.9 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point E)............................................................................................... 100
Figure 5.10 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point F)............................................................................................... 101
CAPÍTULO 6
Figura 6.1 – Mapa geológico modificado por Oliveira & Carneiro (2001), mostrando os pontos amostrados........................................................................................................................................... 108
Figura 6.2 – Fotografias das amostras onde foram coletados os zircões datados: a) Gnaisse da pedreira Kinawa; b,c) Gnaisse da pedreira Corumbá; d) Gnaisse da pedreira Oliveira e, e) Pedreira de rocha ornamental Alemão.............................................................................................................................. 110
Figura 6.3 Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH11. Os zircões pertencem a um gnaisse migmatítico da pedreira Kinawa (Figura 6.1C) .............. 111
Figura 6.4 – Diagrama Concórdia U-Pb para os zircões da mostra AH 11 da Unidade Gnáissica Cláudio ................................................................................................................................................ 112
Figura 6.5 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)........... 113
Figura 6.6 – Diagrama Concórdia U-Pb para os zircões da amostra AH 14 da Unidade Gnáissica Cláudio ................................................................................................................................................ 114
Figura 6.7 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)........... 114
Figura 6.8 – Diagramas Concórdia U-Pb para os zircões da amostra AH 15 da Unidade Gnáissica Cláudio (a, b) e para as amostras AH 14 e AH 15 da Unidade Gnáissica Cláudio ............................. 116
Figura 6.9 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH07. Todos os cristais são de um enderbito da pedreira Oliveira (Figura 6.1 B)............................. 117
Figura 6.10 – Diagrama Concórdia U-Pb para os zircões da amostra AH 07 da Unidade Gnáissica Candeias .............................................................................................................................................. 118
Figura 6.11 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH08 e pertencentes ao charnockito da pedreira Alemão (Figura 6.1 A)........................................... 119
Figura 6.12 – Diagrama Concórdia U-Pb para os zircões da amostra AH 08 da Unidade Gnássica Candeias .............................................................................................................................................. 120
CAPÍTULO 7
Figura 7.1 – Diagrama mostransdo a perda de Pb com relativo ganho de U .................................... 125
Figura 7.2 – Também mostra o distúbio que as rochas foram submetidas desde o Arqueano até o Neoproterozóico/Paleozóico................................................................................................................ 126
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Lista de Tabelas
CAPÍTULO 1
Tabela 1.1 - Cálculo para determinação da razão 87Rb/86Rb ...................................................................8
Tabela 1.2 - Cálculo para determinação da razão 147Sm/144Nd................................................................9
CAPÍTULO 2
Table 2.1 - General characteristics of the rocks types of the study area and their probable ages and tectonothermal events. ......................................................................................................................17
Table 2.2 - Simplified table of the structural elements associated with the tectonothermal events. .....27
CAPÍTULO 3
Table 3.1 - Major (wt%), trace element (ppm) concentrations and selected trace element ratios of Campo Belo Metamorphic Complex. ...............................................................................................42
CAPÍTULO 4
Table 4.1 - Rb-Sr whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex .....................................................................................................................66
Table 4.2 - Sm-Nd whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex. The ε(T1) was calculated fro 2.72Ga (supposed age for the formation of the rock) and ε (T2) was calculated for 2.62Ga (supposed age for the “metamorphic” event) ...................................................68
Table 4.3 - Summary of the Archaean tectonomagmatic events in the southern part of the São Francisco Craton in the light of U/Pb and Sm-Nd data. This table was summarized by Teixeira et al. 2000..............................................................................................................................................75
CAPÍTULO 5
Table 5.1 – Ar-Ar results for amphibolites and gabbros from Campo Belo Metamorphic Complex ......... 87
CAPÍTULO 6
Table 6.1 – Resultados analíticos ................................................................................................................ 109
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Resumo
Os estudos abordados nessa tese envolveram os gnaisses, anfibolitos, gabros e gabronoritos
das unidades gnáissicas, anfibolíticas, supracrustal e máfica fissural. Os resultados obtidos foram
importantes no trato da evolução tectônica do Complexo Metamórfico Campo Belo (CBC) na porção
meridional do Cráton São Francisco.
Os resultados, ora apresentados, foram balisados em petrografia, geologia estrutural,
geoquímica em rocha total (elementos maiores, traços e REE) e geocronologia (Rb-Sr e Sm-Nd em
rocha total, Ar-Ar em anfibólios e biotitas de gnaisses, anfibolitos, gabros e gabronoritos e U-Pb em
zircões de gnaisses). Esses dados mostraram que a região foi submetida a múltiplos episódios
tectonotermais que vão do Arqueano ao Proterozóico.
Os estudos petrográficos mostraram três picos metamórficos bem distintos: o primeiro atingiu
a fácies granulito e afetou as três unidades gnáissicas e anfibolítica. O segundo situa-se na fácies
anfibolito e está bem caracterizado nas rochas máfica-ultramáficas e pelíticas da unidade supracrustal.
O terceiro pico esta caracterizada por um retrometamorfismo regional para fácies xisto-verde.
O padrão estrutural mais significativo que afetou a área estudada está associado ao vigoroso
evento de migmatização e à geração da Zona de Cisalhamento Cláudio (NE/SW com cinemática
dextral). As estruturas mais tardias são o fraturamento crustal de direção NW-SE onde se
posicionaram os diques de gabros e gabronoritos.
Os estudos geoquímicos foram focados apenas nos gnaisses das unidades de Cláudio,
Itapecerica e Candeias. Os resultados mostraram que as rochas da região sofreram múltiplas perdas de
alguns elementos, principalmente de incompatíveis. Os resultados mostraram que o padrão
geoquímico das rochas félsicas, principalmente em se tratando dos gnaisses de fácies granulito
(enderbitos) e charnockitos, mostram particularidades distintas, principalmente quando comparados
com metagranitóides arqueanos ou com a média das crostas superior e inferior e crosta total. Foi
observado também que para alguns dos elementos traços incompatíveis (e.g. Cs, U, Nb, Ta e W), a
mobilidade foi muito alta. Alguns dos elementos (e.g. Cs, Rb e U) são considerados móveis com a
presença de fluidos, porém outros (i.e. Nb, Ta, W) são considerados imóveis. Esses elementos também
se encontram com razões baixas.
Já os dados geocronológicos dos gnaisses, anfibolitos, gabronoritos e gabros mostram
resultados por volta de: 2.9-2.7 Ga (U-Pb), 2.6 Ga (Rb-Sr), 2.05 Ga (U-Pb), 2.0-1.9 Ga (Ar-Ar), 1.7
Ga, 1.0Ga (Ar-Ar), 0.5 Ga (U-Pb).
Este conjunto de informações permitiu caracterizar pelo menos sete eventos tectonotermais
para a região de Cláudio e imediações.
O evento de 2.9-2.7 Ga é interpretado como a idade do protólito das rochas estudadas. O
evento de 2.6 Ga foi caracterizado como um possível evento metamórfico. A esse evento foi indexada
a formação da Zona de Cisalhamento Cláudio.
O evento de 2.05 Ga ficou bem evidenciado nos charnockitos e mostrou que a região esteve
xix
sob eventos que atingiu a fácies granulito de metamorfismo nesse período.
Já os eventos de 2.0-1.9Ga, ~1.7Ga e 1.0 Ga foram bem caracterizados através dos dados de 40Ar/39Ar. As idades de 2,0-1,9 Ga foram correlacionadas ao resfriamento do evento de granulitização
de 2.05Ga e não apenas o soerguimento da crosta siálica. Os eventos de 1.7Ga e 1.0Ga estão
associados ao magmatismo máfico fissural (gabronoritos e gabros respectivamente). Com esses
resultados caracterizam-se pelo menos dois períodos de resfriamento crustal que devem estar
relacionados ao posicionamento / cristalizações desse magmatismo fissural.
O primeiro evento de magmatismo fissural pode ser correlacionado ao evento estateriano/rifte
do Espinhaço. Já o segundo evento de magmatismo, com idade de aproximada de 1,0 Ga, foi o
responsável pela reativação de falhas pré-existentes de direção NW-SE e pode ser correlacionada ao
rifte Macaúba. Em suma, os resultados ora apresentados mostram que a estabilização da crosta siálica
do Cráton São Francisco Meridional, pelo menos na região de Cláudio, ocorreu no final do
Mesoproterozóico e não no Arqueano.
Por fim o evento de 0.5 Ga que representa apenas um intenso distúrbio isotópico com
conseqüente perda de Pb. Os dados ora apresentados foram de grande importância para o
entendimento de algumas das lacunas até então pouco evidentes para a evolução da porção meridional
do Cráton São Francisco. Entretanto, uma evolução similar já foi mostrada para a porção setentrional
do Cráton São Francisco.
xx
Abstract This thesis present the results of gneisses, amphibolites, gabbronorite and gabbros from
Cláudio, Supracrustal and Mafic Fissural Units and display imperative information for the evolution of
the Campo Belo Metamorphic Complex (CBC) in the southern part of the São Francisco Craton.
Using petrography, structural, geochemistry in whole rock, Rb-Sr and Sm-Nd in whole rock,
Ar-Ar for amphibole and biotite, and U-Pb for zircons, we have provide important information about
the timing and tectono-metamorphic history from the Archaean to the Proterozoic.
Based on petrography features, three distinct and anachronic metamorphic peaks were
registered: the first, of highest metamorphic grade related to the granulite facies, is present in the rocks
of the Gneissic Units and of the Amphibolitic Unit. The second, in the amphibolite facies, is observed
in the mafic-ultramafic and clastic-pelitic rocks of the Supracrustal Unit. The third is characterized by
regional retrograde metamorphic processes, which reached the greenschist facies and cover the whole
region.
The mainly and the most expressive structural pattern is characterized by a vigorous regional
migmatization process and by the generation of the NE/SW Claudio Shear Zone, presenting dextral
kinematic movement. The later expressive event is represented by a fissure mafic magmatism placed
in the NW-SE regional structures.
For geochemistry analyses were chosen gneisses from the three units: Cláudio, Itapecerica and
Candeias. By combining major and trace elements evidence it was possible to identify element losses
caused by prograde dehydration in the gneisses rocks (essentially in the charnockites and enderbites).
Those rocks display relative depletion in some of the most incompatible elements (i.e. Cs, Rb, U, Nb,
Ta and W) in multi-element spider diagrams. Some elements (i.e. Cs, Rb and U) have traditionally
been considered mobile with fluids but we found that others (i.e. Nb, Ta, W), which are often
considered immobile, are also deficient in our samples.
The geochronological results of gneiss, amphibolites, gabbronorite and gabbros displayed
results at about 2.9-2.7 Ga (U-Pb), 2.6 Ga (Rb-Sr), 2.05 Ga (U-Pb), 2.0-1.9 Ga (Ar-Ar), 1.7 Ga, 1.0Ga
(Ar-Ar), 0.5 Ga (U-Pb). Combining each of these results with petrography, structural and
geochemistry records it was possible to make link to a distinct tectonic episode that might took place
in the Campo Belo Metamorphic Complex as follow: 2.9-2.7 Ga event is correlated to the protolith
age, the 2.6 Ga is linked to the metamorphic age and might be associated to the deformational event
responsible for the dextral strike-slip Cláudio shear zone.
The 2.05 Ga correspond to high-grade metamorphic event as attested by U-Pb data for
charnockites. It is the peak of the prograde metamorphic event that achieves the granulite facies
metamorphism followed by charnockite formation. As a consequence, the ages at about 2.0-1.9Ga can
be correlated not only with the age of exhumation of the CBC but the age of the retrograde
metamorphic processes from this high-grade metamorphic event at about 2.05 Ga. The data suggests
that the CBC was largely and quickly exhumed after the 2.05Ga metamorphic event.
xxi
The ages at about 1.7 and 1.0Ga can be correlated to the emplacement and crystallization of
the two swarm dykes. The NW-SE structures into which these swarm dykes were emplaced can be
correlated with the Staterian continental rift scenario (1.8-1.6Ga).
The latest thermal evidence in the CBC is the emplacement of the ~1.0Ma gabbros, emplaced
in preexisting NW-SE structures and can be correlated to the Macaubas rift. The results displayed in
this study stand for the final stabilization of the CBC that happened in the Mesoproterozoic and not in
the Achaean.
The 0.5 Ga correspond to no more than the disturbance of the system attested in the U-Pb date.
The new data placed imperative information on the tectonic evolution in the Campo Belo Complex
which appears comparable in many ways to the others Archaean continental crust.
xxii
CAPÍTULO 1 CONSIDERAÇÕES GERAIS
1.1 INTRODUÇÃO
Esta tese apresenta os resultados da cartografia geológica e dos estudos petrográficos,
estruturais, geoquímicos e isotópicos realizados num fragmento da porção meridional do Cráton São
Francisco (Fig. 1a, b). Por outro lado, é a continuidade dos estudos do mestrado do autor (Oliveira
1999).
A área estudada compreende, principalmente, os litotipos do Complexo Metamórfico Campo
Belo e é balizada pelas cidades de Oliveira, Carmópolis de Minas, Cláudio e Itapecerica (Fig. 1.1b).
Subordinadamente afloram uma unidade supracrustal, que pode ser correlacionada ao Supergrupo Rio
das Velhas (Machado Filho et al. 1983), e um enxame de diques máficos. Este complexo insere-se no
contexto geotectônico do Cráton São Francisco Meridional, no estado de Minas Gerais, Brasil
e vários foram os estudos já desenvolvido no Complexo Metamórfico Campo Belo (Silva et al.
1978, Machado Filho et al. 1983, Teixeira 1993, Carneiro et al. 1996 a, b, 1997 a, b, c, 1998 a, b,
Carvalho Júnior et al. 1997, 1998, Fernandes et al. 1997, 1998, Corrêa da Costa et al. 1998, Oliveira et
al. 1998 a, b, 1999, Oliveira 1999, Oliveira & Carneiro 1999, Corrêa da Costa 1999).
Em termos geotectônicos a porção meridional do Cráton São Francisco (Fig. 1.1a) é um
segmento crustal de evolução policíclica, tectonicamente estável em relação aos cinturões móveis do
Ciclo Brasiliano (Alkmim et al. 1993). O acervo geocronológico da região pode ser encontrado em
Teixeira et al. (1996, 1998) que, também, propuseram um modelo de evolução crustal arqueana e
paleoproterozóica, caracterizado por sucessivas etapas de acresção/diferenciação, associados a
processos de retrabalhamento crustal posteriores.
As rochas gnáissicas do Complexo Metamórfico Campo Belo têm composição muito variável
(e.g. granítica, granodiorítica e tonalítica) e podem ser agrupadas em três unidades distintas: Gnaisses
Cláudio, Itapecerica e Candeias (Oliveira et al. 1998 a, b, 1999, Oliveira 1999, Oliveira & Carneiro
1999, 2001). Os gnaisses tipo Cláudio apresentam coloração cinza (localmente com mobilizados
róseos) e composição, predominante, granodiorítica. Os gnaisses tipo Itapecerica apresentam
coloração rósea e composição mais granítica. Já os gnaisses granulíticos tipo Candeias apresentam
coloração esverdeada e composição em geral granodiorítica à granítica. A unidade supracrustal é
composta por peridotito, hornblendito, clorita-hornblendito, anfibolito, granada-sillimanita-xisto,
granada-sillimanita-quartzito, filito grafitoso e formação ferrífera bandada. O enxame de diques
máficos, que descreve grandes lineamentos de direção NW/SE, é formado por gabronorito, gabro e
diabásio (Oliveira et al. 1998b).
Oliveira, 2004 Evolução tectônica de um.........
Figura 1.1 Painel (a) –Mapa Geológico da porção meridional do Cráton São Francisco (modificado de Campos Sales 2004). Lefenda: 1- Coberturas cratônicas Neoproterozoicas indivisas; 2- Supergrupo Espinhaço; 3- Grupos São João Del Rei/Andrelândia; 4—Grupo Dom Silvério; 5- Granitóides Paleoproterozoic; 6- Seqüências Greenstone; 7- Supergrupo Minas; 8- Granitóides Neoarqueanos; 9- Intrusões máficas Neoarqueanas e Mesoproterozoicas; 10- Suíte plutônica ultramáfica Neoarqueanas; 11- Supergrupo Rio das Velhas; 12- Complexos Metamórficos Arqueanos; 13- Falhas e fraturas (CSZ = Zona de Cisalhamento Cláudio; JBSZ = Zona de Cisalhamento Jeceaba-Bom Sucesso; 14- Traços axial dos Eixos de dobras; 15- Contatos litológicos; 16- Cidades. Painel (b) Mapa geológico simplificado da área estudada (modificado de Oliveira 1999, Oliveira & Carneiro 2001). Simbologia: 1) Unidade máfica fissural; 2) Unidade Supracrustal; 3) Unidade gnáissica Candeias; 4) Unidade gnáissica Itapecerica; 5) Unidade gnáissica Cláudio; 6) Contatos inferidos; 7) Foliação; 8) Zona de cisalhamento Cláudio, e 9) Afloramentos escolhidos: A – Pedreira de rocha ornamental Alemão; B – Pedreira de rocha ornamental Oliveira; C - Pedreira de rocha ornamental Lila; D – Pedreira de rocha ornamental
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Itapecerica; E - Pedreira de rocha ornamental Marilan; F - Pedreira de rocha ornamental Kinawa; G - Pedreira de rocha ornamental Corumbá; H – Pedreira de rocha ornamental Fazenda Corumbá; I – Pedreira de rocha ornamental Carmópolis de Minas; J – Afloramento de Gabronorito; K e L – afloramentos de Gabros e M.- afloramento de anfibolito da Unidade Supracrustal
1.2 OBJETIVOS DA TESE
1 - determinar a relação cronoestratigráfica entre as rochas das unidades gnáissicas, seqüência
supracrustal e unidade máfica fissural na região estudada;
2 – determinar, através de estudos isotópicos, a pluralidade dos eventos deformacionais,
acrescionários, de migmatização e de magmatismo, que ocorreram durante o arqueano. Assim como, a
contribuição dos eventos posteriores (e.g. transamazônico e brasiliano) na evolução tectônica da área;
3 - associar os dados obtidos em campo (unidades litodêmicas e a cinemática dos corpos) e
laboratório (petrológicos, geocronológicos e geoquímicos) para modelar a evolução tectônica regional;
4 - comparar os dados obtidos na área com aqueles em terrenos de igual natureza, a nível
mundial, estabelecendo, assim, uma correlação evolutiva para o Craton São Francisco Meridional.
1.3 METODOLOGIA
Como o acervo petrotectônico da região é muito complexo e variado, para elevar o seu
conhecimento geológico, foi necessário executar um estudo amplo envolvendo geologia estrutural,
petrologia, geoquímica e geocronologia para tratar, com maior precisão, a sua evolução tectônica.
O mapeamento geológico, parte da petrografia e parte dos estudos cinemáticos foram
realizados por Oliveira (1999). Nos anos de 2000 e 2001 foram realizadas mais etapas de campo,
totalizando 60 dias. Nessas etapas foram visitados pontos previamente selecionados por Oliveira
(1999). Durante essas etapas foram realizados estudos mais detalhados da cinemática dos corpos, suas
relações no contexto regional e coletadas amostras para estudos geoquímicos e isotópicos.
Das amostras coletadas foi realizado um novo estudo petrográfico e, desse estudo, foi feita
uma seleção para geoquímica e geocronologia.
Ao final do ano de 2000 iniciou-se a preparação de um artigo com informações previamente
obtidas no mestrado juntamente com os dados coletados no doutorado. Ao término de 2001 o
doutorando obteve uma bolsa sanduíche e se transferiu para a Universidade de Queensland na
Austrália para obter os dados geoquímicos em rocha total (elementos maiores, traços e terras raras) e
geocronológicos (U-Pb em zircão, Sr-Nd em rocha total e Ar-Ar em anfibólio e biotita). O
detalhamento de cada uma das etapas está exposto a seguir.
Oliveira, 2004 Evolução tectônica de um.........
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1.3.1 Revisão bibliográfica 1:
Revisão dos estudos realizados no Complexo Metamórfico Campo Belo em termos regionais
(e.g. Silva et al. 1978, Machado Filho et al. 1983) e mesmo ao nível de detalhe (e.g. Teixeira 1993,
Teixeira & Silva 1993, Teixeira & Canzian 1994, Teixeira et al. 1996, 1997, 1999, Carneiro et al.
1996 a, b, 1997 a, b, c, 1998 a, b, Carvalho Júnior et al. 1997, 1998, Fernandes et al. 1997, 1998,
Pinese et. al. 1995, Corrêa da Costa et al. 1998, Oliveira et al. 1998 a, b, 1999, Oliveira 1999, Oliveira
& Carneiro 1999, Corrêa da Costa 1999, Campos et al. 2003).
1.3.2 Revisão bibliográfica 2:
Revisão dos estudos efetuados em terrenos granito-greenstone de igual natureza em outros
crátons com enfoque em dados geocronológicos e geoquímicos (e.g. Tankard 1982, Allen & Condie
1985, Goodwin 1991, MacDonough et al. 1991, Myers 1993, Sarkar et al. 1993, Miller et al. 1994,
Windley 1998, Friend et al. 1996, Mueller et al. 1996, Nutman et al. 1996, Tribuzio et al. 1996,
Moorbath et al. 1997, Collerson & Macdonald 1998, Hamilton 1998, Bebout et al. 1999, Becker et al.
1999, 2000, Bodorkos et al. 2002, Sandiford & MacLaren 2002, Sandiford et al. 2003).
1.3.3 Trabalho de Campo
Os trabalhos de campo tiveram como fundamento a caracterização mais detalhada das 5
unidades litodêmicas definidas por Oliveira et al. (1998 a, b, 1999), Oliveira & Carneiro (1999) e
Oliveira (1999). Nessa etapa, além dos estudos cinemáticos das estruturas, foram coletadas amostras
representativas para estudos petrográficos, geoquímicos e geocronológicos.
1.3.4 Análise petrográfica
As amostras representativas das unidades litodêmicas foram estudadas em microscópio óptico
com a finalidade de se observar texturas, estruturas e associações minerais diagnósticas dos eventos
ígneos e metamórficos regionais. Tais estudos foram de suma importância para definição das unidades
que foram alvo das análises geoquímicas e geocronológicas.
1.3.5 Preparação de amostras
Inicialmente as amostras foram preparadas no LOPAG (Laboratório de preparação de
amostras para geocronologia e geoquímica) do Departamento de Geologia da Universidade Federal de
Ouro Preto.
As amostras sofreram redução granulométrica para concentração dos pesados (geocronologia
U-Pb) e geoquímica (rocha total).
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Posteriormente, as amostras foram submetidas a uma segunda fase de preparação que ocorreu
no Departamento de Geologia da Universidade de Queensland e no ACQUIRE (Advanced Centre for
Queensland University Isotope Research Excellent)/Austrália, conforme procedimentos descritos a
seguir.
1.3.6 Geoquímica:
No Departamento de Geologia da Universidade de Queensland/Austrália as amostras foram
pulverizadas em moinho de ágata e uma alíquota foi separada para o ataque químico.
As análises químicas em rocha total (elementos maiores, traços e elementos terras raras) foram
realizadas utilizando-se de um ICP-EOS (Inductively Coupled Plasma – Optical Emission
Spectroscopy) situado no laboratório do Department of Earth Sciences/University of Queensland e um
ICP-MS (Fisons PQ2 + Plasmaquad Inductively Coupled Plasma-Mass Spectrometer) situado no
ACQUIRE (Advanced Centre for Queensland University Isotope Research Excellent).
1.3.6.1 Metodologia ICP-OES
Do conjunto dos gnaisses estudados, 21 amostras foram selecionadas para as análises
geoquímicas de elemento maiores. As determinações químicas foram analisadas para 10 elementos
maiores (Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SiO2, TiO2) pelo técnico do Laboratório
Mr. Michael Lawrence. O óxido de ferro foi determinado como ferro total (Fe2O3).
Foi utilizado 0,050 g de amostra em pó (secas por 1-2 horas em cadinhos de porcelana a
1050C) as quais foram colocadas, posteriormente, em cadinhos de grafita (primeiramente foi
necessário polir os cadinhos com uma colher). Em seguida, adicionou-se 0,200 g de LiBO2
(Metaborato de lítio) e, então, fez-se a homogeneização do material e levou-se ao forno, por 1 hora, a
uma temperatura de 10000C.
Os béqueres de teflon utilizados foram mantidos em uma vasilha contendo 10% de HNO3 para
limpeza por um período de 24 horas. Após isso, adicionou-se aos béqueres uma mistura de 25 ml de
uma solução de HNO3 a 10%, com 10ppm de lutécio (Lu), como padrão interno e, em seguida, a
amostra fundida (ao final de uma noite, nessa solução, as amostras devem ter uma aparência de flocos
de neve). Se a amostra não for completamente fundida é necessário adicionar 1ml de HF concentrado.
Após a dissolução completa das amostras foi adicionado 25 ml de água destilada, para que a
concentração final alcançasse a relação 50mg/50ml. As curvas de calibração foram obtidas pela
diluição dos padrões JB3 (basalto), JA3 (andesito) e JR2 (riolito). A perda ao fogo foi determinada
Oliveira, 2004 Evolução tectônica de um.........
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aquecendo-se aproximadamente 2 g de amostra a 10000C por cerca de 2 horas.
1.3.6.2 Metodologia ICP-MS
Foram selecionadas 25 amostras de gnaisses para as determinações químicas de elementos
traço e terras-raras (Li, Be, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Sn, Cs, Ba, La, Ce, Pr,
Nd, Sm, Eu, Tb, Gd, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Pb, Th, U). O procedimento analítico foi
orientado pelo responsável técnico do laboratório (Dr. Alan Greig) e seguiu as descrições de Eggins et
al. (1997).
Procedimentos:
Parte 1 - Os béqueres de teflon PTFE (7ml) foram deixados por 1 dia em uma vasilha de vidro
(1000ml) com água destilada e 3ml de detergente. Depois disso, eliminou-se a água e enxaguaram-se
os béqueres com água destilada (3 vezes) que foram colocados na vasilha de vidro (1000ml) e
completada com água destilada. Esse conjunto foi fervido por 1 hora em uma chapa quente dentro de
uma capela e, em seguida, os béqueres foram secos em uma capela de pressão positiva.
Parte 2 – Preparou-se uma solução com água destilada (e.g. 24ml), ácido clorídrico (HCl,
18ml) e ácido nítrico (HNO3, 6ml), que é a composição da aqua regia. Essa solução foi distribuída nos
béqueres de teflon até cobrir a sua base e, em seguida, colocou-se os béqueres dentro de jaquetas de
aço que foram levadas ao forno por 2h (temp. de 178 °C). Retiradas do forno foram resfriados por,
aproximadamente, 2h. Após esse período, os béqueres foram retirados das jaquetas de aço e levados
para uma capela de pressão positiva, onde foram abertos (é necessário precaução ao abrir os béqueres,
pois a pressão interna devido ao vácuo faz com que o material pule para fora). Depois de abertos, os
béqueres foram lavados com água destilada, repetiu-se esse procedimento por duas vezes. Após essa
lavagem, colocou-se 2 gotas de HNO3 nos béqueres e adicionou-se HF (ácido fluorídrico) até a sua
linha mediana (~4ml). Os béqueres assim tratados foram colocados novamente nas jaquetas de aço e
levados ao forno por 1 dia e meio. Após esse período, os béqueres foram lavados com água destilada
(2X) e ficaram limpos para uso.
Ataque químico
Em cada béquer, previamente limpo, adicionou-se 16 gotas de 8NQD de HNO3, pesou-se
aproximadamente 0,10000g de amostra e adicionou-a sobre o ácido e pingou-se mais 16 gotas do
mesmo ácido. Adicionou-se 2ml de HF e colocou as amostras para secarem em uma chapa quente
dentro de uma capela de pressão positiva por 1- 2 horas sem tampar os béqueres. Em seguida, foi
adiciononado-se 16 gotas de 15.8N de HNO3 e 3,5ml de HF e, então, se colocou os béqueres nas
jaquetas de aço que foram levados ao forno por 60h (180°C). Após esse período, abriu-se os béqueres
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que foram secos por aproximadamente 2h. Então, se adicionou 4ml de HCL e, dentro das jaquetas, os
béqueres foram ao forno por mais 24h. Abertos e secos por aproximadamente 2horas, receberam ácido
nítrico concentrado (15,08 N), até cobrir a amostra e foram secos em chapa quente (repetir 1X). Com
pipetas, então foi adicionado 2 X 1000µl de ácido nítrico destilado e 2X 1000µl de H2O destilada.
Fechados e levados para uma capela com pressão positiva e a 80°C ficaram por 24hs. Após isso, os
béqueres, ainda fechados, foram agitados suavemente e girados de ponta a cabeça de modo a agrupar
todas as gotículas das paredes internas. O liquido foi transferido para um tubo de centrifuga e o béquer
foi ainda lavado com H2O destilada (2X), para recuperar o restante do material, e foi centrifugado por
20min. Após esse tempo, se restar resíduo sólido no fundo do tubo, esse material deverá ser
transferido para o béquer e tentar-se-á dissolvê-lo novamente. Se dissolvida, separa-se uma alíquota
para determinação dos elementos traços e terras raras e outra parte para geoquímica isotópica.
Esse procedimento de ataque químico seguiu as descrições de Eggins et al. (1997), exceto que
o elemento Tm não foi usado como padrão interno. Os padrões utilizados foram: BHVO-1 (amostra de
basalto proveniente do Observatório Vulcânico do Havaí - Hawaiian Volcano Observatory) e o W-2
(um diabásio dos laboratórios do United State Geological Society - U.S.G.S.). As concentrações para
W-2 foram derivadas parcialmente de sua relação com o primeiro padrão ou foi baseada em um acervo
de dados de padrões já publicados (A. Greig com. pessoal 2002). O molibdênio não foi analisado
porque as jaquetas de aço de teflon causam contaminação. Baseado em análises repetidas de digestões
múltiplas dos padrões (um total de 54) a reprodutibilidade, bem como o desvio padrão relativo da
maioria dos elementos, é de 1%, exceto para Be e Zn, os quais estão na média de 2-4% e o Sn o qual
apresenta desvio padrão de 15-20%.
1.3.7 Geocronologia:
Quatro métodos geocronológicos diferentes foram utilizados e estão descritos a seguir:
1.3.7.1 Rb-Sr
Uma alíquota da amostra, após ataque químico (conforme procedimento descrito na parte
geoquímica), foi levada para uma coluna de resina de troca catiônica para concentração do Sr
utilizando-se de diferentes concentrações de HCL (1N HCl, 2.5 N HCl, and 6N HCl), conforme
procedimentos descritos por Wendt & Collerson (1999).
As medidas da composição isotópica de estrôncio foram feitas em espectrômetro de massa VG
54-30 (Sector multi-collector mass spectrometer in static mode in the ACQUIRE laboratory at the
University of Queensland).
Oliveira, 2004 Evolução tectônica de um.........
A reprodutibilidade para o método geocronológico do Sr do ACQUIRE é controlada pela
repetição de análises do padrão internacional EN1 e NBS-987 (carbonato de estrôncio), cujo valor é de
0.710251±20 (2σ) medido por um período de 7 anos.
A razão 87Rb/86Sr foi determinada utilizando-se da concentração em ppm do Rb e Sr obtidas
no Fisons PQ2+ Plasmaquad Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) conforme
metodologia descrita anteriormente. Os cálculos efetuados encontram-se na Tabela 1.1.
Os valores das razões 87Sr/86Sr foram normalizadas em função da relação 86Sr/88Sr igual a
0,1194 tendo sido utilizadas nos cálculos as constantes recomendadas por Steiger & Jaeger (1978).
Tabela 1.1 – Cálculo para determinação da razão 87Rb/86Sr.
A B C D E F G H I
Rb Moles Rb 87Rb Sr Moles Sr 86Sr Rb/Sr 87Rb/86Srppm ppm
A1/A4 B1*A3 F9 D1/E1 F1*D3 A1/D1 C1/G11 AO-1 211.79 2.4780 0.6898 131.34 87.6058 1.4992 0.1478 1.6125 4.666123 87Rb 0.2784 86Sr 0.0986
abundância
5 88/88 1 88 0.8256 87.9056 A5/A9 = 0.8131 F5*D5 = 71.47626 84/88 0.0067 84 0.0056 83.9134 A6/A9 = 0.0055 F6*D6 = 0.46057 86/88 0.1194 86 0.0986 85.9093 A7/A9 = 0.0971 F7*D7 = 8.34048 87/88 0.1037 87 0.0702 86.9089 A8/A9 = 0.0843 F8*D8 = 7.32869 SUM (A5:A8)= 1.2299 A9/A9 = 1.0000 SUM (H5:H8)= 87.6058
4
87/88 é o valor da amostra (e.g AO-1) medida no espectrômetro vezes o valor de 86/88
87.44245
Quantidade de isotopos
AO1 Sr - atomic WtAmostra
Peso atômico do Rb 85.4678 Peso atômico
do Sr
Cálculo para determinação da razão 87Rb/86Sr
1.3.7.2 Sm-Nd
Depois de coletado os REEs da coluna de Sr, o material foi seco e a ele adicionou-se uma
fraca concentração de HCL e esse concentrado foi levado para uma coluna de resina de troca catiônica
para separação do Nd. Diferentes concentrações de HCL (1N HCl e 2.5 N HCl) foram utilizadas para
obter o Nd.
A concentração isotópica do neodímio foi medida no espectrômetro de massa VG 54-30
(Sector multi-collector mass spectrometer in static mode in the ACQUIRE laboratory at the University
of Queensland).
A reprodutibilidade para Nd também é controlada pela repetição de análises do padrão
internacional La Jolla and Ames metal Nd padrão com valores 143Nd/144Nd = 0.511861±11 (2σ) e
0.511977±12(2σ). Na determinação da razão Sm-Nd foi seguido os mesmos passos para Rb-Sr.
Maiores detalhes para separação dos elementos Sr e Nd ver Wendt & Collerson (1999).
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Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
A razão 147Sm/144Nd foi determinada utilizando-se da concentração em ppm do Sm e Nd
obtidas no Fisons PQ2+ Plasmaquad Inductively Coupled Plasma-Mass Spectrometer (ICP-MS)
conforme metodologia descrita anteriormente. Os cálculos efetuados encontram-se na Tabela 1.2.
Tabela 1.2 – Cálculo para determinação da razão 147Sm/144Nd
A B C D E F G H
Sm Moles Sm 147Sm Nd Moles Nd 144Nd 147Sm/144Nd Sm/Ndppm ppm
A1/A4 B1*A3 D1/D4 E1*D3 C1/F1 A1/D11 AO-1 0.88 0.0059 0.0009 5.61 0.0389 0.0093 0.0954 0.157723 147Sm 0.15 144Nd 0.2384 At wt Sm 150.3656 At wt Nd 144.2379
Amostra
Cálculo para determinação da razão 147Sm/144Nd
Os erros experimentais usados para calcular as regressões lineares utilizando-se do Isoplot
foram de 0,5% para 87Rb/86Sr e 0.1% para 147Sm/144Nd baseados nos erros dos padrões medidos no
laboratório e 26ppm para 87Sr/86Sr e 143Nd/144Nd baseados nos erro externo baseado nos padrões NBS
SRM 987 e EN-1 medidos nos últimos 7 anos no ACQUIRE.
1.3.7.3 Ar-Ar
Sete amostras foram escolhidas para determinação Ar-Ar, as quais separou-se uma quantidade
de 20 cristais de biotitas e/ou anfibólios utilizando-se de uma lupa binocular. Esses cristais foram
lavados, primeiro em água destilada e depois em álcool absoluto para eliminar as impurezas
superficiais, impregnadas nos cristais que foram colocados para secar. Depois de secos, uma média de
10 cristais foi colocada num disco de alumínio juntamente com padrões de sanidina. Selou-se o disco e
o mesmo foi encaminhado para a irradiação no Oregon State University Triga Reator-USA (CLICIT
Facility) por um período de 14 horas, utilizando-se do “Fish Canyon sanidine neutron fluence
monitors”. Após essa irradiação, resguardando o período correto de resfriamento das amostras, deu-se
início às análises 40Ar/39Ar, pelo método de passo incremental de laser no Laboratório de
geocronologia - UQ-AGES (the University of Queensland Argon Geochronology in Earth Science
Laboratory).
Os procedimentos seguiram as descrições de Vasconcellos (1999a, b) e Vasconcelos et al.
2002
1.3.7.4 U-Pb
Os zircões concentrados inicialmente no LOPAG foram reconcentrados na Universidade de
Queensland, utilizando-se líquidos densos. Primeiro utilizou-se o bromofórmio para eliminar
impurezas (e.g. feldspatos e quartzo). Em seguida foi utilizado o iodeto de metileno para separar a
9
Oliveira, 2004 Evolução tectônica de um.........
apatita de zircão e, por fim, utilizou-se o separador magnético Frantz para obter o concentrado final de
zircão.
A partir desse concentrado utilizou-se um microscópio binocular para catar os melhores
cristais de zircão que foram montados numa pastilha (mount) contendo em torno de 30-40 zircões das
5 amostras estudadas e um padrão (Fig. 1.2). A pastilha foi submetida a catodoluminescência e, então,
definiu-se os melhores cristais a serem datados. As razões isotópicas foram obtidas num equipamento
CAMECA IMS1270 Ion Microproble do Swedish Museum of Natural History, na Suécia. Para
maiores detalhes sobre os métodos analíticos ver Whitehouse & Russel (1997) e Whitehouse et al.
(1999).
Figura 1.2 – Mapa esquemático dos zircões das 5 amostras selecionadas (a numeração das amostras selecionadas está em negrito).
1.3.8 Tratamento dos dados:
Os programas utilizados para construção dos vários digramas, figuras e tabelas foram os seguintes:
• Excel 2000 para construção dos diagramas de geoquímica e diagramação das tabelas.
• Corel Draw 9 (para confecção de algumas das figuras)
• Freehand 8 (confecção das figuras Ar-Ar)
• Isoplot versão 3.0 (Aplicativo geocronológico para Microsoft Excel, Ludwig 1998).
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Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
11
• EndNote (programa de referências bibliográficas)
Para o cálculo dos parâmetros isotópicos de estrôncio e neodímio foram utilizados os
programas de Centro de Pesquisas Geocronológicas da USP.
1.3.9 Elaboração da Tese:
Os subseqüentes capítulos dessa tese são artigos publicados e submetidos ou a submeter.
Sendo assim, cerca de 90% dela foi elaborada em inglês. Especificamente, o capítulo 2, já publicado,
trata os dados petrográficos e estruturais (Oliveira, A.H. and Carneiro, M.A., 2001. Campo Belo
Metamorphic Complex: Tectonic evolution of an Archean sialic crust of the southern São Francisco
Craton in Minas Gerais (Brazil). Anais da Academia Brasileira de Ciências, 73(3): 397-415).
O capítulo 3 trata o comportamento geoquímico, com ênfase nas rochas charnockíticas e
enderbíticas. Esse artigo foi submetido à Lithos (Oliveira, A.H., Kamber, B.S., Collerson, K.D. and
Carneiro, M.A. Origin of charnockites and enderbites inferred from incompatible element loss during
prograde dehydration).
O capítulo 4 trata a geocronologia Rb-Sr e Sm-Nd. Esse artigo foi submetido aos Anais da
Academia Brasileira de Ciências (Oliveira, A.H., Carneiro, M.A., Kamber, B.S. and Collerson, K.D.
Recent advances among Sr and Nd concerning to the tectonic evolution of Campo Belo Metamorphic
Complex, southern portion of the São Francisco Craton, Brazil).
O capítulo 5 trata a geocronologia Ar-Ar. Esse artigo foi submetido ao Journal of South
American Earth Sciences (Oliveira, A.H., Vasconcelos, P.M. Carneiro, M.A and Carmo, I.O.
Implications for Transamazonian 40Ar/39Ar ages for the Campo Belo Metamorphic Complex, Southern
Sao Francisco Craton, Brazil).
O capítulo 6 trata a geocronologia U-Pb e o capítulo 7 das considerações finais. Pretende-se
submeter um artigo com esses dois capítulos ao Precambrian Research, mas isso só após a defesa
dessa tese...
Oliveira, 2004 Evolução tectônica de um.........
12
CAPÍTULO 2 CAMPO BELO METAMORPHIC COMPLEX: TECTONIC
EVOLUTION OF AN ARCHAEAN SIALIC CRUST OF THE SOUTHERN SÃO FRANCISCO CRATON IN MINAS GERAIS
(BRAZIL)
2.1 ABSTRACT
Systematic geological studies performed in the study area allowed characterize six lithodemic
units: three gneissic, one amphibolitic, one supracrustal and one fissure mafic. The rock mineral
assemblage and the structural record of these lithodemic units indicate that the study area was affected
by five tectonothermal events. The structural pattern of the first and oldest event occurred under
granulite facies conditions and reveals an essentially sinistral kinematics. The second event, showing
dominant extensional characteristics, is related to the generation of an ensialic basin filled by the
volcano-sedimentary sequence of the supracrustal lithodemic unit. The third event, which is the most
expressive in the study region, is characterized by a vigorous regional migmatization process and by
the generation of the Claudio Shear Zone, presenting dextral kinematic movement. The four events is
represented by a fissure mafic magmatism (probably two different mafic dike swarm) and finally, the
fifth event is a regional metamorphic re-equilibration that reached the greenschist facies, closing the
main processes of the tectonic evolution of the Campo Belo Metamorphic Complex.
Key Words: Craton, Archean, tectonic evolution, lithodemic units, metamorphic complex.
2.2 INTRODUCTION
The São Francisco Craton is a vast Precambrian platform (Almeida 1977, Alkmim et al.
1993), which encompasses part of Minas Gerais and Bahia States (Fig. 2.1). Its southern portion (Fig.
2.1) presents significant expositions of Neoarchean granite-greenstone terranes; its tectonic evolution
started in the Mesoarchean (Teixeira et al. 1996, 1998, Carneiro et al. 1998a). From that period on the
first sialic crusts and supracrustal sequences were generated by means of successive
accretion/differentiation stages associated with crustal reworking processes (Teixeira 1993, Teixeira &
Silva 1993, Teixeira & Canzian 1994, Noce 1995, Carneiro et al. 1996 a, 1997 a, b, 1998 a, b, Pinese
1997, Teixeira et al. 1996, 1997, 1999).
The apex of these processes took place in the Neoarchean, during the Rio das Velhas
Tectonothermal Event (Carneiro et al. 1998a). Other events are also represented in this crustal
segment. They are: two events of tectonothermal nature in the Mesoarchean [3.2 and 2.9 Ga, (Teixeira
et al. 1996, 1998)]; a migmatization event [2.86 Ga., (Noce 1995)]; a fissure mafic magmatism event
[2.658 Ga., (Pinese 1997)] and a felsic magmatism event [2.65 Ga., (Noce 1995)]. Due to this
succession of events, the tectonic relationships between the generated (or reworked) units were
Oliveira, 2004 Evolução tectônica de um.........
progressively masked. Therefore, a complex structural pattern was imprinted in the lithodemic units
that constitute the metamorphic complex, their supracrustal sequences and intrusive bodies that crop
out in the study area.
Divinópolis
CláudioCarmópolisde MinasItapecerica
Oliveira
Candeias Conselheiro Lafaiete
Ouro Preto
Barbacena
Campo Belo
Lavras
Studied Area
0 10 20 30Km
BRAZIL
Figure 2.1 Geologic map of southern São Francisco Craton (modified from Machado Filho et al. 1983). 1) Porto dos Mendes Granite; 2) São João del Rey Supergroup; 3) Minas Supergroup; 4) Formiga Granite; 5) Rio das Velhas Supergroup; 6) Divinópolis Metamorphic Complex, and 7) Barbacena Metamorphic Complex.
Recent research, involving regional (1:200,000) and detailed (1:10,000) mapping performed in
the Campo Belo, Oliveira, Itapecerica, Cláudio, Carmópolis de Minas regions has characterized new
lithodemic units and presented new facts to the tectonic evolution of the southern São Francisco
Craton (Carneiro et al. 1996a, b, 1997a, b, c, 1998a, b, Carvalho Júnior et al. 1997, 1998, Fernandes et
al. 1997, 1998, Fernandes & Carneiro 2000, Corrêa da Costa et al. 1998, Corrêa da Costa 1999,
Oliveira et al. 1998a, b, 1999, Oliveira 1999, Oliveira & Carneiro 1999).
In this work the results of one of these studies (Oliveira 1999) are presented, carried out in the
Cláudio, Itapecerica, São Francisco de Paula, Oliveira and Carmópolis de Minas region, involving
lithostructural mapping at the 1:200,000 scale (Figs. 2.1 and 2.2).
14
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Monsenhor João Alexandre
Itapecerica60
70
3740
40
60
20
20
20
30
85 4050
45 80
50
40
25
20
30
251030
30
20
80
60
45
30
60
8065
3545
20
50 75
70
80
4045
75
60
70
6045
50
6545
45
60
50
30
3015
20
40
60
60
30
30
40
87
50
5580 7040
85
50
6055
55
25
40
60
25 40
3540
70
45
45
45
8055
3075
16
10
40 40
45
20
2536
0505
1 2 3 4 5 6 7 8
Carmoda
Mata
Oliveira
Carmópolisde M inas
Cláudio
São Franciscode Paula
0 2 4 6km
25 30
40
9
B
F
A
60
40
C
E
I
H
D
J
A
G
20°15´00´´
44°1
5´00
´´ 20°15´00´´
44°3
0´00
´´44
°30´
00´´
20°45´00´´
N44
°15´
00´´
20°45´00´´
Figure 2.2 Geologic map of the study area (modified from Oliveira 1999), showing the different lithodemic units: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A - Fernão Dias road; B - Corumbá dimension stone quarry; C - Kinawa dimension stone quarry; D – Morro da antena; E - Vista Bela farm; F – Carmo da Mata dimension stones quarry; G – Marilan dimension stones quarry; H – Lila dimension stones quarry; I – Alemão dimension stones quarry; J – Oliveira dimension stones quarry.
2.3 GEOLOGIC CONTEXT
The sialic crust of the southern São Francisco Craton is constituted by gneisses, granitoids,
amphibolites, mafic and ultramafic rocks, schists and quartzites that were grouped by (Machado Filho
et al. 1983), in the Divinópolis and Barbacena metamorphic complexes (Fig. 2.1). Locally, remains of
the supracrustal sequences are found, correlated with the Rio das Velhas or Minas Supergroups.
The geographic distribution of the Divinópolis Metamorphic Complex, according to (Machado
Filho et al. 1983), occurs in the vicinity of Divinópolis, Itaúna, Formiga and Passa Tempo cities (Fig.
2.1). In the remaining region, rocks from the Barbacena Metamorphic Complex would predominate.
However, (Teixeira et al. 1996) consider these two complexes in the Campo Belo region (Fig. 2.1) as
a single unit, naming it Campo Belo Metamorphic Complex. According to (Machado Filho et al.
1983), the majority of the lithotypes listed above presents three principal deformation and/or fracturing
directions: NS; NW/SE and NE/SW.
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Oliveira, 2004 Evolução tectônica de um.........
16
The tectonic meaning of these directions according to (Machado Filho et al. 1983) is the
following: 1) NS direction, of more plastic character, would be responsible for the compression of
supracrustal sequences in a tight syncline; 2) the NW/SE structures displace the N/S structures,
slightly metamorphosed basic dikes were emplaced along this trend; 3) the last direction is related to
the generation of the Atlântico Mobile Belt, at the southeastern-eastern margin of the Southern São
Francisco Craton.
2.4 GEOLOGY OF THE AREA
Using satellite imagery and aeromagnetometric map interpretation and by means of regional
geologic sections (Oliveira 1999) it was possible to characterize six lithodemic units (Tab. 2.1 and Fig.
2.2). The older three of these units are gneissic, the fourth is amphibolitic (cannot be mapped at the
scale adopted), the fifth is a supracrustal sequence (of greenstone belt type), and the youngest is a
mafic dike swarm.
2.4.1 Gneissic Units
The petrographic distinction of the gneissic units (Fig. 2.2) was mainly based on their color
and composition. The Cláudio Unit rocks are gray and their composition is predominantly
granodioritic. Those from the Itapecerica Unit are light pink and their composition is predominantly
granitic. Finally, the Candeias Unit rocks are green and their composition is granitic to granodioritic.
The geologic contacts between the gneissic units are masked by the thick weathering mantle
and by the superposition of tectonothermal processes taking place since the Mesoarchean. However,
the gneissic units have distinct outcrop domains. As an example, the case of the Itapecerica Gneissic
Unit is given, its rocks cropping out in the surroundings of the city it borrows the name from and it is
characterized by intense migmatization, which imprinted an average mineralogic composition of
granitic nature. Such migmatization is more pronounced in the western portion of its geographic
occurrence.
Three pegmatite vein families are associated with the gneissic units. The first family is formed
by mobilizates concordant with the gneissic foliation and presents centimetric thicknesses and variable
(centimetric to metric) lengths. These mobilizates are essentially composed of feldspars and small
quantities of mica. The mobilizates are usually light pink and sometimes white.
The second pegmatite family is discordant from the gneissic foliation and is centimetric- to
metric-thick, with veins reaching some meters in length. It is in general coarse-grained and light pink,
formed by feldspar phenocrysts and less abundant centimetric mica flakes. The third family is
represented by pegmatite veins that crosscut either the gneissic foliation or the other two families.
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
These are centimetric-thick and metric-long veins. They are fine- to medium-grained and fill NS-
trending fracture planes.
Table 2.1 General characteristics of the rocks types of the study area and their probable ages and tectonothermal events.
Unit Lithology Rock-forming Minerals AgesTectono-thermal Events
quartzite Quartz, sillimanite and garnet
schist Sill imanite, quartz, mica and garnet
amphibolite Clinoamphibole, plagioclase and quartz
ultramaficOrthopyroxene,
olivine, clinoamphibole
Amphibolitic amphibolite Hornblende, plagioclase and quartz Probable age: 3,38-3,0 Ga1 (?)
E1, (E2?), E3, (E4?) and E5.
Candeias Gneiss
Gneiss of granodioritic to
granitic composition
Plagioclase, microcline, quartz and
hypersthene
Itapecerica Gneiss Gneiss of granitic composition
Microcline, plagioclase and quartz
Cláudio GneissGneiss of
granodioritic composition
Plagioclase, microcline and quartz
References: 1(Teixeira et al . 1996b), 1(Teixeira et al . 1998), 2(Carneiro et al . 1998b), 3(Pinese 1997).
Supracrustal (Rio das Velhas
Supergroup)
E3, (E4?) and E5
E4 and E5
gabbro
E1, (E2?), E3, E4 and
E5
Banded iron formation
Quartz and opaque minerals
Probable ages for the mafic-ultramafic magmatism,
followed by sedimentation (2.8 Ga2).
Probable age for the protolith (3.38-3.0 Ga1), and probable age for the migmatiza-tion
(2.75-2.72 Ga2).
Fissure Mafic
gabbronorite plagioclase, ortho- and clinopyroxenes Ages for the intrusions: 2.658
Ga3 and 1.875 Ga3.Plagioclase,
clinopyroxene
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Oliveira, 2004 Evolução tectônica de um.........
18
d can alter to biotite. Opaque minerals, zircon
and apatite are accessories, as well as rutile, tourmaline and allanite, which are rare. As secondary
mineral te occur.
n chlorite. Amphibole is rarer, fine-grained and alters to biotite. Common
accessories are zircon, apatite and opaque minerals. Sericite, chlorite, carbonate and epidote are
secondary minerals.
2.4.1.1 Cláudio Gneissic Unit
The rocks of this unit crop out in the eastern portion of the study area (Fig. 2.2) and present
granodioritic to dioritic, locally granitic, compositions. They are gray, fine- to medium-grained,
banded and migmatized. In some outcrops light pink pegmatitic mobilizates occur, giving a rosy tint to
the rock. Regionally, the color of these gneisses can vary, so that eastwards (Fig. 2.2), it is
predominantly gray, diminishing the percentage of light pink felsic mobilizates. The texture of the
Cláudio gneisses is predominantly granoblastic to granolepidoblastic. The crystals vary from
subidioblastic to idioblastic, medium- to fine-grained, being more rarely coarse-grained. Intergrowth
textures are common (e.g. perthite, myrmekite and sometimes antiperthite). In general, plagioclase is
the main constituent of these rocks (30-40%). The crystals are predominantly medium-grained and
those showing polysynthetic twinning vary from subhedral to euhedral. Quartz is in general the second
constituent in volume (20-30%). The microcline percentage is also variable, sometimes being
comparable to the other two constituents. However, it is more common to be subordinated (15-30%).
As varietal mineral, biotite (5-10%) shows light to dark or greenish brown pleochroism, altering to
chlorite. It has zircon inclusions and sometimes it is associated with opaque minerals. Amphibole
(hornblende) is rare and shows greenish pleochroism an
s chlorite, sericite, carbonate and epido
2.4.1.2 Itapecerica Gneissic Unit
The rocks from the Itapecerica Gneissic Unit crop out in the NW portion of the area and
present granitic composition are light pink, migmatized and show inequigranular to equigranular
textures and are medium- to fine-grained. In some outcrops closer to the contact with the Candeias
Gneissic Unit (Fig. 2.2), the rocks are greenish. In other localities the gneiss is grayish, as the outcrops
located east of Itapecerica city. Pegmatitic or amphibolitic dikes constituting boudins are common
features. The texture of the Itapecerica Gneissic Unit rocks is granoblastic to granolepidoblastic with
subidioblastic to idioblastic, fine- to medium-grained crystals. Microcline is the predominating
mineral (30-40%), plagioclase second in abundance (15-30%) and quartz is subordinated to feldspar
(±20%). Biotite (<10%) is a varietal mineral and presents dark brown to greenish pleochroism, locally
altering to pale-gree
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
19
k. Locally, biotite alters to chlorite.
Opaque minerals, zircon and apatite are the commonest accessories of the rock. Chlorite is the
commo rowing biotite and pyroxene.
, black and strongly
foliated. In most places, the amphibolites are found in the form of disrupted thin boudins, representing
disrupte
reach 10% of the volume and alters to amphibole.
Accessory minerals are zircon, opaque minerals and apatite. Besides biotite, secondary minerals are
chlorite sericite.
Ultramafic rocks (metaperidotite, chlorite-amphibole-schist and hornblendite), amphibolite,
2.4.1.3 Candeias Gneissic Unit
The gneisses of this unit crop out in the SW portion of the area (Fig. 2.2). In the most
deformed domains the rocks present granodioritic composition (opdalites). However, in the more
homogeneous domains the composition is granitic (charnockites). In general, these rocks are green,
slightly migmatized, deformed and medium- to coarse-grained. The mineralogic banding, in some
places, is difficult to be observed due to the great homogeneity of the bodies, except for the
hyperstene-biotite-gneiss and biotite-gneiss where the biotite planar orientation is more enhanced.
Close to the contact with the Itapecerica Gneissic Unit, light pink pegmatitic mobilizates are found.
The texture of the Candeias-type gneisses varies from granolepidoblastic to granoblastic and they are
subdioblastic to idioblastic, predominating granoblastic. Antiperthitic plagioclase (30-45%) is the
main constituent, quartz (20-25%) and the microcline content (15-20%) varies from sample to sample
but can reach up to 40%. Hyperstene, when present, is medium-grained, has low birefringence and
pleochroism varying from light pink to pale green. It alters to amphibole, biotite and chlorite. Biotite,
sometimes showing symplectitic intergrowth with quartz, presents light brown, sometimes greenish,
pleochroism. In general, this mineral defines the banding of the roc
nest secondary mineral, overg
2.4.2 Amphibolitic Unit
The rocks of this unit are fine-grained, melanocratic, dense, phaneritic
d and metamorphosed dikes emplaced in the rocks of the gneissic units.
The texture of the amphibolites varies from nematoblastic to granolepido-nematoblastic. The
crystals are fine- to medium-grained, strongly oriented and altered. Green hornblende (±50%),
sometimes altered to biotite, is a main mineral. Locally, biotite schist (or biotitite) results from
retrograde metamorphism and shearing. Plagioclase is the second constituent in volume (20-30%).
Quartz appears subordinated to the other minerals and its content varies from 5 to 10% of the rock
volume. Biotite, sometimes chloritized, is the product of amphibole transformation. Pale yellow
clinopyroxene, present in some rocks, can
and amphibole, epidote and
2.4.3 Supracrustal Unit
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20
e-schist, garnet-sillimanite-quartzite and banded iron formation were characterized in
the study area.
ed by positive magnetic anomalies as can be observed in aeromagnetometric
maps (Oliveira 1999).
nd generally result from spinel transformation.
Chlorite is sometimes present as secondary mineral.
ine hornblende mass (±
70%). A small amount of opaque minerals represents the accessory minerals.
dominantly medium-grained granonematoblastic
hornblende crystals (± 95%) and opaque minerals.
and clinopyroxene occur occasionally. The main accessories are zircon,
opaque minerals and apatite.
garnet-sillimanit
Regionally, the Supracrustal Unit (Fig. 2.2) can be correlated with the Rio das Velhas
Supergroup, which crops out in the Quadrilátero Ferrífero in Minas Gerais. In some places, they are
strongly deformed, metamorphosed and sometimes cataclastic. In the field, the domain of the
ultramafic rocks is mark
The metaperidotite is a holocrystalline rock with inequigranular, anhedral to subhedral
crystals and grain-size varying from fine to coarse. It shows cumulatic and/or mesh texture. Enstatite
makes up 50% to 60% of the total volume of the rock. Anhedral clinopyroxene and olivine crystals
can reach up to 20% of the rock volume. The hornblende percentage varies from 20% to 50%. Spinel
can reach 10% of the rock volume and appears as granular crystals of high relief and color varying
from green to greenish-brown. Serpentine, rare in the majority of the thin sections, is found as fibro-
lamelar aggregates, resultant from the alteration of olivine and pyroxenes. The opaque minerals can
reach 5 to 10% of the rock. They are isotropic a
The chlorite-amphibole-schist is a greenish rock with porphyroblastic texture. Chlorite sums
up 30% of volume and appears as poikilitic porphyroblasts embedded in a f
The hornblendite is composed of pre
The amphibolite is a fine- to medium-grained rock, melanocratic, dense, phaneritic, black and
strongly foliated. The amphibolite is, in general, found in contact with ultramafic rocks, mainly in the
surroundings of Cláudio, in the supracrustal unit domain (Fig. 2.2). The main foliation trend is
NE/SW, accompanying the direction of the magnetic anomaly observed in the aeromagnetometric
maps (Oliveira 1999). The rock texture varies from nematoblastic to granonematoblastic. The crystals
are fine-to medium-grained. Green hornblende (± 40%), sometimes alters to biotite and chlorite.
Plagioclase (± 35%) is the second constituent in volume and alters to sericite and epidote, and quartz is
subordinated (± 10%). Biotite
The garnet-sillimanite-schist is a fine- to medium-grained, dark gray, strongly mylonitized
magnetic rock. The rock texture varies from granolepidoblastic to granonematoblastic. The crystals
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21
minerals, alteration products after plagioclase. White
mica is also formed from sillimanite and biotite.
mica is the secondary mineral in the rock and it is the alteration product
after sillimanite and biotite.
and strongly
recrystallized. Magnetite contents vary from 30 to 40 % in terms of modal composition.
2.4.4 Fissure Mafic Unit
same lineament small diabase dikes can be observed as a dark, dense and fine-grained rock.
vary from subidioblastic to granoblastic. Quartz is the main constituent (± 40%). The crystals are
elongated (ribbons) and follow the rock foliation. Plagioclase (± 15%) is the second main constituent.
Sillimanite in variable proportions (up to 70%) presents yellow polarization color, is fine- to medium-
grained and the crystals are fibrous (more common) or prismatic. It alters to white mica as like biotite
(± 10%). Garnet (± 5%) is a light brown mineral with dark-gray polarization color, almost isotropic
with opaque minerals, quartz and white mica inclusions. Opaque minerals, zircon and rutile are the
commonest accessories in this rock. Rutile is translucent brown and can be associated with biotite.
White mica, epidote and carbonate are secondary
The garnet-sillimanite-quartzite is a fine-grained rock, pale greenish yellow to white,
mylonitized and with subvertical foliation. The texture varies from granolepidoblastic to
granonematoblastic and the grain-size varies from fine to medium. Quartz (± 70%) is the main rock
constituent. The crystals are elongated (ribbons) accompanying the foliation. Sillimanite (± 20%)
presents yellow polarization color, grain-size varying from fine to medium with fibrous crystals
(fibrolite) oriented according to the foliation. It alters to white mica. Garnet (± 3%) is a light brown
mineral and has dark gray polarization color. Rare biotite shows light to dark brown pleochroism, is
fine-grained, with zircon inclusions, and alters to white mica. Opaque minerals, zircon and rutile are
common accessories. White
Finally, the banded iron formation is a magnetic rock, of predominantly fine grain-size,
composed by magnetic minerals and quartz. This rock presents granoblastic texture and the contact
between grains is elongated due to deformation. Quartz (± 60%) is the main constituent of the rock,
with elongated crystals in the form of ribbons; it is fine- to medium-grained
This unit is represented by NW/SE-trending dikes (Fig. 2.2). These dikes have varied
thickness, from metric to dozens of meters and their length can reach dozens of kilometers. The dikes
are subvertical and commonly develop ramifications involving the gneissic massifs. Compositionally,
the dikes are composed of gabbronorite, gabbro and diabase. The gabbronorite and the gabbro are
medium- and coarse-grained, possibly representing variations of crustal emplacement during
magmatic crystallization. These rocks are in general dark-colored to greenish, and present massive
structure and phaneritic texture. However, when very fine-grained, they can be aphanitic. Along the
Oliveira, 2004 Evolução tectônica de um.........
22
The gabbronorite shows ophitic, subophitic to intergranular textures, with subhedral to
euhedral crystals, and is medium- to fine-grained. Plagioclase is the most abundant mineral in volume
(40-60%). (Clino- and ortho-) pyroxene contents vary from 20 to 35% in volume. In general, augite
predominates over hyperstene and is predominantly medium-grained, presents variable polarization
color (from high to low), the crystals are poikilitic, altering to amphibole. Hyperstene is light pink,
pleochroic, from medium- to fine-grained. Hornblende with green pleochroism is the alteration
product of pyroxenes (uralitization), reaching up to 5% of the rock. Quartz appears in small quantities,
with clean, anhedral, fine-grained crystals. Biotite is rare and reddish, pleochroic. Zircon and opaque
minerals are accessory minerals. White mica, epidote, carbonate and chlorite are secondary minerals.
The gabbro presents ophitic, subophitic or intergranular textures. The medium- to fine-
grained crystals vary from subhedral to euhedral. Plagioclase is the most abundant mineral in volume,
varying from 30-50%. Augite contents, second component in volume, vary from 20 to 30%. The
crystals are poikilitic and alter to amphibole. Hornblende is the uralitization product after pyroxene,
being its content in volume variable between 5 and 15%. When present, microcline is rare, showing
cross-hatch twinning and is associated with myrmekite. Quartz appears in small quantities. Biotite is
also rare, reddish and pleochroic. Zircon, opaque minerals and apatite are accessory minerals. White
mica, epidote and chlorite are secondary minerals observed in some of the rocks.
2.5 METAMORPHISM
Three distinct and anachronic metamorphic peaks are registered in the rocks of the study area.
The first, of highest metamorphic grade related to the granulite facies, is present in the rocks of the
Gneissic Units and of the Amphibolitic Unit. The second, predominantly of the amphibolite facies, is
observed in the mafic-ultramafic and clastic-pelitic rocks of the Supracrustal Unit. The third is
characterized by regional retrograde metamorphic processes, which reached the greenschist facies and
cover the whole region.
The main paragenesis of the Cláudio, Itapecerica and Candeias Gneissic Units is constituted
by plagioclase ± quartz ± (antiperthitic) microcline ± amphibole ± hyperstene ± biotite. As secondary
paragenesis sericite, carbonate, chlorite and epidote occur. Antiperthite and hyperstene indicate that
these rocks underwent metamorphic conditions of the granulite facies. On the other hand, the
secondary paragenesis indicates greenschist-facies retrograde metamorphism.
The Amphibolitic Unit, enclosed in these gneissic units, also presents high-grade paragenesis,
with the presence of hyperstene, and retrograde metamorphic features of the greenschist facies,
characterized by the presence of chlorite.
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23
However, Supracrustal Unit rocks show slightly different parageneses. Except for the
Supracrustal Unit ultramafic rocks, where green Al2O3-richer spinels would indicate metamorphic
conditions compatible with the amphibolite-granulite facies (Paktunç 1984), the mineral assemblages
of the schists (quartz ± plagioclase ± sillimanite ± garnet) and quartzites (quartz ± sillimanite ± garnet
± biotite) indicate that the metamorphism in these rocks reached at most the amphibolite facies.
Afterwards, as show the processes of sillimanite alteration to white mica and plagioclase
sericitization, these assemblages were re-equilibrated to greenschist facies conditions. However, if in
the Supracrustal Unit rocks metamorphism reached the amphibolite or even granulite facies, this was
note the case in the Fissure Mafic Unit rocks. These rocks have preserved igneous textures, showing
only metamorphic re-equilibration in the greenschist facies that, in fact, is common to all units
described in this paper.
So, two significant considerations can be expressed: a) the chronostratigraphic character of the
Fissure Mafic Unit that, due to its igneous textures and metamorphic parageneses at most of the
greenschist facies, indicates their crustal emplacement after the tectonothermal events of higher
metamorphic facies that affected the older units; b) the metamorphic facies variation between rocks of
the Gneissic, Amphibolitic and the Supracrustal Units. Two hypotheses can be put forward to explain
this matter. The first hypothesis would imply in the existence of two tectonothermal events. The first
one, of the granulite facies, would be previous to the generation of the Supracrustal Unit rocks and the
second one, locally reaching the granulite facies, would follow the generation of the Supracrustal Unit
rocks. The second hypothesis would favor the existence of a single event of the granulite facies that
occurred after the generation of the Supracrustal Unit rocks. Isotopic evidence supports the first
hypothesis, as attested (Teixeira et al. 1996, 1998, Carneiro et al. 1998b), which respectively defend a
metamorphic event of the granulite facies in the Mesoarchean and another of amphibolite facies in the
Neoarchean (Rio das Velhas Tectonothermal Event). Therefore, it is believed that the first possibility
is more realistic to explain the tectonic evolution of the region, according to the reasons exposed in the
following items.
2.6 STRUCTURAL ANALYSIS
The rocks of the study region present distinct structural features (Fig. 2.3, Fig. 2.4, Table 2.2).
For example, it is observed in the supracrustal unit rocks: 1) a mylonitic foliation (Sm) of
anastomosing character; 2) structures S/C; 3) S and Z intrafolial folds, tight and/or isoclinal sheath
mesofolds; 4) foliation sigmoids; 5) rotated quartz veins, and 6) tension gashes.The gneissic unit rocks
present, besides the structural features described above, the following ones: 7) a gneissic foliation (Sg)
of (local) mylonitic character and anastomosing pattern; 8) shear bands (Sb1 and Sb2); 9) pegmatitic
Oliveira, 2004 Evolução tectônica de um.........
24
vein folds and rotated amphibolite boudins. Local normal and reverse folds and a regional mega-
structure here named Cláudio Shear Zone were characterized. The area also shows intense NW-SE-
trending crustal fracturing along which mafic dike swarms were emplaced (Fig. 2.2).
2.6.1 Foliations
The gneissic foliation (of mylonitic character) presents steep to low-angle dips. This foliation
is characterized by a fine compositional banding represented by the orientation of biotite, sericite and
amphibole. In general the foliation is anastomosing.
The (mylonitic) foliation Sm observed in the supracrustal unit rocks presents steep to low-
angle dips and is characterized by the orientation of the following minerals: pyroxene (in the
ultramafic rocks); amphibole (in the amphibolites); elongated quartz (ribbons ) and sillimanite (in the
quartzites); biotite, sericite and sillimanite (in the schists).
The structural style and strain of the foliation deformation indicate that they could have
originated from the same deformation phase. Sg and Sm foliations present inflections along the study
area so that the main trend is NE-SW (Figs. 2.3A and B). Locally, a change in the foliation direction is
observed (from NS to NW-SE). The NW-SE predominates on the NS (Fig. 2.3A). The lateral relation
between these directions can be seen in some key outcrops (points 9A, 9D, 9G, 9I; Fig. 2.2).
In these points the predominant foliation (NE-SW) is folded and its trajectory in folds inflects
to NS and NW-SE. The last trend represents one of the flanks of the fold of 2 to 3 meters of amplitude.
Thus the several trends characterized in the region as a whole reflect the principal foliation inflection
(NE/SW). In general the foliation dip is steep (70° to 80°), mainly along the Cláudio Shear Zone (Fig.
2.2). However outside the Cláudio Shear Zone domain medium to low-angle dips (50° a 20°) are
observed. The mineral lineation (lm) also presents a certain variation: from strike (Kinawa quarry,
point 9C, Fig. 2.2) to oblique (Corumbá quarry, point 9B; Fig 2.2) and dip (in the quartzites, point
9D). In these three cases, the foliation planes present NE-SW trends.
2.6.2 Shear bands
Shear bands Sb1 and Sb2 are subvertical and displace the foliation of the gneissic units, as seen
in horizontal and vertical planes of the quarries of the region. The foliation Sb1 is oriented according to
NW/SE and E-W trends (Figs. 2.3C and D) and the foliation planes are filled by pegmatitic
mobilizates. The NW/SE trend presents a dextral kinematic component whereas the E-W trend has a
sinistral component (seen in the horizontal plane). Foliation Sb2 is oriented along the ENE/WSW and
NW/SE trends (Figs. 2.3E and F). Sb2 does not present pegmatitic mobilizates in its planes. The
kinematics of the ENE/WSW foliation has a sinistral component and that of the NW/SE plane a
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
dextral component (seen in the horizontal plane). These two foliations form a pair R’ and X (anti-
Riedel and secondary, Fig. 2.4H). Shear bands Sb1 and Sb2 are different in the following aspects: the
first group presents pegmatitic mobilizates along its planes, lacking in the second group. Besides, the
kinematic trends of both groups are opposite (Figs. 2.3C; 2.3D; 2.3E and 2.3F).
0.5 % 1.0 % 1.5 % 2.1 % 2.6 % 3.1 % 3.6 % 4.1 %
1.2 % 2.4 %
3.7 %
4.9 %
6.1 %
4.5 % 9.1 %
13.6 % 18.2 %
22.7 % 27.3 % 31.8 %
5.9 %
11.8 %
17.6 % 23.5 % 29.4 % 35.3 %
41.2 %
11.1 %
22.2 %
33.3 %
44.4 %
A B
C D
E F
4.3 %
8.7 %
13.0 %
17.4 %
21.7 %
25
Figure 2.3 Stereographic diagrams representing the polar projections of the foliations of the study area: A) General foliation poles of the area (Sg and Sm), of NE/SW NS and NW/SE directions. Number of foliation measurements: 388; Maximum: 310/40, 156/40 and 200/37. B) Foliation poles Sg and Sm and mineral lineation of the Cláudio Shear Zone domain. Number of foliation measurements: 246; Maximum: 310/40 and 155/40. Number of lineation measurements: 21 C) Polo to foliation of Dextral Sb1 (Shear bands) of NW/SE direction. Number of foliation measurements: Maximum: 040/82 and the rosette represents the foliation direction. D) Polo to foliation of Sinistral Sb1 (Shear bands) of E-W direction. Number of foliation measurements: 22; Maximum: 130/82 and 344/83 and the rosette represents the plane direction. E) Polo to foliation of Sinistral Sb2 (Shear
Oliveira, 2004 Evolução tectônica de um.........
bands) of ENE/WSW direction. Number of foliation measurements:17 ; Maximum: 218/88 and 040/85. F) Polo to foliation of Dextral Sb2 (Shear bands) of NW/SE direction. Number of foliation measurements: 9; Maximum: 170/83 and 350/85.
X
R'
Sb2
A B
D
F
H
S 150/70 C (130/75)
''Sb ''2
110 290Anf. Sb2
0 10cm
PH
PH
0 20cm
0 20cm
0 4mPV
E
60 250
Peg.Diab.
0 5m
PV
N
280 100
PH
C
Anf.
Peg.
''Sb ''=2 140/75
PH
0 20cm
PH
Sg=140/70
0 10cm
PH
Sg=150/63
CS
GT
Figure 2.4 Kinematic indicators (A-G) and schematic models explaining one of the event active in the study area (H). A) Structure S/C in gneiss of Cláudio Unit, with Event 1 sinistral movement; B) Sigmoid foliations of the Supracrustal Unit schists with dextral movements; C) Amphibolite boudin rotated clockwise and broken by shear zones (shear bands Sb2, X); D) Asymmetric folded pegmatite vein, Z-shaped, with dextral movement in the gneisses; E) (Brecciated) diabase dike in Cláudio Unit gneisses, displacing pegmatite vein and the gneissic foliation, denoting Event 3 Phase 2 normal movements; F) Shear bands (Sb2 , R’); G) Rotated amphibolite boudins, vertical plane view, with reverse movements; H) Schematic model representing Event 4, sinistral, indicating shear bands (Sb2), reverse faults and the intrusion of the Fissure Mafic Unit. Symbology: PH –Horizontal plane; PV – Vertical plane; Peg. – Pegmatite; Anf. - Amphibolite; Diab. - Diabase, and R’ and X (anti-Riedel and secondary).
2.6.3 Structures S/C
Structures S/C [S150/70 C(130/75), seen in the horizontal plane AC] can be observed in
restricted points in the area (points 9C and 9B; Fig 2.2). This foliation dimensions are centimetric and
the kinematics indicates sinistral movements (Figs 2.4A).
26
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Table 2.2 Simplified table of the structural elements associated with the tectonothermal events
Tectono-thermal Events
Foliation
Kinematic indicators
Faults and Shear
Zones
Metamorphism
(M)
Associated
Rocks E5 M3
greenschist facies
E4
Foliation sigmoids, “shear bands” (Sb2, R’ and X)
Reverse fault
Fissure Mafic Unit
E3
Gneissic foliation (of mylonitic character or not) and Sm foliation (of the Supracrustal Unit)
“Z” folds, foliation sigmoids, amphibolite and rotated and displaced pegmatite veins, “shear bands” (Sb1)
Cláudio Shear Zone (ZCC) and normal fault
M2 Amphibolite facies
Migmatization and deformation of the previous units
E2
(?)
(?)
(?)
(?)
Mafic-ultramafic magmatism and sedimentation of the Supracrustal Unit
E1
Gneissic foliation (?), fold with double vergence and S/C
“S” and sheath folds, S/C structures
(?)
M1
Granulite facies
Formation of the protolith of the Gneissic and Amphibolitic Units
2.6.4 Folds
Folds are observed locally and can be described as: intrafolial, asymmetric and tight S and Z;
sheath mesofolds (rare) and tight and/or isoclinal (with flanks inverted or not). The most common
folds are S and Z (the latter being more prominent). The S and Z intrafolial folds are important in the
characterization of two of the phases of tectonic evolution of the study area.
2.6.5 Foliation Sigmoids
Foliation Sigmoids are observed both in schists and gneisses and characterize movements with
dextral directional components (seen in plane AC), where the foliation dip is steep and of high strain
[e.g. 130/80 (lm=225/40)]. Locally, sigmoids indicative of normal movements with an oblique
component are observed (seen in vertical plane AC).
27
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28
2.6.6 Pegmatites
In the gneissic unit domain Z-folded pegmatitic mobilizates are observed indicating dextral
movement seen in plane AC. Locally appear veins denoting normal movements.
2.6.7 Quartz Veins
Both in the supracrustal unit rock and the gneissic unit domains, rotated centimetric quartz
veins are observed. These veins indicate dextral movement, seen in plane AC. Locally, veins
indicating reverse movement occurs.
2.6.8 Amphibolite Boudins
In the domains of the three gneissic units centimetric to metric amphibolite boudins occur
disrupted, strongly deformed and rotated, in general showing dextral movement. Locally there are
boudins indicating sinistral movement.
2.6.9 Faults
Faulting systems can be observed in several key outcrops (points 9A, 9C, 9D, 9G, 9H and 9J;
Fig. 2.2). At the sides of the Fernão Dias highway, a (NW/SE) fault plane is filled with volcanic
material that precedes or is coeval to the fault generation, appearing as a breccia. The fault plane cuts
the gneissic banding, is subvertical and displaces pegmatite veins, indicating normal movement (Fig.
2.4E). In the Montueira Ridge, close to Carmópolis de Minas, a fault with the same trend is observed,
yielding a steep slope. The reverse faulting of local nature does not present regional correlation of
major magnitude. The indicators for this movement are the (sinistral) Sb2 foliation, seen in the vertical
plane, and the foliation sigmoids seen in the gneisses, schists and quartzites.
Finally, the mega-structure that characterizes the Cláudio Shear Zone, firstly revealed by
aeromagnetometric images (Oliveira 1999), describes a roughly NE-SW alignment (Fig. 2.2). Along
the Cláudio Shear Zone are observed migmatized gneisses, ultramafites, amphibolites, quartzites and
schists showing high deformation strain. The foliation inside the mega-structure shows medium- to
high-angle dips and mylonitic character. Its fabric describes NE/SW (Fig. 2.3B) and subordinately NS
and NW/SE trajectories (Fig. 2.3A), characterizing regional foldings.
The general fabric is represented by “S” tectonites composed of pegmatitic mobilizates
concordant with the foliation. Other features presented by this mega-structure are: Z intrafolial folds,
foliation sigmoids, amphibolite boudins, pegmatite and rotated quartz veins with dextral movement.
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29
The meaning of the variation of the different (strike, oblique or dip) mineral lineation trends, contained
in the foliation planes is still an open issue in the regional context.
2.7 TECTONIC EVOLUTION
It is known that the period between 3.6 and 2.5 Ga was responsible for the worldwide
formation of granite-greenstone terranes, with their apex around 3.0-2.6 Ga (e.g. Windley 1976,
Hamilton et al. 1979, Tankard 1982, Goodwin 1991, Myers 1993, Sarkar et al. 1993, Worden et al.
1995, Byerly et al. 1996, Friend et al. 1996, Kroner et al. 1996, Collerson & Macdonald 1998,
Hamilton 1998).
In the case of the southern São Francisco Craton, it was in the Neoarchean (from 2.78 to 2.70
Ga) that this crustal segment had its main accretion stage and crustal reworking (e.g. Noce 1995,
Machado et al. 1996, Carneiro et al. 1998a).
Considering that in the southwestern portion of the study area in the Campo Belo region the
gneissic units present zircon U-Pb ages of the order of 3.2 Ga. (Teixeira et al. 1996, 1998), it is
possible to infer that the beginning of the crustal evolution of the studied segment started in the
Mesoarchean.
Thus, on a primordial sialic crust formed by the protoliths of the gneissic and amphibolitic
units, a tectonothermal event took place, preserving some kinematic and petrologic records that
characterize the event 1.
2.7.1 Event 1
It is the oldest event or the event that was not totally obliterated by the successive events
imprinted in the rocks of the study region. Its deformational structures are foliations of S/C-type,
sinistral-rotated amphibolitic boudins and centimetric intrafolial S folds that are observed locally (Fig.
2.4). Systematically, the rocks that were affected by this event have relic parageneses of the granulite
facies. Later migmatization processes related to Event 3 developed on this fabric.
2.7.2 Event 2
Similarly to the Quadrilátero Ferrífero, where the Rio das Velhas Supergroup crops out, the
study area also presents remmants of the greenstone-belt type supracrustal sequences, with ultramafic
and clastic-pelitic rocks (Fig. 2.2). In the case of the Quadrilátero Ferrífero, consider that the
sedimentation and magmatism responsible for the formation of the Rio das Velhas Supergroup rocks
would be in operation around 2.8 Ga., in an ensialic-type basin (Carneiro et al. 1998b). This would be,
Oliveira, 2004 Evolução tectônica de um.........
30
in principle, the situation of the study area where, during Event 2, of essentially extensional
characteristics, the ensialic basin would be installed for sedimentation and magmatism of the
Supracrustal Unit (or Rio das Velhas Supergroup).
2.7.3 Event 3
As said before, it was between 2.78 and 2.70 Ga that in the Southern São Francisco Craton the
main accretion and crustal reworking phase took place (e.g. Carneiro et al. 1998a, b, Machado et al.
1996, Noce 1995). Such evolution was related to the Rio das Velhas Tectonothermal Event. The peak
of this event occurred around 2.78 – 2.77 Ga (Machado & Carneiro 1992, Carneiro et al. 1998b), when
the zircons of the Alberto Flores Gneiss from the Bonfim Metamorphic Complex suffered
overgrowing, indicating isotopic re-equilibration under high amphibolite metamorphic facies
conditions. These metamorphic conditions have correspondence in the study region, where an ample
and vigorous migmatization process is observed, imposed on the rock fabric generated during Event 1.
In structural terms, the migmatitic rocks originated during this process present the same kinematic
elements of the Supracrustal Unit rocks.
Two important considerations derived from this fact: a) migmatization follows the
Supracrustal Unit sedimentation and magmatism; b) due to the tectonic intensity of this process, the
presence of high amphibolite or granulite facies parageneses, common in the Supracrustal Unit rocks
of the study region but lacking in the Quadrilátero Ferrífero Archean supracrustal rocks, would be
explained. In hierarchic terms, Event 3 can be divided in two phases. To Phase 1 would correspond the
migmatization process and the structural elements mainly found in the Cláudio Shear Zone (Fig. 2.2),
which are: asymmetric Z folds, foliation sigmoids, pegmatitic vein folds and rotated amphibolite
boudins indicating dextral movement.
Associated with the final phase of this dextral directional flow brittle-ductile shear zones are
observed, here represented by Sb1 foliations (shear bands, Figs. 2.3C and 2.3D), filled by pegmatitic
mobilizates. NW/SE- trending Sb1 presents a dextral component and the less conspicuous E/W-
trending Sb1 has a sinistral component. To Event 3, Phase 2 would correspond a crustal relaxation
phase with the generation of normal-component faults, observed in the Supracrustal Unit quartzites
that crop out south of Cláudio and in the gneissic rocks of the sides of the Fernão Dias highway, north
of Carmópolis de Minas.
The lineation contained by the foliation planes is steep (dip to slightly oblique) and the
kinematic indicators associated with this phase are rotated pegmatite veins and fault planes filled by
brecciated volcanic material (Fig. 2.4G). This phase is not expressive, does not obliterate the previous
one and its kinematics does not have regional correspondence.
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31
2.7.4 Event 4
The initial phase of this event had a character flow brittle-ductile, here represented by Sb2
foliations was generated (shear bands, Figs. 2.3E and 2.3F) with forming a par R’ and X (R’ anti-
riedel and secondary, Fig. 2.4H) of NE/SW and NW/SE directions and high-angle dips. These
foliations crosscut the mylonitic gneissic foliation (points 9B, 9C, 9F, 9G and 9H; Fig 2.2). Seen on
the XZ plane, these foliations indicate reverse movements. With the final phase the emplacement of
the 2.658 Ga old, NW/SE-trending fissure mafic magmatism could be correlated (e.g. Pinese et al.
1995, Pinese 1997) and the 2.612 Ga granitic magmatism, interpreted as the final stage of the Archean
platform consolidation (Noce 1995). Regionally this event can be correlated to the Rio das Velhas
Event 2, with sinistral component (Endo 1997).
2.7.5 Event 5
The fifth event is a regional metamorphic re-equilibration to the greenschist facies, later than
the fissure mafic magmatism. Lacking more precise chronostratigraphic indicators to limit its
minimum age, it is suggested that Event 5 be of Transamazonian age or younger.
2.8 CONCLUSIONS
From the geologic results discussed above and geochronologic data of the southern São
Francisco Craton, the following conclusions can be formulated:
1) the protoliths of the gneisses from the three lithodemic units and the amphibolitic unit were
emplaced in the crust in the Mesoarchean and reworked in this period and mainly in the Neoarchean;
2) the volcano-sedimentary sequence of the supracrustal unit was deposited before Event 3,
which is the main migmatization event in the area;
3) the main deformational events in the area essentially occurred in the Neoarchean. Such
conclusion is based on Sm-Nd geochronologic data (Pinese 1997), obtained from dikes of
gabbronoritic composition that crystallized from magma 2.658 ± 44 Ma ago. Such dikes are not
deformed and only present its original magmatic paragenesis re-equilibrated to the greenschist facies.
Thus, as in the study area, similar dikes are found intrusive in the gneissic and supracrustal units that
have relic high-grade metamorphic parageneses, showing that the fissure mafic unit was placed in the
crust after the main Archean tectonothermal events of the Campo Belo Metamorphic Complex.
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CAPÍTULO 3 ORIGIN OF CHARNOCKITES AND ENDERBITES INFERRED FROM
INCOMPATIBLE ELEMENT LOSS DURING PROGRADE DEHYDRATION.
3.1 ABSTRACT
Charnockites and enderbites of theCandeias Unit in the Campo Belo Metamorphic Complex,
in the southern part of the São Francisco Craton display unusual trace element patterns when
compared to typical Archaean granitoids of both upper and lower continental crust. Combination of
major and trace element geochemical evidence allowed detecting element losses caused by prograde
dehydration. Studied rocks from the Campo Belo Complex display relative depletion in some of the
most incompatible elements (i.e. Cs, Rb, U, Nb, Ta and W) in multi-element spider diagrams. Some
elements (i.e. Cs, Rb and U) have traditionally been considered mobile with fluids but we found that
others (i.e. Ta, W), which are often considered immobile, are also deficient in our samples. The
difference in averages of charnockites and enderbites studied and the average of Barberton Archaean
granitoids, upper continental crust (UCC) and lower crust (LC) are also well expressed by ratios of
some of the most incompatible elements. For example, the average of Nb/Ta in UCC is 12 and the
average in LC is 9.17, while our charnockites and enderbites yield 17.53 and 21.54, respectively
similar to many eclogites. Another unusual aspect of the rocks studied concerns the abundance of the
heat producing elements, which display the expected low U but higher K and Th concentrations than
UCC and lower crust. Therefore charnockites and enderbites are not important components of the
lower crust and metamorphic dehydration is not the reason for the low heat production in the LC.
Finally, characteristic loss of highly incompatible elements can be used as a fingerprint of
charnockites and enderbites that formed by metamorphic dehydration of granitoids.
Keywords: Campo Belo Complex, dehydration, incompatible elements, charnockites,
enderbites
3.2 INTRODUCTION
Orthopyroxene-bearing granites, granodiorites and tonalites are called charnockites, charno-
enderbites and enderbites, respectively (Holland 1900, Tilley 1936, Le Maitre et al. 1989, Frost et al.
2000). The term “charnockite series” was first used to describe a suite of rocks of granitic composition
containing orthopyroxene from Madras (Holland 1900). The original acid end-member of “charnockite
series” (i.e. hypersthene granite) was essentially defined as charnockite by Holland (1900). Tilley
(1936) defined the term enderbite when studying unusual rocks from Enderby Land (Antarctica),
containing hornblende or biotite, which is uncommon in the “charnockites”, described by Holland
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(1900). “Charnockite series” occur from the Archaean to Phanerozoic, but are more common in
Archaean terrains. They have been studied in many places around the world including India (e.g.
Madras), Enderby Land-Antarctica (Napier Complex), Sri Lanka, Southern Africa (e.g. Limpopo
Belt), Brazil (e.g. Complexo Jequie), and Eastern Siberia. There is a profusion of scientific literature
covering the charno-enderbite theme yet no true consensus exists about the origin of at least some
charnockite occurrences including classic localities. Charnockites have at least three possible,
divergent origins. These are: (i) granitic rocks metamorphosed to the granulite facies (metamorphic)
(i.e. Friend 1981); (ii) granitoid rocks in which pyroxene crystallized directly from an anhydrous
magma (igneous) (i.e. Jordt-Evangelista 1996, Zhao et al. 1997, Percival & Mortensen 2002), and (iii)
granitoid rocks that recrystallised during CO2 flushing (arrested) (i.e. Harris & Bickle 1989, Santosh et
al. 1991a, b, Satish-Kumar & Santosh 1998). Not surprisingly then, there are many controversies and
discussions about the origin of many particular charnockite occurrences.
A good example of ongoing uncertainty is the origin of “arrested charnockite” in Chilka Lake
(India). Dobmeier & Raith (2000) postulated that it formed as a result of localized synkinematic fluid
migration and was genetically linked to the host leptynite in which it is enclosed. These authors also
proposed that the charnockites are completely unrelated to enderbite layers in the same outcrops. In a
comment, Bhattacharya & Sen (2002) claimed that many rocks interpreted by Dobmeier & Raith
(2000) as enderbite are, in reality, charnockites or charno-enderbites. Hence, Bhattacharya & Sen
(2002) proposed a common origin for charnockite and enderbite.
Friend (1981) claimed that the majority of charnockites crystallized from anhydrous magmas.
However, Bohlender et al. (1992) distinguished between two origins of charnockite in the southern
marginal zone of the Limpopo belt: one is magmatic with intrusive relationships and preserved
igneous textures; the other is metamorphic with preserved relict banding. Those authors based their
evidence on field relationships, petrography and bulk rock geochemistry. Frost et al. (2000) defined
charnockites (sensu lato) as rocks that have crystallized under low water activities and they further
claimed that such rocks are not restricted to a particular geologic time, a specific chemical
composition or a certain tectonic environment.
It is be beyond the scope of this study to summarise all conflicting views on charnockite and
enderbite petrogenesis. The few cases quoted here are sufficient to illustrate that since “charnockite
series” were first recognized by Holland (1900) these rocks have been a matter of continuos
controversy. In this study we will use geochemical data, based on interpreted fluid-mobility of
elements, to distinguish igneous from metamorphic “charnockite series”. To this end, we next discuss
recent geochemical evidence that we believe to be useful for identifying metamorphic dehydration.
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35
3.3 BRIEF REVIEW OF RELEVANT GEOCHEMICAL RESEARCH
During metamorphic dehydration some major and trace elements are very mobile and tend to
be lost to escaping fluids. In a classic study of calcareous sediments from the Vassalboro Fromation,
Ferry (1983) showed the mobility of some major elements with increasing metamorphic grade. Ferry
(1983) discussed that during metamorphism, decarbonation and dehydration cause mass transport of
CO2, and H2O, which are accompanied by loss of K and Na. Much experimental work has since been
published in which effects of dehydration, trace element concentration, mineral/fluid partitioning and
fO2 was studied (e.g. Brenan et al. 1995a, b, 1998). The behaviour of major and trace elements and
REE during prograde and retrograde metamorphism has also been studied extensively in natural
metamorphic rocks (Allen & Condie 1985, Miller et al. 1994, Tribuzio et al. 1996, Bebout et al. 1999,
Becker et al. 1999, 2000). The combined observations from those studies have helped to improve
understanding of continental crust formation. Brenan et al. (1995a) studied the fluid/mineral partition
coefficients of some minerals from the upper mantle (e.g. garnet, amphibole, clinopyroxene and
olivine) to investigate the effects of fluids on trace element transport within the mantle. The most
important result obtained by experimental and empirical studies is that certain trace elements are
transported into the mantle more easily than similarly compatible chemical twins following slab
dehydration.
Brenan et al. (1995a) reported data for more lithophile behavior of B, Be and Li than could be
expected from relative incompatibility during progressive dehydration of oceanic crust. This explains
the overabundance of these elements in island arc lavas. In addition, the behavior of rare earth and
other trace elements from high-pressure rocks (e.g. eclogites) has been studied to better understand the
chemical inventory of continental crust as a whole (Miller et al. 1994, Brenan et al. 1995a, Tribuzio et
al. 1996, Bebout et al. 1999, Becker et al. 1999). Miller et al. (1994) and Brenan et al. (1995b)
proposed that the enrichment of Pb in continental crust is directly related to the transfer of Pb from
slabs in basalt-derived fluids during prograde metamorphism. An important principle established in
these studies is the use of trace element ratios between elements of similar compatibility in MORB-
melting (e.g. Pb/Ce or Pb/Nd). Tribuzio et al. (1996) presented results of REE redistribution during re-
equilibration of Fe-gabbros under blueschist and eclogite facies. They demonstrated that there is
incorporation of REE into lawsonite and titanite in blueshist facies and allanite-epidote in eclogite
facies. This shows the additional possibility of element transfer between metamorphic minerals.
Becker et al. (1999) proposed that there is preferential loss of some elements during subduction of
altered oceanic basalts (e.g. K, Ba, Cs, Rb, U, and Pb) but that other elements are largely immobile
(Th, Sr, Nd, Sm, Y, the HFSE, Cr, Co and Ni). As a result it was claimed that eclogites retain all Nb
despite extensive dehydration.
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36
Allen & Condie (1985) examined in southern India the relationship between prograde and
retrograde gneiss-charnockite. They identified that there is a wide scatter of element abundances and
element ratios evidently caused by prograde and retrograde reactions. In the case of prograde
reactions, there is a small loss of Y, REE and Mg relative to Fe and gains of Ta, Pb and CO2. On the
other hand, retrogressed gneisses are enriched in Pb, Th, Hf, Rb, Zn relative to Co, Nb relative to Ta
and Hf relative to Ta. In both cases, the fluid phase contained a mixture of CO2, H2O and halogens.
In summary, study of metamorphic dehydration has demonstrated the preferential mobility of
certain elements, which are overabundant in continental crust. However, no systematic treatment,
using high-quality data, of dehydration-induced element depletion has been published for charno-
enderbites. In this paper we will show that despite the complexities of trace element transfer, it is
possible to identify systematic features that are clearly related to prograde dehydration.
A complication of any empirical study is that high-grade rocks are typically retrogressed (i.e.
re-hydrated) at least to some extent. Because it is unknown whether the retrograde fluids carried with
them fluid-mobile trace elements (such as Pb) the possibility of re-enrichment needs to be considered.
In view of this, we selected for this study extremely fresh samples, many of which were obtained in
quarries exposing unweathered fresh rock faces (Fig. 3.1a and b). Next we will describe the relevant
aspects of regional geology and sample characteristics.
3.4 SÃO FRANCISCO CRATON: RELEVANT GEOLOGIC RECORD
The crystalline basement of the São Francisco Craton (Fig. 3.2a) comprises Archaean medium
to high-grade metamorphic rocks, and granite-greenstone associations (Cordani et al. 2000). The
geological evolution of the Southern part of São Francisco Craton involved four metamorphic
complexes: Bação, Bonfim, Belo Horizonte and Campo Belo (Fig. 3.2a). They are composed
essentially of TTG gneisses, mafic intrusions and greenstone belts (e.g. Machado & Carneiro 1992,
Teixeira 1993, Teixeira et al. 1996, Carneiro et al. 1998a, b, Noce et al. 1998).
This study focused on rocks from Campo Belo Metamorphic Complex. Samples were
collected around the cities Oliveira, São Francisco de Paula, Itapecerica, Cláudio and Carmópolis de
Minas (Fig. 3.2b). Over the last 10 years, the general geological framework was established with
mapping, structural, geochemistry and geochronological studies (Machado Filho et al. 1983, Teixeira
& Figueiredo 1991, Teixeira 1993, Teixeira et al. 1996, Carneiro et al. 1998 a, b, Noce et al. 1998,
Teixeira et al. 1998, Correa da Costa 1999, Oliveira 1999, Fernandes & Carneiro 2000, Oliveira &
Carneiro 2001, Teixeira et al. 2000).
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a
b
Figure 3.1 Panel (a) Photograph of massive charnockite with incipient foliation taken in Alemão dimension stone quarry (Candeias Unit). Panel (b) Photograph taken at Marilan dimension stone quarry (Itapecerica Unit) illustrating the quality of rocks collected from unweathered core. The rocks from this outcrop are pink to grey, strongly migmatized but of exceptional physical quality.
According to these authors, the region is formed predominantly by metamorphic rocks and
subordinately by igneous rocks, which jointly comprise geological records from the meso to neo-
Archaean. The dominant rock-type is migmatitic gneiss with schieren and stromatic structures and
mafic enclaves. After the first substantial sialic crust and supracrustal sequences were generated,
successive accretion and differentiation stages associated with crustal reworking affected this area
during the Archaean and Proterozoic (Teixeira et al. 1996).
37
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38
Oliveira (1999) and Oliveira & Carneiro (2001) subdivided the studied area into a number of
lithological units (Cláudio, Itapecerica and Candeias Unit, Supracrustal Unit and Mafic Unit, Fig.
3.2b) whose features are summarised below.
3.5 SAMPLES SELECTION
For the present study, samples were selected from the Cláudio, Itapecerica and Candeias Units
(Oliveira 1999, Oliveira & Carneiro 2001). From each unit fresh samples were taken from different
outcrops and within outcrops care was taken to subsample visibly different gneiss varieties (Fig. 3.2b
(outcrops A-I)). Specifically, we chose 6 samples from Candeias unit (charnockites and enderbites
from 2 outcrops; A and B), 7 samples from Itapecerica Unit (migmatized gneisses and granitoid from
3 outcrops; C, D and E) and 9 samples from Claudio Unit (gneisses, migmatized gneisses, and
granitoid from 4 outcrops; F, G, H and I).
3.6 PETROGRAPHY FEATURES
The charnockites in the studied area crop out along Oliveira and São Francisco de Paula Town
(Fig. 3.2b). The rocks were collected at Oliveira stone quarry and Alemão stone quarry (Fig. 3.2b, A
and B). In most domains the rocks exhibit massive textures with incipient foliation. These rocks are
olive green, slightly to strongly migmatized, and medium- to coarse-grained. The mineralogical
banding is difficult to identify macroscopically giving a homogeneous aspect to the bodies. The
texture of the gneisses varies from granolepidoblastic to granoblastic. The grains are predominantly
medium-sized and show intergrowths of the perthite, myrmekite and antiperthite types. The variety
from the Oliveira outcrop (samples 7A and B, Table 3.1) is slightly to strongly deformed. The main
constituents from this group are antiperthitic plagioclase (30-45%), quartz (30%), microcline (10-
20%), biotite (<10%), amphibole (~1%), hypersthene (~1-3%), and the commonest accessories are
apatite, zircon, and an opaque mineral. In some thin sections hypersthene is absent. The charnockite
from Alemão quarry (samples 8A, B, C and D, Table 3.1) is only slightly deformed; massive in hand
specimen and with grains that vary from medium to coarse (Fig. 3.1b). Under the microscope,
microcline (40-50%) is the main constituent followed by plagioclase (25-30), quartz (20-25), biotite
(~5), hypersthene, and rare amphibole.
Accessories are represented by apatite, zircon, and opaques. Chlorite is sometimes found as a
secondary mineral, overgrowing biotite. In general, plagioclase is medium-grained, with subhedral to
anhedral crystals. Quartz is medium- to fine-grained, anhedral, with undulatory extinction and
generation of sub grains. Microcline shows perthite intergrowth and pericline twinning in subhedral to
anhedral grains. Hypersthene, when present, is medium-grained, has low birefringence and
pleochroism varying from light pink to pale green. In some samples it is altered to amphibole, biotite
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
and chlorite along the rims. Biotite, sometimes showing symplectitic intergrowth with quartz, has light
brown pleochroism. The main accessories are opaques, zircon and apatite.
Figure 3.2 – Panel (a) -Geological map of the southern São Francisco Craton (modified by Campos Sales 2004). Key: 1- Neoproterozoic undivided cratonic cover; 2- Mesoproterozoic Espinhaço Supergroup; 3- Mesoproterozoic(?) São João Del Rei/Andrelândia Groups; 4-- Mesoproterozoic (?), Dom Silvério Group; 5- Paleoproterozoic granitoids; 6- Paleoproterozoic indiscriminate greenstone-type sequences; 7- Paleoproterozoic
39
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40
Minas Supergroup; 8- Neoarchean granitoids; 9- Neoarchean and Mesoproterozoic gabbroic and dioritic rocks (sills and dikes); 10- Neoarchean ultramafic plutonic suite; 11- Neoarchean Rio das Velhas Supergroup; 12- Archean metamorphic complexes partially reworked on Proterozoic time; 13- Faults and fractures (CSZ = Cláudio Shear Zone; JBSZ = Jeceaba-Bom Sucesso Shear Zone); 14- Fold axes; 15- Lithologic contacts; 16- Cities. Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) showing the following features: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Alemão dimension stone quarry; B – Oliveira dimension stone quarry; C - Lila dimension stone quarry; D – Itapecerica dimension stone quarry; E - Marilan dimension stone quarry; F - Kinawa dimension stone quarry; G - Corumbá dimension stone quarry; H – Fazenda Corumbá dimension stone quarry and, I – Carmópolis de Minas dimension stone quarry.
The samples from Itapecerica Unit are pink and locally dark gneiss bodies. This unit is
strongly migmatized and the rocks are fine to medium grained (Fig. 3.1a). The rocks are
predominantly of granitic composition but can vary to granodioritic. The main constituents of these
rocks are microcline followed by quartz, plagioclase, biotite, apatite, zircon, opaques and epidote. The
samples from the dark parts (maybe the protolith) are granodioritic to tonalitic in composition with
abundant amphibole and some pyroxene.
The Claudio Unit samples are gneisses of granodioritic to tonalitic, locally granitic
compositions. They were chosen for comparison with charnockites. They are predominantly grey but
in some outcrops (e.g. Kinawa, Fig. 3.2b-F), they are locally pink due to the occurrence of pegmatite
fluids. At Fazenda Corumba stone quarry the rocks vary from dark, grey to white (granodioritic to
granitic). The rocks are fine- to medium-grained, slightly to strongly migmatized. In general,
plagioclase is the main constituent of these rocks followed by quartz, microcline, biotite, amphibole
(not common), zircon, apatite, titanite (not common), opaques, white-mica, chlorite and epidote. Some
samples show antiperthitic intergrowth. The main petrographic differences of the units are:
compositional (i.e. presence of hypersthene), grade of migmatization and color of the bodies (i.e.
green, pink, grey, dark and white).
3.7 ANALYTICAL METHODS
Whole rock samples where analysed for major and trace elements. Major element contents
were analysed by ICP-OES (Inductively coupled plasma optical emission spectrometry). These
analyses were carried out at Earth Science Department of the University of Queensland.
ICP-OES
Samples were analysed for all 10 major oxides at the Department of Earth Sciences,
University of Queensland by Mr Michael Lawrence. Major oxides were determined by a Lithium
Metaborate fusion of silicate rocks, followed by analysis using Inductively Coupled Plasma-Optical
Emission Spectroscopy (ICP-OES). Iron is determined as total ferric iron and expressed as Fe2O3T.
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Procedure - 50mg of powdered rock sample was weighed into graphite crucibles and mixed
with 200mg of LiBO2. The samples were then fused for 60 mins at 1000°C. Fused samples were
transferred to 100mL Teflon beakers containing 25mL of 10% HNO3 solution containing a 10ppm Lu
internal standard. The samples were stirred, covered and allowed to sit until they were fully dissolved
(approximately 120-180 mins, although in some cases overnight). 25mls of deionised water was then
added to bring the final concentration of silicate rock to 50mg/50ml.
All 10 major oxides are determined directly from the solution using matched silicate reference
materials as calibrating standards. Calibration standards were prepared in the same manner. Accurate
calibration curves were achieved by fusing 25mg of the calibration standard to provide a 3-point
calibration curve. Where possible, the concentration of each element is determined at a number of
wavelengths to ensure that interferences are minimised. The calibration standards and another
reference standard were run as unknowns at then end of each batch for quality control purposes. The
Lu signal is monitored to allow for correction of instrument drift.
ICP-MS
Rare earth element (REE) and trace element abundances were analysed by ICP-MS
(Inductively coupled plasma mass spectrometry).
The trace elements were determined by ICPMS at the Advanced Centre for Queensland
University Isotopic Research Excellence (ACQUIRE). Sample preparation and analytical procedures
used were identical to those of Eggins et al. (1997) except that Tm was not used as an internal standard
and W-2, a U.S.G.S. diabase standard, was used as the calibration standard. Our preferred
concentrations for W-2 and the measured concentrations and relative standard deviations for AGV-1
(an average of 26 analyses of 9 digestions analysed over four years) are shown in Table 1.
Concentrations for W-2 were derived partly by analysing it relative to synthetic standards (Li, Cr, Ni,
Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, and U) or are based on an assessment of published
standard data (A. Greig, pers. comm.). Results are reported in Table 3.1.
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Table 3.1 Major (wt%), trace element (ppm) concentrations and selected trace element ratios of Campo Belo Metamorphic Complex.
42
034260
49
.7
.5
33
01
Gneiss UnitRock typeSample ID AO-07A AO-07B average AO-08A AO-08B AO-08C AO-08D average
SiO2 67.45 68.73 68.09 73.11 73.41 74.33 75.02 73.97TiO2 0.49 0.39 0.44 0.22 0.21 0.20 0.23 0.22Al2O3 14.71 15.64 15.17 13.46 13.36 13.44 13.34 13.40Fe2O3 4.16 3.08 3.62 1.74 1.69 1.48 1.66 1.64MnO 0.06 0.04 0.05 0.03 0.03 0.03 0.03 0.MgO 1.75 1.08 1.42 0.46 0.44 0.33 0.45 0.CaO 3.23 3.27 3.25 1.63 1.61 1.56 1.62 1.Na2O 4.23 4.69 4.46 3.46 3.35 3.29 3.35 3.36K2O 1.60 1.57 1.59 4.05 4.01 4.27 4.28 4.15P2O5 0.12 0.23 0.17 0.06 0.03 0.04 0.04 0.04
Cs 0.68 0.51 0.59 0.34 0.33 0.32 0.32 0.33Rb 58.06 49.68 53.87 96.15 96.70 99.37 98.64 97.71Ba 533 462 497 1083 1080 1122 1143 1107Th 6.02 8.60 7.31 25.31 27.16 24.67 20.87 24.50U 0.47 0.40 0.44 0.52 0.49 0.52 0.45 0.Nb 11.59 9.17 10.38 5.59 5.22 5.94 5.66 5.60Ta 0.78 0.41 0.60 0.23 0.24 0.29 0.27 0.26W 0.02 0.01 0.01 0.02 0.03 0.03 0.03 0.03La 28.03 36.15 32.09 53.90 44.81 52.41 46.60 49.43Ce 51.96 62.64 57.30 101.93 83.75 98.41 87.78 92.97Pr 5.69 6.31 6.00 10.72 8.80 10.24 9.17 9.73Pb 13.16 15.20 14.18 27.04 27.21 29.82 27.47 27.88Sr 377.28 405.84 391.56 163.55 160.54 157.90 155.97 159.49Nd 20.26 20.86 20.56 35.76 29.33 33.87 30.25 32.30Be 1.66 1.66 1.66 0.89 0.82 0.75 0.78 0.81Zr 131.38 166.29 148.84 114.02 101.61 143.93 125.39 121.24Hf 3.24 4.30 3.77 2.97 2.67 3.81 3.38 3.21Sm 3.83 3.10 3.47 5.78 4.76 5.52 4.86 5.23Eu 1.00 1.03 1.01 0.76 0.73 0.74 0.75 0.75Gd 3.45 2.36 2.91 4.50 3.72 4.14 3.56 3.98Tb 0.52 0.31 0.42 0.57 0.48 0.54 0.46 0.51Dy 2.92 1.58 2.25 2.79 2.39 2.57 2.22 2.49Ho 0.58 0.29 0.44 0.51 0.45 0.47 0.41 0.46Y 15.24 7.30 11.27 12.88 11.43 11.79 10.08 11.54Er 1.52 0.73 1.13 1.20 1.09 1.10 0.96 1.09Tm 0.22 0.10 0.16 0.15 0.14 0.15 0.13 0.14Yb 1.32 0.59 0.96 0.90 0.85 0.87 0.76 0.84Lu 0.19 0.09 0.14 0.13 0.13 0.13 0.12 0.13Li 22.26 22.85 22.55 18.62 18.00 17.12 17.69 17.86
Hf/W 200 487 301 125 102 117 129 118Th/U 12.8 21.3 16.8 49.1 55.2 47.6 46.9 49Nb/Ta 14.9 22.3 17.4 24.0 21.4 20.4 20.8 21La/W 1727 4094 2561 2266 1711 1611 1774 1818Cs/Rb 0.012 0.010 0.011 0.004 0.003 0.003 0.003 0.003Sr/Nd 18.6 19.5 19.0 4.6 5.5 4.7 5.2 4.9Ce/Pb 3.95 4.12 4.04 3.77 3.08 3.30 3.20 3.La/Yb 21.2 61.3 33.5 60.1 52.5 60.2 61.6 58.5Th/Cs 8.87 16.93 12.32 73.66 81.70 77.96 65.79 74.83Cs/Th 0.11 0.06 0.08 0.01 0.01 0.01 0.02 0.Eu/Eu* 0.82 1.10 0.94 0.43 0.51 0.45 0.53 0.48
Major element (WT%)
Trace element (ppm)
Selected trace element ratios
CandeiasEnderbite Charnockite
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Gneiss UnitRock type Dark Mig.Sample ID AO-01 AO-03 AO-10 AO-02 average AO-09A AO-9B average AO-04
SiO2 71.92 70.68 72.31 71.83 71.69 72.01 72.01 69.46TiO2 0.14 0.26 0.16 0.23 0.20 0.17 0.17 0.50Al2O3 13.00 14.34 13.89 13.69 13.73 12.91 12.91 15.45Fe2O3 0.91 1.91 1.35 1.69 1.47 1.99 1.99 3.09MnO 0.01 0.03 0.03 0.03 0.03 0.04 0.04 0.04MgO 0.03 0.28 0.04 0.06 0.10 0.04 0.04 0.55CaO 0.49 3.19 1.17 1.25 1.52 0.77 0.77 2.72Na2O 2.53 3.84 3.78 3.03 3.29 3.30 3.30 4.85K2O 7.10 3.96 4.54 5.79 5.35 5.17 5.17 2.00P2O5 0.04 0.08 0.01 0.04 0.04 0.03 0.03 0.12
Cs 0.92 1.06 1.38 0.96 1.08 2.39 1.60 2.00 2.33Rb 211.79 137.34 200.38 187.25 184.19 304.37 247.24 275.81 134.34Ba 1133 646 693 746 804 278 238 258 810Th 3.77 29.26 29.66 41.12 25.95 51.72 35.02 43.37 18.56U 0.60 2.11 4.08 2.21 2.25 27.38 21.98 24.68 2.39Nb 5.53 12.66 17.39 8.48 11.01 37.27 26.64 31.96 7.72Ta 0.19 0.31 0.93 0.17 0.40 1.90 1.18 1.54 0.48W 0.01 0.01 0.08 0.01 0.03 0.07 0.04 0.05 0.10La 10.10 49.45 37.36 109.65 51.64 58.98 42.43 50.70 40.65Ce 17.24 94.61 72.66 216.28 100.20 127.38 91.86 109.62 79.04Pr 1.73 10.17 8.03 23.01 10.73 14.49 10.37 12.43 8.39Pb 36.40 29.16 29.82 39.57 33.74 39.99 33.84 36.91 27.28Sr 131.34 141.10 142.91 104.19 129.89 52.38 44.10 48.24 155.16Nd 5.61 34.78 28.16 75.75 36.07 50.11 35.67 42.89 27.61Be 0.45 1.65 2.11 0.88 1.27 2.58 1.80 2.19 2.06Zr 14.24 176.11 135.34 252.42 144.53 178.68 143.89 161.28 196.93Hf 0.46 4.82 4.40 7.58 4.31 6.07 4.91 5.49 6.27Sm 0.88 6.70 6.50 12.52 6.65 12.23 8.42 10.33 4.62Eu 0.76 0.87 0.65 0.88 0.79 0.41 0.33 0.37 0.70Gd 0.66 5.44 6.62 8.90 5.41 13.00 8.72 10.86 3.56Tb 0.09 0.72 1.18 1.10 0.78 2.41 1.65 2.03 0.48Dy 0.50 3.50 7.39 5.00 4.10 15.05 10.39 12.72 2.28Ho 0.11 0.61 1.59 0.81 0.78 3.16 2.20 2.68 0.39Y 2.81 14.60 49.08 18.91 21.35 92.09 61.79 76.94 10.16Er 0.32 1.40 4.64 1.75 2.03 8.97 6.23 7.60 0.91Tm 0.05 0.17 0.74 0.21 0.29 1.37 0.95 1.16 0.12Yb 0.32 0.97 4.74 1.20 1.81 8.26 5.73 6.99 0.74Lu 0.05 0.15 0.70 0.18 0.27 1.18 0.82 1.00 0.12Li 15.86 21.27 20.15 16.58 18.46 41.56 28.94 35.25 23.75
Hf/W 33 442 53 591 143 89 121 101 65Th/U 6.3 13.9 7.3 18.6 11.5 1.9 1.6 1.8 7.8Nb/Ta 28.6 41.5 18.7 49.7 27.5 19.6 22.6 20.7 16.1La/W 738 4531 448 8557 1710 867 1044 933 424Cs/Rb 0.004 0.008 0.007 0.005 0.006 0.008 0.006 0.007 0.017Sr/Nd 23.4 4.1 5.1 1.4 3.6 1.0 1.2 1.1 5.6Ce/Pb 0.47 3.24 2.44 5.47 2.97 3.19 2.71 2.97 2.90La/Yb 31.2 50.7 7.9 91.6 28.6 7.1 7.4 7.2 54.7Th/Cs 4.08 27.48 21.50 42.79 23.98 21.60 21.82 21.69 7.98Cs/Th 0.24 0.04 0.05 0.02 0.04 0.05 0.05 0.05 0.13Eu/Eu* 2.88 0.42 0.30 0.24 0.39 0.10 0.12 0.11 0.50
Itapecerica
Trace element (ppm)
Fine Migmatite
Table 1-continued
Selected trace element ratios
Major element (WT%)
Migmatite (pink + Grey)
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Gneiss Unit ItapecericaRock type Dark GneissSample ID AO-06 AO-11 AO-15A AO-15B AO-17 AO-18 average
SiO2 61.45 70.98 73.16 73.61 72.58TiO2 1.01 0.36 0.25 0.18 0.26Al2O3 15.39 15.66 13.48 13.70 14.28Fe2O3 6.41 3.00 2.11 1.29 2.14MnO 0.11 0.05 0.04 0.02 0.04MgO 2.54 0.70 0.20 0.45CaO 3.43 3.40 1.19 1.16 1.92Na2O 4.18 4.64 2.99 3.27 3.64K2O 2.66 1.62 5.31 5.39 4.11P2O5 0.76 0.08 0.04 0.00 0.04
Cs 3.87 1.42 1.92 1.75 1.13 1.27 1.50Rb 240.35 62.30 176.96 188.79 107.08 152.42 137.51Ba 527 336 856 703 444 373 543Th 24.53 8.44 32.67 39.96 19.03 32.98 26.61U 6.02 1.09 4.78 5.25 2.45 6.60 4.03Nb 30.72 11.18 12.66 13.82 13.32 8.60 11.91Ta 2.61 0.50 0.51 0.43 0.94 0.18 0.51W 0.12 0.08 0.09 0.04 0.03 0.04 0.06La 75.71 27.25 61.08 74.34 57.59 29.90 50.03Ce 157.71 51.56 121.64 150.37 97.50 55.80 95.37Pr 18.57 5.75 13.24 16.35 9.89 6.49 10.35Pb 15.91 14.67 31.49 34.51 18.95 34.38 26.80Sr 347.78 230.42 132.67 121.70 208.17 66.45 151.88Nd 69.31 20.72 44.18 54.70 32.68 22.57 34.97Be 3.12 2.06 1.65 1.68 2.25 1.31 1.79Zr 209.89 257.71 213.25 229.29 333.04 153.50 237.36Hf 5.06 6.43 5.89 6.57 7.73 4.76 6.28Sm 13.18 4.42 8.11 9.95 5.41 4.57 6.49Eu 2.18 1.04 0.76 0.77 1.19 0.58 0.87Gd 10.82 4.34 6.12 7.44 4.37 3.71 5.20Tb 1.64 0.65 0.82 1.01 0.64 0.47 0.72Dy 9.14 3.46 3.90 4.86 3.35 2.14 3.54Ho 1.78 0.66 0.67 0.83 0.64 0.35 0.63Y 46.82 15.98 17.87 20.44 18.37 9.09 16.35Er 4.61 1.56 1.59 1.86 1.70 0.78 1.50Tm 0.65 0.20 0.20 0.23 0.24 0.09 0.19Yb 3.79 1.13 1.13 1.27 1.49 0.54 1.11Lu 0.52 0.16 0.17 0.19 0.22 0.09 0.17Li 63.59 28.29 17.41 19.37 28.36 15.74 21.84
Hf/W 41 77 62 148 300 133 110Th/U 4.1 7.8 6.8 7.6 7.8 5.0 6.6Nb/Ta 11.7 22.6 24.7 32.2 14.1 46.8 23.2La/W 611 325 644 1676 2237 835 879Cs/Rb 0.016 0.023 0.011 0.009 0.011 0.008 0.011Sr/Nd 5.0 11.1 3.0 2.2 6.4 2.9 4.3Ce/Pb 9.91 3.52 3.86 4.36 5.14 1.62 3.56La/Yb 20.0 24.2 54.0 58.8 38.7 54.9 45.1Th/Cs 6.33 5.92 16.98 22.82 16.80 25.94 17.74Cs/Th 0.16 0.17 0.06 0.04 0.06 0.04 0.06Eu/Eu* 0.54 0.71 0.31 0.26 0.72 0.41 0.44
Migmatite
Trace element (ppm)
Selected trace element ratios
Table 1-continuedCláudio
Major element (WT%)
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45
02
63
87
28
1812
Gneiss UnitRock typeSample ID AO-20A AO-20B average AO-14 AO-16 average AO-12 AO-19 average
SiO2 72.05 72.05 64.72 68.68 66.70 66.75 73.44 70.09TiO2 0.40 0.40 1.15 0.61 0.88 0.05 0.03 0.04Al2O3 15.08 15.08 14.10 14.59 14.35 17.41 13.15 15.28Fe2O3 2.81 2.81 6.93 4.86 5.90 2.06 0.28 1.17MnO 0.06 0.06 0.10 0.07 0.08 0.03 0.01 0.MgO 0.68 0.68 1.50 1.00 1.25 -0.15 -0.15CaO 2.65 2.65 2.60 3.09 2.84 2.27 1.00 1.Na2O 4.56 4.56 3.86 4.32 4.09 4.11 2.83 3.47K2O 2.27 2.27 2.59 2.10 2.34 5.72 5.83 5.78P2O5 0.13 0.13 0.42 0.18 0.30 0.10 0.01 0.06
Cs 3.03 2.98 3.00 4.14 4.19 4.16 2.17 1.66 1.92Rb 84.89 93.28 89.08 214.28 147.94 181.11 119.78 162.83 141.31Ba 398 525 461 498 170 334 1399 970 1185Th 15.80 9.77 12.79 31.20 20.51 25.85 16.49 14.90 15.70U 1.78 3.54 2.66 8.13 3.54 5.83 5.91 1.84 3.Nb 12.03 12.57 12.30 40.40 49.45 44.92 2.61 2.44 2.52Ta 0.82 0.86 0.84 3.26 2.52 2.89 0.35 0.32 0.34W 0.05 0.05 0.05 0.23 0.06 0.15 0.12 0.08 0.10La 38.95 32.14 35.55 98.49 49.49 73.99 20.45 23.31 21.88Ce 80.29 65.37 72.83 193.19 101.43 147.31 39.44 46.48 42.96Pr 9.57 7.89 8.73 21.24 11.69 16.46 4.41 5.05 4.73Pb 17.79 18.71 18.25 19.64 18.17 18.91 35.17 32.13 33.65Sr 289.05 298.55 293.80 133.48 97.31 115.39 256.11 153.69 204.90Nd 37.19 30.60 33.90 73.02 42.25 57.63 16.02 17.02 16.52Be 2.05 1.92 1.99 4.27 3.09 3.68 1.60 1.43 1.51Zr 169.35 154.27 161.81 567.68 327.29 447.48 163.65 10.19 86.92Hf 4.64 4.14 4.39 13.25 9.08 11.17 4.48 0.37 2.43Sm 8.75 7.51 8.13 13.80 9.86 11.83 3.46 3.18 3.32Eu 1.20 1.28 1.24 1.38 0.82 1.10 1.12 0.72 0.92Gd 8.61 8.19 8.40 13.32 10.79 12.06 3.17 2.32 2.74Tb 1.27 1.42 1.34 2.20 1.86 2.03 0.43 0.31 0.37Dy 6.63 8.61 7.62 13.11 10.74 11.93 2.09 1.51 1.80Ho 1.17 1.72 1.45 2.70 2.06 2.38 0.38 0.25 0.32Y 28.87 43.52 36.20 76.51 53.21 64.86 9.57 6.69 8.13Er 2.72 4.41 3.57 7.44 5.10 6.27 0.88 0.62 0.75Tm 0.34 0.60 0.47 1.10 0.64 0.87 0.11 0.08 0.10Yb 1.87 3.19 2.53 6.62 3.33 4.97 0.67 0.51 0.59Lu 0.25 0.41 0.33 0.94 0.42 0.68 0.10 0.07 0.09Li 43.79 45.51 44.65 45.51 66.78 56.15 14.15 5.94 10.04
Hf/W 102 91 97 58 147 77 38 4 24Th/U 8.9 2.8 4.8 3.8 5.8 4.4 2.8 8.1 4.1Nb/Ta 14.7 14.6 14.6 12.4 19.7 15.6 7.5 7.5 7.5La/W 856 707 781 429 801 508 172 280 216Cs/Rb 0.036 0.032 0.034 0.019 0.028 0.023 0.018 0.010 0.014Sr/Nd 7.8 9.8 8.7 1.8 2.3 2.0 16.0 9.0 12.4Ce/Pb 4.51 3.49 3.99 9.84 5.58 7.79 1.12 1.45 1.La/Yb 20.8 10.1 14.0 14.9 14.9 14.9 30.5 45.8 37.1Th/Cs 5.22 3.28 4.26 7.53 4.90 6.21 7.59 8.95 8.Cs/Th 0.19 0.31 0.23 0.13 0.20 0.16 0.13 0.11 0.Eu/Eu* 0.41 0.50 0.45 0.31 0.24 0.28 1.01 0.77 0.90
Table 1-continuedCláudio
Selected trace element ratios
Dark gneiss GranitoidGray gneiss
Trace element (ppm)
Major element (WT%)
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3.8 RESULTS
The dataset obtained in this study is given in Table 3.1. Based on the rock classification
diagram in Na2O x CaO x K2O space (Glikson 1979), the studied rocks classify as thondhjemites,
tonalites, granodiorites, adamellites and granites (Fig. 3.3a). The orthopyroxene-bearing rocks from
Candeias Unit fall into fields 1 and 4 (tonalite and granite composition) and thus classify as enderbite
and charnockite, respectively. The rocks from Itapecerica and Claudio Units, petrographicaly defined
as dark and grey gneisses, have chemistry typical of Archaean TTG (samples 4, 6, 11, 16, 17 and
20A). Sample 11 is a migmatite from Claudio Unit.
1 2
3
45
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10Yb
La/Y
b
0
1
2
3
4
5
6
7
8
9
10
60 65 70 75 80SiO2
MgO
+Fe
O+M
nO2
3
0
1
2
3
4
5
6
78
9
10
0 2 4 6 8%K2 O
Na O2 K O2
CaO
a b
c d
Na
O+K
O-C
aO2
2
Figure 3.3 (a) Chemical classification diagram in Na2O x CaO x K2O space (Glikson 1979) used to divide the studied rocks into: 1 – tonalities, 2 - granodiorites, 3 - adamellites, 4 – granites, and 5 – thondhjemites. (b) (MgO + Fe2O3 + MnO) vs SiO2 showing the expected trend from fractionation of ferromagnesian silicates and Fe-oxides. (c) (Na2O + K2O – CaO) vs K2O diagram shows compositional relationship for Archaean gneisses and granitoids, in which studied samples plot along the calc-alkaline trend. (d) Plot of La/Yb vs Yb comparing rocks from Candeias, Claudio and Itapecerica Units. The trend shows that studied rocks vary from high La/Yb and low Yb to low La/Yb and high Yb contents but the majority of our rocks (mainly charnockite series) are falling into the Archaean TTG field, characterized by HREE-depletion. Solid diamonds = Candeias Unit, solid square = Itapecerica, and open triagle = Claudio Unit.
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3.8.1 Similarities and differences from our rocks compared with typical Archaean
granitois from Barberton normalized by N-MORB
The average of rocks from Candeias, Itapecerica and Cláudio Units in N-MORB normalized
spidergrams (Fig. 3.4 a, b and c), indicate that the three units display some similarities in terms of
fractionation. Otherwise, they record some interesting aspects in terms of concentration of several
elements that will be explained in detail for each graph, principally when they are compared with
typical Archaean granitoids from Barberton.
In Harker variation diagrams, the suites from the three units form moderately coherent trends
typical of differentiation of calc-alkaline series. In (MgO + FeO + MnO) versus SiO2 (Fig. 3.3b), an
excellent negative correlation is obtained attesting to the removal of ferromagnesian silicates
(pyroxene + amphibole) ± oxides. Successive fractionation of sodic plagioclase followed by K-
feldspar explains the strong trend in (K2O + Na2O – CaO) vs K2O space (Fig. 3.3c). Migmatite
samples tend to scatter most widely but in general the studied samples appear to have preserved their
major element coherence.
The similarity with other Archaean granitoids extends to REE systematics. The majority of
samples are strongly depleted in heavy REE (HREE) and some samples are also enriched in light REE
(LREE). These yields steep MORB-normalised REE patterns (Fig. 3.4 a, b and c) and La/Yb vs Yb
systematics characteristic of Archaean TTG (Fig. 3.3 d, Martin 1986). A few samples, particularly
some of the migmatites and grey gneisses, are not as depleted in HREE. In the case of the migmatites,
this may be the result of segregation of major REE hosts (mainly apatite; Ayres & Harris 1997) but in
the grey gneisses, high relative HREE abundance is a primary feature. There is variety in the relative
abundance of Eu (e.g. Fig. 3.4) which tightly correlates with the abundances of Al2O3 and Sr. This
strongly suggests that plagioclase removal has depleted the more potassic melts preferencially in Eu
(compared to Sm and Gd).
In multielement spider diagrams the studied rocks also show general similarity with typical
Archaean granitoids and upper continental crust. When elements are arranged in order of decreasing
MORB-melting incompatibility, all studied rocks display a strong negative slope. Some of the typical
continental anomalies (positive Pb and Li; negative Nb and Ta) are also well-developed. However, our
main thesis is that the studied rocks, despite all similarities with typicall TTG, show very remarkable
deviations that are not easily appreciated on chondrite or MORB-normalised plot.
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Candeias
0.1
1
10
100
1000
N-M
OR
B no
rma
lised
Itapecerica
0.1
1
10
100
1000
N-M
OR
B n
orm
alis
ed
0.01
0.1
1
10
100
1000
N-M
OR
B n
orm
alis
ed
a b
c
Cs
Rb
Ba Th U Nb
Ta W La Ce
Pr Sr Nd
Zr Hf
Sm Eu Gd
Ti Tb Dy
Ho
Y Er Tm Yb Lu
Cs
Rb
Ba
Th
U Nb
Ta W La Ce
Pr
Sr
Nd
Zr
Hf
Sm
Eu
Gd
Ti
Tb
Dy
Ho
Y Er
Tm
Yb
Lu
Cláudio
Cs
Rb
Ba
Th
U Nb
Ta W La
Ce
Pr
Sr
Nd
Zr
Hf
Sm
Eu
Gd
Ti
Tb
Dy
Ho
Y Er
Tm
Yb
Lu
Figure 3.4 N-MORB-normalized trace element patterns of average gneisses from A – Candeias, B – Itapecerica, and C – Claudio Units. Elements arranged in order of decreasing incompatibility in MORB-melting (after Sun & McDonough 1989). Normalizing values were taken from Sun & MacDough (1989) except W where the average of Indian, Pacific and Atlantic MORB was calculated from data presented by Newson et al. (1996). All patterns of all rock types from the three units display strong fractionation of LREE over HREE and many other typical features of Archaean TTG gneisses. Enderbites (solid black diamonds) from Candeias unit have virtually identical patterns to charnockites s.s. (solid gray squares). No significant differences are found for the three gneiss types of the Itapecerica Unit (solid dark grey diamond represents pink migmatite, open square represents fine migmatite, and solid grey triangles is the dark gneiss, believed to be the protolith of the migmatites). In Claudio Unit, the four differentiated gneiss types have patterns similar in shape but there is a difference in total abundance of trace elements, particularly the HREE. The most depleted rocks are granitoids (solid grey circles), which are believed to have formed as a result of migmatization. The most enriched gneisses are grey migmatites (solid grey diamond) while grey gneisses (open diamonds) and dark gneisses (solid grey triangles) are intermediate. The similarity of all studied gneisses to typical Archaean granitoids is evident in panel (a) from comparison with average upper continental crust (open circles) of McLennan (2001) and average 3.0-3.2 Ga Barberton granitoids (solid dark triangle; Kleinhanns et al. in press). However, appreciable differences exist in some of the most incompatible elements (see Figs. 3.5 and 3.6).
3.8.2 Deviations of expected trace element abundances
Closer inspection of the multi-element spider diagrams on Fig. 3.5 reveals that the studied
rocks display irregular abundance of some of the most incompatible trace elements, when compared to
the patterns of typical upper continental crust or averages of Archaean granitoids. These features are
present in most of the studied rock types but best expressed by the charnockites and enderbites of the
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49
Candeias unit (Figs. 3.5 and 3.6a). It can be seen that the Candeias gneisses are depleted in Cs, Rb,
and U by a factor of 10-50. While this could be attributed to mobility of these LILE during low T
processes (but we emphasise that these are unaltered, extremely fresh samples), the irregularities in W
and Ta are certainly not due to weathering or alteration. Indeed, both charnockites and enderbites show
negative slopes in their Nb-Ta tie lines, which imply superchondritic Nb/Ta ratios (higher than
MORB). This is a rare feature in terrestrial rocks. Equally, the W/La ratios of the Candeias gneisses
are more than a factor of 10 lower than in average Barberton highlands granitoids.
While these features are most evident in the orthopyroxene-bearing Candeias gneisses, many
lithologies from Itapecerica and Claudio share similar properties (Fig. 3.6b and c). General depletion
in W, U, and Ta is found in all Itapecerica gneisses, while Cs and Rb deficits are more variable. The
Claudio unit gneisses are all depleted in U and W but the behaviours of Ta, Rb and Cs are more akin
to typical TTG.
While it might be argued that these deviations are trivial, we point out that logarithmic
chondrite or MORB normalisation is not suitable to expose details of the most incompatible elements
at continental crust abundance. This point is illustrated on Fig. 3.5, where the identical data are
normalised to typical upper continental crust. In this normalisation, the deficiencies are readily
appreciated. The Candeias gneisses (enderbites and charnockites) show the typical depletion in HREE,
some irregularities in fluid-mobile elements like Pb and Be but very strong negative anomalies in W,
Ta, U, Rb and Cs.
Focussing on the most incompatible elements only (Fig. 3.6) it becomes obvious that average
granitoid gneisses from Barberton show a much smoother pattern with only a small anomaly in Cs.
These rocks do show a pronounced negative W anomaly, which we attribute to the uncertainty in
upper crustal W abundance (i.e. an artefact of normalisation). However, the important point is that the
Candeias gneisses are a factor of 10 times more depleted in W still. The depletions in Ta, U, Rb and
Cs are also easily exposed. Gneisses from Itapecerica show equally deep W and Cs deficits but are less
coherent in their depletion of Ta and U. No Rb-depletion is evident. The averages of Claudio unit
gneisses are apparently less depleted in these trace elements. However, individual samples can also
show comparable depletions, while other show slight enrichment (relative to the average) in Ta, U and
Rb.
Oliveira, 2004 Evolução tectônica de um.........
Candeias
0.001
0.01
0.1
1
10
Cs
Rb
Ba
Th
U Nb Ta
W
La
Ce Pr
Pb Sr
Nd
Be Z r
Hf
Sm
Eu
Gd Tb
Dy
Ho Y
Er
Tm
Yb
L u
Li
Upp
er C
rust
nor
mal
ised
Figure 3.5 – Upper Continental Crust normalization of same data (average of Candeias Unit) as shows in Fig. 3.4. Normalizing values were taken from McLennan (2001). This demonstrates clearly that certain elements (see below) are underabundant in comparison with typical Archaean granitoids as can be seen in detail in Fig. 3.6. The fractionation of HREE is evident in Candeias Unit. No major differences are found between the three units with the exception of the fine gneiss from Itapecerica Unit (open square). In all units unusual deficiencies in the following elements are evident: Cs, U, Ta, and W while Be, Rb and maybe Sr are moderately depleted.
3.9 DISCUSSION
The combination of geochemical features that characterise charnockites and enderbites from
the Campo Belo Metamorphic Complex is unusual when compared to most published studies of
granitoid rocks (Figs. 3.4, 3.5, 3.6). However, we are not aware of a single published geochemical
study that reports the full suite of incompatible trace elements for orthopyroxene-bearing granitoids
and it remains to be seen whether the features discovered here are typical of charnockites and
enderbites in general. Regardless, at this stage the main issue is to evaluate the likely processes that
could have caused these features and to draw conclusions regarding the petrogenesis of the studied
rocks.
Two competing scenarios need to be tested: (i) either the relative depletion in some of the
most incompatible elements was caused by metamorphic processes, most likely by loss to a fluid that
escaped during prograde dehydration or, (ii) the peculiar trace element patterns were caused by
fractional crystallisation during a purely igneous history in water-undersaturated magma chambers.
The textural evidence from outcrop and microscopic study is not conclusive in this matter. The
charnockites do not show a pervasive deformation fabric (as shown in Fig. 3.1b) but this might simply
reflect post-deformational thermal annealing.
50
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Candeias
0.001
0.01
0.1
1
10
Cs Rb Ba Th U Nb Ta W La Ce Pr Pb Sr
Upp
erC
rust
norm
alis
edItapecerica
0.01
0.1
1
10
Cs Rb Ba Th U Nb Ta W La Ce Pr Pb Sr
Upp
erC
rust
Nor
mal
ised
a
Cs
Rb
U Ta
W
Sr
b
Cláudio
0.0
0.1
1.0
10.0
Cs Rb Ba Th U Nb Ta W La Ce Pr Pb Sr
Upp
erC
rus
tnor
mal
ised
c
Figure 3.6 – Detail of Upper Continental Crust normalized average patterns of Candeias, Itapecerica, and Claudio Units. Panel (a) highlights depletion of Cs, Rb, U, Nb, Ta, W and Sr by comparison with average of typical Archaean granitoid rocks from Barberton (Kleinshanns et al. 2004). For instance, the average from Barberton shows small depletion Cs, W and possibly Sr. when compared with Itapecerica and Claudio Units some similarities and differences are evident (panels b and c). Fine migmatite from Itapecerica Unit is enriched in Th, U and Nb and it is depleted in Ba and Sr. The dark gneiss shows similar characteristics as Barberton granites while grey migmatites display the same trend as Candeias charnockites (panel c). Details from Claudio Unit show that granitoids are the most depleted. Dark gneiss is slightly enriched in U, Nb and Ta. Fine migmatite is enriched in Th and U. Migmatite and grey gneiss have similar trends as Candeias charnockite.
In the following, we will demonstrate that correlations exist between the extents of depletion
in incompatible elements (e.g. Cs, U, Ta), which seem unrelated to indicators of fractional
crystallisation. From these observations, we will develop our preferred model that dehydration-related
element loss during the prograde path of a high-grade metamorphic event superimposed these features
upon igneous protoliths.
When the depletion of U is expressed relative to Th (i.e. elevated Th/U ratios) we find a strong
positive correlation with the depletion of W relative to La (Fig. 3.7a). With the exception of
charnockites (but not enderbites) from the Candeias unit all samples indicate that W and U were lost
together. Because the relative incompatibility of W is poorly known we also expressed W-depletion
relative to Hf (Fig. 3.7b), which is significantly more compatible than La. Again a strong positive
correlation is found (omitting the high Th/U charnockites from the Candeias unit) pointing to coeval 51
Oliveira, 2004 Evolução tectônica de um.........
52
loss of U and W. When the relative U-depletion is compared to that of Cs (in a Th/U vs. Th/Cs
diagram, Fig. 3.7c) all samples, including the Candeias unit charnockites, define a strong positive
linear correlation. An interesting feature is that the charnockites have very high Th/U and Th/Cs ratios,
indicating that the igneous protoliths were already enriched in Th. This explains why these samples
plot off the trends in the previous two diagrams. In Fig. 3.7d it can be seen that most rocks from the
three units display elevated Nb/Ta ratios, which are generally much higher than average continental
crust (Nb/Ta = 12; McLennan 2001). It is also evident that relative loss of Ta is generally related to
loss of W. While the correlation is not as strong as those between the previously studied ratios, certain
lithologies (e.g. the Candeias unit enderbites) show very strong positive colinearity. A very strong
positive correlation is again found when the relative losses of Cs and Rb are compared (Fig. 3.7e).
This indicates that the samples most depleted in the most incompatible element Cs (relative to Rb) are
also the most depleted in Rb, when compared to Th. In summary, the relative depletions in Cs, Rb, U,
Ta and W are systematic, with few exceptions, across all lithologies in three different tectonic units
that underwent the same metamorphic history. Could these correlated features be of magmatic origin
caused by fractional crystallization?
In Figure 3.8 various trace element systematics are shown that are generally expected to form
during fractional crystallisation. The studied sample suite shows the effects of crystallisation of
plagioclase, K-feldspar and amphibole/clinopyroxene. A very strong trend is found between the extent
of (negative) Eu anomaly and the Sr/Nd ratio (Fig. 3.8a) indicating that the samples most depleted in
Eu are also lacking Sr (relative to REE), which is best explained by prior removal of plagioclase. This
is corroborated by the observation that the Eu-depleted samples are also relatively poor in Al (Fig.
3.8b). This trend is caused largely by plagioclase (see e.g. the enderbite samples from Candeias unit),
while K-feldspar cause only small (negative) covariation as borne out by the position of the
charnockite datapoints. A well-defined positive correlation is found between indicators of alkalinity
and Pb content (Fig. 3.8c), which most likely shows that the incompatible Pb is incorporated into K-
feldspar during the latest stages of crystallisation. There is an increase in the Ce/Pb ratio with
increasing abundance of combined Fe, Mn and Mg oxides. This shows that Pb was concentrated into
the remaining melt as clinopyroxene and amphibole crystallised while Ce was preferentially
incorporated into the liquidus phases (probably including apatite). In summary, the majority of
samples show geochemical features that are expected for calc-alkaline fractionation trends.
The important observation, in our view, is that no significant correlation exists between the
various differentiation indicators (i.e. Eu/Eu*, Sr/Nd, Al2O3, Na2O+K2O-CaO, Ce/Pb and
MgO+Fe2O3+MnO) and the anomalous trace element ratios (Th/U, La/W, Hf/W, Th/Cs, Nb/Ta and
Cs/Rb). Namely, the average r2 among these ratios is (0.194; calculated by converting negative
correlations into positive r2). The only significant (i.e. r2>0.3) are found with the Cs/Rb and Th/Cs
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
53
ratios, while ratios not involving Cs have extremely low degree of correlation (typically r2<0.15). Thus
we rule out that the observed incompatible trace element depletions were caused by fractional
crystallization. Rather, we believe that these features were caused by selective element loss to a
metamorphic fluid phase. This is supported by the fact that many of the most dehydrated rocks
(enderbites and charnockites) are the most depleted in those elements.
The average ratios used in Fig. 3.7 (Th/U, La/W, Hf/W, Th/Cs, Nb/Ta, Cs/Rb and La/Yb)
highlight just how different the studied samples are from average upper continental crust (UCC). In
UCC, these ratios are 3.82, 15, 2.9, 2.33, 12, 0.04 and 13.64 respectively (McLennan 2001). By
comparison the averages for enderbites and charnockites are: 16.8 and 49.7, 2561 and 1818, 301 and
118, 12.3 and 74.8, 17.5 and 21.6, 0.011 and 0.0033 and 33.6 and 58.5 (Table 3.1). While it is
generally appreciated that Cs, Rb and U can be mobile during dehydration (or aqueous alteration), W
and certainly Ta are generally considered to be relatively immobile. However, it has already been
shown (Kamber & Collerson 2000) that Ta is significantly more fluid-mobile than its geochemical
twin Nb and the coherent depletion in Rb, Cs, U, Ta and W suggests that in the studied rocks, these
elements were lost to a fluid, whose chemistry favoured their incorporation, while other fluid-mobile
elements, such as Pb, were evidently less affected (Fig. 3.8c). It is unknown how widespread these
geochemical characteristics are among metamorphic rocks, largely because very few complete datasets
exist in the literature. In addition, the studied rocks are remarkably unaltered (compared to common
metamorphic rocks) and were therefore not affected by retrograde fluids, which can obscure the
prograde element depletion (Sorensen 1997). Regardless, from the present evidence at least two
implications can be drawn.
Charnockites and enderbites that formed by complete metamorphic dehydration of granitoids
can be expected to show characteristic and systematic depletions in certain highly incompatible trace
elements. The resulting chemistry should thus allow their distinction from primary igneous water-
undersaturated charnoenderbites. Berger et al. (1995) postulated that charnockites and enderbites from
the Northern Marginal Zone of the Limpopo Belt represent primary igneous rocks. Here we use their
Th and U data to test this hypothesis building on our observation that metamorphic charnockites are
strongly depleted in U. The average U content of chanockite and enderbite analysed (by isotope
dilution) by Berger et al. (1995) is 2.4 and 2.8 ppm, respectively. This is much higher than the
contents found in our study (0.49 and 0.44 ppm). Indeed, the average Th/U ratios determined by
Berger et al. (1995) are 5.02 and 4.16 (charnockite and enderbite, respectively). These are within error
of Archaean UCC (Kramers & Tolstikhin 1997) and indicate no preferential U-loss, supporting Berger
et al. (1995) hypothesis. Our study demonstrates the need for more complete and accurate trace
element data of metamorphic rocks.
Oliveira, 2004 Evolução tectônica de um.........
54
With regard to the composition of the lower crust, the decoupling of the heat producing
elements K, Th and U observed in our rocks is of significance. It is clear, from estimates using
xenoliths and terrain models that the lower crust is on average neither charnoenderbitic in composition
nor of completely dehydrated nature. Many models for lower crust formation exist but they all agree
that the lower crust is less felsic than the upper crust (e.g. Rudnick & Fountain 1995). For example,
according to Rudnick (1992) basaltic underplating supplies heat to melt the lower crust, generating
granitoid liquids with negative Eu anomalies. The disparitity between upper and lower crust in bulk
composition is also reflected in the distribution of heat producing elements (chiefly K, Th and U).
McLennan (2001) estimated that concentrations of K, Th and U in the upper continental crust are 2.8
(weight percent), 10.7 (ppm) and 2.8 (ppm), while the lower crust has much lower concentrations of
0.53, 2, and 0.53, respectively. The lower crustal estimate, which is calculated from mass balancing
upper crustal and total crustal abundances, is quite robust because the total heat production in the crust
and the distribution of heat producing elements are accurately known for many continental areas (e.g.
Rudnick & Fountain 1995, Sandiford & MacLaren 2002, Sandiford et al. 2003). The heat producing
element abundance decreases strongly with increasing depth in the crust. By implication, the lower
crust is strongly depleted in heat producing elements compared to the upper crust, a prediction that has
been confirmed by chemical analyses of deep crustal sections and xenoliths derived from the deep
crust (e.g. Rudnick & Fountain 1995, Kramers and Tolstikhin 1997, Sandiford & MacLaren 2002).
Hence the question arises how lower continental crust ended up with its low inventory of K, Th and U.
As our enderbites and charnockites have K, Th and U concentrations of 1.32 and 3.45 (weight
percent); 7.31 and 24.5 (ppm), and 0.44 and 0.49 (ppm) the insight gained from our study is of
relevance for this issue. Three different types of models have been proposed for the lower crust:
(i) The lower crust is low in K, Th and U because it consists largely of gabbroic cumulates
rich in plagioclase (Kramers & Tolstikhin 1997). In this case, no mechanism for K, Th and U removal
is required. However, Murphy et al. (2003) found that lower crustal xenoliths did not have Pb-isotope
compositions that could be expected for low (Th&U)/Pb cumulates, but that those which are
unradiogenic required removal of Th and U long after formation of igneous protoliths.
(ii) The second hypothesis is that continental crust reorganises itself until it reaches a thermal
state compatible with heatflow across the crust/mantle boundary. A crustal section in which too much
heat is produced in the lower section will experience melting of the lower/middle crust. This happens
due to i) conductive dissipation following voluminous mid crustal plutonism, ii) accretion of material
with high radiogenic heat production at the base of the crust during continental collision or iii) because
of sediment-loading of high U, Th, K upper crustal rocks (e.g. Bodorkos et al. 2002, Sandiford &
MacLaren 2002, Sandiford et al. 2003). As the incompatible elements K, Th and U readily partition
into the melt, they are transported into upper crustal levels via granite plutons.
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
55
(iii) The third hypothesis, originally advocated by O’Nions et al. (1979) envisages
metamorphic K, Th and U loss. The relatively fluid-mobile elements (including the heat producing
nuclides) could be transported in aqueous fluids that escaped from the rocks during prograde
dehydration. However, this proposal, which was founded on Pb-isotope data of granulites, is not
supported by our observations from the Campo Belo granulites. Our enderbites, charnockites and
migmatites are very depleted in U only, while Th and K remained relatively immobile. Selective loss
of U also appears unlikley in view of the relatively high Th/U ratio of the upper crust (Hemming &
McLennan 2001).
The combined evidence thus favours crustal differentiation by melting of sections of lower
crust that contained too much K, Th and U. The last pulse of K-rich granitoids that marks final
stabilisation of many Archaean cratons is also observed in the Campo Belo terrains and the
metamorphic event experienced by our granulites may ultimately have been related to cratonisation
but it did not contribute significantly to the redistribution of heat producing elements.
3.10 SUMMARY
Unusually fresh, unaltered late Archaean enderbites, charnockites and migmatites from the
Campo Belo complex were used for a comprehensive geochemical study that has yielded the
following results and conclusions:
(i) All samples preserve many major element and trace element characteristics of the pre-
metamorphic protoliths, which are identified as typical TTG. Features include steep REE patterns,
high Sr/Y ratios, and general enrichment/depletion typical of calc-alkaline fractionation series.
However, all studied rocks but in particular charnockites and enderbites show very peculiar depletions
in some of the most incompatible elements.
(ii) Some of the depleted elements (Rb, Cs, U) are generally regarded as fluid mobile and their
low abundance in the studied rocks is thus compatible with metamorphic dehydration. There is
minimal retrograde or later low-grade overprint of the studied rocks and we attribute the loss of Cs, Rb
and U to escape of aqueous fluids during prograde metamorphism that culminated at granulite grade.
(iii) Depletions in Cs, Rb and U are, however, also correlated with depletions in elements (Ta,
W) that are not generally regarded as fluid-mobile. Depletion in Ta (but not Nb) has previously been
demonstrated for eclogites and must have been caused by the same processes that led to loss of U, Cs
and Rb. Hence, under certain circumstances Ta and W can also be fluid mobile.
Oliveira, 2004 Evolução tectônica de um.........
Figure 3.7 Selected trace element rations of gneiss from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) unit: (a) Th/U vs La/W, (b) Th/U vs Hf/W, (c) Th/U vs Th/Cs, (d) Cs/Rb vs Cs/Th, and (e) Nb/Ta vs Hf/W. All diagrams show positive covariation, which reflect systematic depletion in certain elements as discussed in the text.
56
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0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 5 10 15 20 25Sr/Nd
Eu/E
u*
0
2
4
6
8
10
12
0 10 20 30 40 50Na O+K O -CaO2 2
Pb
(ppm
)
0
2
4
6
8
10
12
0 2 4 6 8MgO + Fe O + MgO2 3
Ce/P
b
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
12 13 14 15 16 17 18Al O2 3
Eu
/Eu
*
a b
c d
Figure 3.8 Selected relationship between major and trace element from Candeias (solid diamonds), Itapecerica (solid square) and Claudio (open triangle) samples: (a) Eu/Eu* vs Sr/Nd, (b) Eu/Eu* vs Al2O3, (c) Pb vs (Na2O + K2O – CaO), and (d) Ce/Pb vs (MgO + Fe2O3 + MnO). All diagrams are showing the expected trend from fractionation of plagioclase (a, b); plagioclase and K-feldspar (c); and ferromagnesian silicates and Fe-oxides (d).
(iv) These relative element depletions are powerful fingerprints for study and classification of
charnockites and enderbites. Our study demonstrates the need for comprehensive datasets. Only
limited literature data is available. Using our new criteria, we confirm an igneous origin for
charnockites and enderbites from the Limpopo Belt of southern Africa (i.e. lack of U depletion).
(v) Finally, because element loss during metamorphic dehydration is only effective at
depleting U but not Th and K, this process cannot be implied for the low heat production rate of the
lower crust. Hence, the lower crust must be inherently depleted in these elements or have lost the
combination of the three elements to granites by partial melting.
57
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CAPÍTULO 4 RECENT ADVANCES AMONG Sr AND Nd CONCERNING TO THE
TECTONIC EVOLUTION OF CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN PORTION OF THE SÃO FRANCISCO
CRATON, BRAZIL.
4.1 INTRODUCTION
Strontium and neodymium isotopic signatures can potentially provide imperative information
on the evolution of polymetamorphic Archaean terrains. However, they may be poorly correlated
when the Rb-Sr and Sm-Nd systems reflect strong disturbances that occurred during younger events,
long after the formation of the rocks (e.g. metamorphic dehydrations).
Previous Rb-Sr and Sm-Nd studies on Archaean rocks have documented large behavior
divergences in these isotopic systems when undergoing deformational and/or metamorphic events (e.g.
Cameron et al. 1981, Collerson 1983, Tobisch et al. 1994, Ayres & Harris 1997).
As reported by Cameron et al. (1981) “the Rb-Sr isochrones from polymetamorphic terrains
display a wide scatter as a consequence of”: 1) open-system behavior of the samples during later
metamorphism; 2) Variation in the initial 87Sr/86Sr owing to heterogeneity of the protoliths; and 3)
Inclusion of rocks of different primary ages.
Collerson et al. (1983) says: “Nd and Sr isotopic data from high-grade gneissic complexes
metamorphosed under granulite facies conditions may yield equivocal and potentially misleading
interpretations of isochrons, significance of model ages, and estimates of crustal residence time”.
Hickman (1984), studying sheared and unsheared gneisses from the Limpopo mobile belt in
South Africa, reported the behavior of the Rb-Sr systematic. He showed that the unsheared gneisses
preserved their properties and, in contrast, the sheared zones displayed an age of metamorphic age.
The composition of the sheared gneiss was reset to that of the younger granites.
Moorbath et al. (1986) stated that the Sm-Nd system has a relatively small degree of Sm-Nd
fractionation, whereas it is greater for Rb-Sr. This means that the former is more susceptible to errors
than Rb-Sr isochrones.
Moorbath et al. (1997) pointed out that the interpretation of the Sm-Nd system is valid if the
analyzed rock(s) has (have) not been subject to open-system behavior or Nd-isotope homogenization.
It is the scope of this study to display new Rb-Sr and Sm-Nd whole-rock data for the gneisses
(migmatized or not) and the amphibolite of the gneissic unit (Fig. 4.1b). In addition, we are going to
Oliveira, 2004 Evolução tectônica de um.........
60
point out that the Rb-Sr and Sm-Nd can give reliable information on the metamorphic age and the
estimated age of rock formation, even if the Rb-Sr and Sm-Nd systems had been clearly affected by an
earlier event as attested by Oliveira et al. (submitted to Lithos). We are going to show the correlation
between strictly cogenetic rocks and a combination of “non-cogenetic” group of rocks. The relative
chronology between cogenetic and non-cogenetic rocks discussed in this paper is largely based on
observations made during fieldwork and geochemical behavior.
We are now going to review relevant published geochronological data for the area, whose
interpretation has certainly displayed a complex evolution for the southern São Francisco Craton
Basement (Fig. 4.1a).
4.2 GEOCHRONOLOGICAL OVERVIEW
The study area encompasses a sialic fragment of the Campo Belo Metamorphic Complex. In
regional terms, the area belongs to the southern portion of the São Francisco Craton, which is a
polycyclically evolved crustal segment, tectonically stable in relation to the Brasiliano Cycle mobile
belts (Alkmim et al. 1993).
The rocks of this region belong to the Divinópolis and Barbacena metamorphic complexes
(Machado Filho et al. 1983) that were later grouped in the Campo Belo Metamorphic Complex by
Teixeira et al. (1996). Besides the Complex, a mafic dike swarm and a supracrustal unit also crop out;
the latter, according to Machado Filho et al. (1983), can be correlated with the Rio das Velhas
Supergroup.
The geochronological record of the region can be found in Teixeira et al. (1996, 1998), who
also proposed an Archaean and Paleoproterozoic crustal evolution model characterized by successive
accretion/differentiation stages associated with subordinated crustal reworking processes.
The history of the Campo Belo Metamorphic Complex is compatible with the evolution of the
Bonfim and Belo Horizonte Metamorphic Complexes (Machado et al. 1992, Teixeira 1993, Teixeira &
Silva 1993, Teixeira et al. 1996, 1998, Pinese 1997, Carneiro et al. 1998a).
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Figure 4.1 Panel (a) -Geological map of the southern São Francisco Craton (modified by Campos Sales 2004). Key: 1- Neoproterozoic undivided cratonic cover; 2- Mesoproterozoic Espinhaço Supergroup; 3- Mesoproterozoic(?) São João Del Rei/Andrelândia Groups; 4-- Mesoproterozoic (?), Dom Silvério Group; 5- Paleoproterozoic granitoids; 6- Paleoproterozoic indiscriminate greenstone-type sequences; 7- Paleoproterozoic Minas Supergroup; 8- Neoarchean granitoids; 9- Neoarchean and Mesoproterozoic gabbroic and dioritic rocks (sills and dikes); 10- Neoarchean ultramafic plutonic suite; 11- Neoarchean Rio das Velhas Supergroup; 12- Archean metamorphic complexes partially reworked on Proterozoic time; 13- Faults and fractures (CSZ = Cláudio Shear Zone; JBSZ = Jeceaba-Bom Sucesso Shear Zone); 14- Fold axes; 15- Lithologic contacts; 61
Oliveira, 2004 Evolução tectônica de um.........
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16- Cities. Panel (b) Geological map of the study area (modified from Oliveira 1999; Oliveira & Carneiro 2001) showing the following features: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Alemão dimension stone quarry; B – Oliveira dimension stone quarry; C - Lila dimension stone quarry; D – Itapecerica dimension stone quarry; E - Marilan dimension stone quarry; F - Kinawa dimension stone quarry; G - Corumbá dimension stone quarry; H – Fazenda Corumbá dimension stone quarry and, I – Carmópolis de Minas dimension stone quarry
Teixeira et al. (1998), dating zircons (ion probe U/Pb) from migmatites of the Campo Belo
Complex, defined a polyphase Archaean history. They proposed three major geological events in that
area. The first took place at 3,205±17 Ma and would mark the formation of a granitoid crust. A second
period of intrusions occurred at 3,047±25 Ma, and the third event is recorded at 2,839±17 Ma. The
youngest age was considered to be the crystallization age of the neosome (migmatization event,
Teixeira et al. 1998).
Machado & Carneiro (1992) and Carneiro et al. (1998a, b) defined the Rio das Velhas orogeny
(2,780¯2,700 Ma) that represents the most important event that affected the Campo Belo Complex.
According to these authors, mafic-ultramafic intrusions were emplaced between ca. 2.75 and 2.66 Ga
in association with distensional phases of the Rio das Velhas orogeny. Endo & Machado (1998) also
defined discontinuities or distensional phases at the margins of the Rio das Velhas Greenstone Belt
during this period.
According to Oliveira (1999), Oliveira & Carneiro (2001) and Oliveira et al. (1998a, b, 1999)
the study region can be further divided in five lithodemic units: three gneissic, one supracrustal and
one fissural mafic. The gneissic rocks of the Campo Belo Metamorphic Complex vary widely in
composition (e.g. granitic, granodioritic and tonalitic) and can be grouped in three distinct units:
Cláudio, Itapecerica and Candeias Gneisses.
Thus, as the petrologic-tectonic picture is very complex and varied, a comprehensive study
including structural geology, petrography, geochronology and geochemistry is necessary to deal with
its tectonic evolution in a more precise way, completing the geologic mapping, petrographic definition
and kinematic study by Oliveira (1999) and Oliveira & Carneiro (2001), contributing to a better
geological knowledge of the region.
To point out these advances, new geological data obtained from a cratonic fragment located
among the cities of Itapecerica, Cláudio, Monsenhor João Alexandre, Carmópolis de Minas, Oliveira
and São Francisco de Paula (Fig. 4.1) are presented for gneisses, granitoids, and amphibolite rocks
from the gneissic unit.
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
63
4.3 SAMPLE SELECTION AND ANALYTICAL PROCEDURES
During fieldwork, eight quarries were selected for further laboratory work (Fig. 4.1). From
each quarry a group of rocks was carefully sampled from fresh fronts (about 20kg for each group) for
Rb-Sr and Sm-Nd analyses.
Sr and Nd isotope measurements were carried out at the Advanced Centre for Queensland
University Isotopic Research Excellence (ACQUIRE), using a VG 54-30 Sector multi-collector mass
spectrometer in static mode. Procedures were identical as described by Wendt et al. (1999). The long-
term (7 years) reproducibility of the NBS SRM 987 Sr and La Jolla and Ames metal Nd standards are 87Sr/86Sr = 0.710251±20 (2σ) and 143Nd/144Nd = 0.511861±11 (2σ) and 0.511977±12(2σ),
respectively.
Rb and Sm ratios were obtained from trace element analyses. The trace elements were
determined by ICPMS at the Advanced Centre for Queensland University Isotopic Research
Excellence (ACQUIRE). Sample preparation and analytical procedures used were identical to those of
Eggins et al. (1997) except that Tm was not used as an internal standard and W-2, a U.S.G.S. diabase
standard, was used as the calibration standard. Our preferred concentrations for W-2 and the measured
concentrations and relative standard deviations for AGV-1 (an average of 26 analyses of 9 digestions
analysed over four years) are shown in Table 1. Concentrations for W-2 were derived partly by
analysing it relative to synthetic standards (Li, Cr, Ni, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th,
and U) or are based on an assessment of published standard data (A. Greig, pers. comm.).
4.4. RESULTS
The ages reported in this study were calculated using the ISOPLOT program (Ludwig 1998).
Experimental (2 sigma) errors used in the regression analyses were 0.5% for 87Rb/86Sr and 0.1% for 147Sm/144Nd (based on standard reproducibility) and 26ppm for 87Sr/86Sr and 143Nd/144Nd (external
error based on NBS SRM 987 and EN-1 measurements over 6 years) at ACQUIRE. The Rb-Sr and
Sm-Nd isotopic data presented are from: 5 gneisses from Marilan quarry; 1 gneiss from Lila quarry, 1
gneiss from Itapecerica quarry 2 enderbites from Oliveira quarry; 3 charnockites from Alemão quarry;
2 gneisses from Kinawa quarry; 2 gneisses from Corumbá quarry; 4 gneisses from Fazenda Corumbá
Quarry; 3 amphibolites from Kinawa quarry and 1 amphibolite from Corumbá quarry (Figs 4.2 and
4.3, Tables 1 and 2).
All gneisses are slightly to strongly migmatized and deformed. The amphibolites occur as
strongly deformed boudins. Some isochron diagrams were chosen to illustrate the comparison between
a strictly cogenetic group of rocks with a combination of “non-cogenetic” group of rocks.
Oliveira, 2004 Evolução tectônica de um.........
Figure 4.2 Panel (a-h) – a) gneisses from Marilan quarry; b) gneiss from Lila quarry, c) gneiss from Itapecerica quarry; d) enderbites from Oliveira quarry; e) charnockites from Alemão quarry; f) gneisses from Kinawa quarry; g) gneisses from Corumbá quarry; h) gneisses from Fazenda Corumbá Quarry.
64
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Figure 4.3 Panel (a-b) – a) amphibolites boudins from Kinawa quarry and b) amphibolite boudins from Corumbá quarry. The samples were collected in assorted fraction alond the boudin outcrop.
65
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Table 4.1 Rb-Sr whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex.
.
Field number Gneiss Unit Rb Sr 87Rb/86Sr 87Sr/86Sr Error(ppm) (ppm) 2sigma
AO-1 211.79 131.34 4.6661 0.8686 5AO-2 187.25 104.19 5.2003 0.9034 4AO-3 137.34 141.10 2.8181 0.8137 5AO-4 134.34 155.16 2.5069 0.8017 5AO-6 240.35 347.78 2.0010 0.7728 5AO-9 304.37 52.38 16.8168 1.3260 4
AO-10 200.38 142.91 4.0594 0.8600 4
AO-7A 58.06 377.28 0.4456 0.7192 4AO-7B 49.68 405.84 0.3493 0.7176AO-8A 96.15 163.55 1.7021 0.7764 5AO-8B 96.70 160.54 1.7285 0.7772AO-8C 99.37 157.90 1.8063 0.7788
AO-11 62.30 230.42 0.7829 0.7369 5AO-12 119.78 256.11 1.3541 0.7529 5AO-14 214.28 133.48 4.6480 0.8748 4AO-15 176.96 132.67 3.8616 0.8484 4AO-16 147.94 97.31 4.4013 0.8770 4AO-17 107.08 208.17 1.4893 0.7612 5AO-18 152.42 66.45 6.6402 0.9555 5AO-19 162.83 153.69 3.0676 0.8204 5AO-20 84.89 289.05 0.8503 0.7363 5
AO-25A 10.40 126.09 0.2388 0.7112AO-25B 7.80 135.03 0.1673 0.7085AO-26 47.01 138.32 0.9841 0.7403AO-27 165.79 69.90 6.8660 0.9574AO-28 58.31 116.91 1.4441 0.7447
Itapecerica
Candeias
Cláudio
Cláudio amphibolites
Uncertainties reported for each ratio are 2 σ(mean)
66
Table 4.2 - Sm-Nd whole-rock analytical data for representative samples from the Campo Belo Metamorphic Complex. The ε(T1) was calculated fro 2.72Ga (supposed age for the formation of the rock) and ε (T2) was calculated for 2.62Ga (supposed age for the “metamorphic” event).
Field number Gneiss Unit Sm Nd 147Sm/144Nd 143Nd/144Nd Error ε(0) TDM (Ma) T (Ga) E (T)(ppm) (ppm) 2 sigma Rock formation
AO-1 0.88 5.61 0.0954 0.51067 20 -38 3088AO-2 12.52 75.75 0.1000 0.51080 10 -36 3042AO-3 6.70 34.78 0.1165 0.51114 10 -29 3023AO-4 4.62 27.61 0.1011 0.51085 8 -35 3006AO-9 12.23 50.11 0.1476 0.51161 9 -20 3388
AO-10 6.50 28.16 0.1396 0.51144 -23 3380
AO-7A 3.83 20.26 0.1143 0.51096 7 -33 3239 2.76 -3.6AO-7B 3.10 20.86 0.0898 0.51065 7 -39 2974AO-8A 5.78 35.76 0.0977 0.51074 7 -37 3061 2.05 -10.8AO-8B 4.76 29.33 0.0981 0.51072 6 -38 3109 2.05 -11.1AO-8C 5.52 33.87 0.0985 0.51069 6 -38 3148 2.05 -11.7
AO-11 4.42 20.72 0.1289 0.51116 7 -29 3460 2.74 -5.1AO-12 3.46 16.02 0.1306 0.51116 9 -29 3535AO-14 13.80 73.02 0.1142 0.51100 9 -32 3184 2.65 -4.1AO-15 8.11 44.18 0.1110 0.51093 10 -33 3183AO-16 9.86 42.25 0.1411 0.51148 9 -23 3366AO-18 4.57 22.57 0.1224 0.51118 8 -28 3160AO-19 3.18 17.02 0.1131 0.51100 7 -32 3141AO-20 8.75 37.19 0.1422 0.51155 9 -21 3258
AO-25A 5.10 18.35 0.1682 0.51211 -10 3214AO-26 6.21 26.22 0.1431 0.51159 8 -20 3191AO-27 1.55 6.26 0.1493 0.51165 10 -19 3373AO-28 5.43 20.07 0.1637 0.51195 12 -13 3437
CHUR - Goldstein et al. (1984) 0.51264
Itapecerica
Cláudio
Amphibolite Cláudio
Candeias
4.4.1 Outline of petrography features
The samples from Itapecerica Unit are pink and locally dark gneiss bodies (Fig. 4.3, panel a, b
and c). This unit is strongly migmatized and the rocks are fine to medium grained. The rocks are
predominantly of granitic composition but can vary to granodioritic. The main constituents of these
rocks are microcline followed by quartz, plagioclase, biotite, apatite, zircon, opaques and epidote. The
samples from the dark parts (maybe the protolith) are granodioritic to tonalitic in composition with
abundant amphibole and some pyroxene.
The Claudio Unit samples are gneisses of granodioritic to tonalitic, locally granitic
compositions. They are predominantly grey but in some outcrops (e.g. Corumbá, Fig. 4.2 e, sample
AO 15), they are locally pink due to the occurrence of pegmatite fluids, the same as seen in the
Kinawa quarry (not showed in the pictures).
At Fazenda Corumba stone quarry the rocks vary from dark, grey to white (granodioritic to
granitic). The rocks are fine- to medium-grained, slightly to strongly migmatized. In general,
plagioclase is the main constituent of these rocks followed by quartz, microcline, biotite, amphibole
(not common), zircon, apatite, titanite (not common), opaques, white-mica, chlorite and epidote. Some
samples show antiperthitic intergrowth. The main petrographic differences of the units are:
compositional (i.e. presence of hypersthene), grade of migmatization and color of the bodies (i.e. dark,
green, grey, and white).
The charnockites and enderbite in the studied area crop out along São Francisco de Paula
Town (charnockites) and Oliveira Town (enderbites), (Fig. 4.1b, A and B). The rocks were collected at
Alemão stone quarry and Oliveira stone quarry (Fig. 4.2 G and H). In most domains the rocks exhibit
massive textures with incipient foliation. These rocks are olive green, slightly to strongly migmatized,
and medium- to coarse-grained. The mineralogical banding is difficult to identify macroscopically
giving a homogeneous aspect to the bodies. The texture of the gneisses varies from granolepidoblastic
to granoblastic.
The charnockite from Alemão quarry (samples 8A, B, C and D, Table 4.1 and Figs 4.1A and
4.2 h) is only slightly deformed; massive in hand specimen and with grains that vary from medium to
coarse (Fig. 4.2h). Under the microscope, microcline is the main constituent followed by plagioclase,
quartz, biotite, hypersthene, and rare amphibole.
The variety from the Oliveira outcrop (samples 7A and B, Table 4.1 and 4.2, Figs 4.1b and
4.2g) is slightly to strongly deformed. The main constituents from this group are antiperthitic
plagioclase, quartz, microcline, biotite, amphibole, hypersthene, and the commonest accessories are
apatite, zircon, and an opaque mineral. In some thin sections hypersthene is absent.
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
4.4.2 Rb-Sr Sytematics
The isochron diagram displayed at Fig. 4.3a shows a combination of 3 migmatized, cogenetic
rocks from Marilan quarry (samples 2, 3 and 4, Itapecerica Unit, Fig. 4.1b-point E and Fig 4.2a). The
samples presented at Fig. 4.3a yield a model I isochron equivalent to an age of 2,609 ± 27 Ma and an
initial 87Sr/86Sr of 0.7072 ± 0.0012 (MSWD = 0.34). The analogous group of migmatized, cogenetic
gneisses (samples 16, 17 and 18, Cláudio Unit, Fig. 4.1b, point H) from Fazenda Corumbá quarry
displays a model I isochron (Fig. 4.3b) equivalent to an age of 2,603 ± 15 Ma and an initial 87Sr/86Sr of
0.70511 ± 0.00049 (MSWD = 0.85). When the sample 19 is integrated it exhibits a similar age
correlation (Fig. 4.3c). It shows a Model 3 Solution (±95%-conf.) on 4 points with an age of 2,623 ±
280 Ma and initial 87Sr/86Sr =0.706 ± 0.018 (MSWD = 70).
The regression lines displayed at Figs. 4.3a and 4.3b are not totally precise for the reason that
only three samples were used for each regression. However the results are representative in terms of a
regional scenario. For the Marilan group, samples 1 and 6 were not used, because they present
unrealistic initial ratios. Model ages for samples 2, 3 and 4 are coherent with the time of rock
formation (Table 4.1). Machado & Carneiro (1992) reported similar ages (about 2.75Ga) for zircons
from the Rio das Velhas orogeny (2,780¯2,700 Ma).
Model ages listed in table 4.1 for both Marilan and Fazenda Corumbá quarries are very
similar. The initial ratio for sample 16 may indicate the source for a crustal component melt. On the
other hand, samples 17, 18 and 19 display intermediate initial ratios that indicate a range of
intermediate melt sources.
Three-point isochrons are considered to be of uncertain reliability. We will then combine
cogenetic and “non-cogenetic” samples from different quarries, taking some from the same gneissic
unit and with similar composition. The combination of cogenetic and “non-cogenetic” samples from
Marilan (samples 2, 3, and 4), Lila (sample 9) and Itapecerica (sample 10) quarries, which belong to
the Itapecerica Unit and show approximately the same composition, display a robust regression
isochron, corresponding to an age of 2,571 +33/-65 Ma and an initial 87Sr/86Sr of 0.7079 +0.0038/-
0.00081 (Fig. 4.4a)
The same combination above, excluding sample 9 which yields high 87Rb/86Sr and 87Sr/86Sr
concentrations, display a Model 1 Solution (±95%-conf.) for 4 points, equivalent to an age of 2,603 ±
23 Ma and initial 87Sr/86Sr of 0.7074 ± 0.0011 (MSWD = 0.60, Probability = 0.55, Fig. 4.4b). The
robust isochron displayed in Fig. 4.4c is a combination of samples 2, 3, 4, 9 and 10 and one sample
from the Kinawa quarry (11). The results show an age equivalent to 2,593 +13/-67 Ma and initial 87Sr/86Sr of 0.7077 + 0.0022/-0.00061. 69
Oliveira, 2004 Evolução tectônica de um.........
0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
1.5 2.5 3.5 4.5 5.587Rb/86Sr
87Sr
/86S
r
Age = 2609 ± 27 MaInitial 87Sr/86Sr =0.7072 ± 0.0012
MSWD = 0.34
data-point error ellipses are 2σ
0.72
0.76
0.80
0.84
0.88
0.92
0.96
1.00
0 2 4 687Rb/86Sr
87Sr
/86Sr
Age = 2603 ± 15 MaInitial 87Sr/86Sr =0.70511 ± 0.00049
MSWD = 0.85
data-point error ellipses are 2σ
8
a b
Figure 4.3 Rb-Sr isochron diagram showing the nearby correlation between Marilan quarry and Fazenda Corumba quarry. The three point isochron are considering be of uncertain realiability.
The same group of samples displayed at fig. 4.4c, excluding sample 9, display Model 1
Solution (±95%-conf.) for 5 points, resulting an age of 2,603.5 ± 8.9 Ma and initial 87Sr/86Sr of 0.7074
± 0.00021 (MSWD = 0.40, probability=0,75, Fig. 4.4d). The robust regression line shown in Fig. 4.4e
is a combination of samples 2, 3, 4, 10 and 11 and migmatites from the Corumbá quarry (samples 14
and 15). This isochron exhibits an age of 2,556 +50/-87 Ma and an initial 87Sr/86Sr of 0.7079 +0.0038/-
0.0018. This group, excluding sample 9, shows a robust regression line of 2,597 +12/-270 Ma and
initial 87Sr/86Sr of 0.7075 +0.0078/-0.0022 (Fig. 4.4f).
The information is further constrained when rocks from fig. 4.4e are grouped with migmatites
from the Fazenda Corumbá quarry (samples 16, 17, 18 and 19). They display a robust regression for
11 points of 2,597 ± 56/-110 Ma with initial 87Sr/86Sr =0.7074 ± 0.0015/-0.0034 (4.4g). The
combination of all rocks, excluding samples 8 and 9, show similar information and displays a robust
regression isochron for 17 points corresponding to an age of 2,613 +75/-90 Ma and an initial 87Sr/86Sr
of 0.7043 +0.0031/-0.0021 (Fig. 4.4 h).
Model ages and initial ratios listed in table 4.1 are variable when analyzed independently, but
geologically possible.
70
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
0.7 8
0.8 0
0.8 2
0.8 4
0.8 6
0.8 8
0.9 0
0.9 2
1.5 2 .5 3.5 4.5 5.587Rb/86Sr
87S
r/86 Sr
0.6
0.8
1.0
1.2
1.4
0 4 8 12 16 2087Rb/86Sr
87S
r/86S
r
Age = 2571 + 33/-65 Ma
Initial 8 7Sr/86Sr =0.7079 ±+0.0038/-0.00081
Age = 2603 ± 23 Ma
Initial 87Sr/86 Sr =0.7074 ± 0.0011MSWD = 0.60, Probability = 0.55
0.6
0.8
1.0
1.2
1.4
0 4 8 12 16 2087Rb/86Sr
87Sr
/86Sr
Age = 2593 +13/-67 MaInitial 87Sr/86Sr =0.7077 +0.0022/-0.00061
a b
cdata-point error ellipses are 2σ
data-point error e llipses are 2σ data-point error ellipses are 2σ
0.70
0.74
0.78
0.82
0.86
0.90
0.94
0 2 4 687Rb/86Sr
87Sr
/86Sr
Age = 2603.5 ± 8.9 MaInitia l 8 7Sr/8 6Sr =0.70738 ± 0 .00021
MSWD = 0.40, probability = 0.75
data-point error ellipses are 2σ
d
0 .6
0 .8
1 .0
1 .2
1 .4
0 4 8 12 1 6 2087Rb/86Sr
87Sr
/86Sr
data-point error ell ipses are 2σ
e
0.70
0.74
0.78
0.82
0.86
0.90
0.94
0 2 4 687Rb/86Sr
87Sr
/86Sr
data-point error ellipses are 2σ
f
Age = 2597 +12/-270 MaInitial 87Sr/86Sr =0.7075 +0.0078/-0.00022
Age = 2556 +50/-87 MaInitia l 87Sr/86Sr =0.7079 +0.0038/-0.00018
0.65
0.75
0.85
0.95
0 2 4 6 887Rb/86Sr
87Sr
/86Sr
Age = 2597 +56/-110 M aIn itia l 87Sr/86Sr =0.7074 +0.0015/-0.0034
data-point error ellipses are 2σ
g
0 .6 5
0 .7 5
0 .8 5
0 .9 5
1 .0 5
0 2 4 6 887Rb/86Sr
87S
r/86 Sr
Age = 2613 +75/-90 MaInitial 87Sr/ 86Sr =0.7043 +0.0031/ -0.0021
data-point error ellipses are 2 σ
h
Figure 4.4 Rb-Sr isochron diagram showing in close proximity correlation between the three gneisses units.
In general the combinations of gneisses (migmatites) from all units display a similar evolution.
The majority of the samples yield Neoarchean ages or metamorphic ages and Mesoarchaean protoliths.
71
Oliveira, 2004 Evolução tectônica de um.........
Most of the model ages support the Rio das Velhas orogeny postulated by Machado & Carneiro
(1992). The population of three cogenetic amphibolites from the Kinawa quarry shows an age of 2,695
± 130 Ma and an initial 87Sr/86Sr of 0.70189 ± 0.00075 (MSWD = 5.7) (Fig. 4.5a, point F, Cláudio
Unit). The same amphibolites from the Kinawa quarry, when grouped with one “non-cogenetic”
amphibolite from the Corumbá quarry, display a Model I isochron of 2566 ± 54 Ma with an initial 87Sr/86Sr of 0.7027 ± 0.0023 and MSWD of 72 (Fig. 4.5b, point G). The combination of these samples
with another amphibolite from the Carmópolis de Minas quarry gives an unrealistic result, as shown in
fig. 4.5c. Initial ratios for samples 25a, b and 26 from the Kinawa quarry indicate the source for a
mantle component melt. Nevertheless, the model ages are not conclusive. For samples 27 and 28, the
initial ratios are not convincing and display unreliable results. However the combination of these rocks
displays an interesting errochron, similar to the evolution from the gneisses.
0 .70
0 .71
0 .72
0 .73
0 .74
0 .75
0.0 0.2 0.4 0.6 0.8 1.0 1.287Rb/86Sr
87S
r/8 6S
r
Age
= 26 95 ± 130 M aInitial
87Sr/86Sr =0.70189 ± 0 .00075
MSWD
= 5.7
data-po int error e llipses are 2σ
0.65
0.75
0.85
0.95
1.05
0 2 4 6 887Rb/86Sr
87Sr
/86Sr
Age
= 2566 ± 54 MaIn itial
87Sr/86Sr =0.7027 ± 0.0023
MSWD
= 72
data-po int error ellipses are 2 σ
0.65
0.75
0.85
0.95
1.05
0 2 4 6 887Rb/86Sr
87S
r/86S
r
Age = 2569 +130/-1900 MaInitial 8 7Sr/ 86Sr =0.7023 ± 0.0082
data-point error ellipses are 2 σ
a b
c
Figure 4.5 Rb-Sr isochron diagram showing in close proximity correlation between the three gneisses units.
72
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
73
4.4.3 Sm-Nd Systematics
Sm-Nd “errochrons” for the same group of rocks analyzed for Rb-Sr will be presented,
excluding sample 17 (Sm-Nd data not available). Certain combinations of cogenetic and “non-
cogenetic” gneisses result in poor correlations and exhibit great scatter of MSWD values. Therefore,
apart from the high age errors, the results appear to be similar to the Rb-Sr results. The “errochron”
diagram presented at Fig. 4.6a reveals a group of 3 cogenetic rocks from the Marilan quarry (samples
2, 3 and 4). This result yields a Model III Solution errochron equivalent to an age of 3,014 ± 270Ma
and an initial 143Nd/144Nd of 0.50882 ± 0.00021 (MSWD = 2). The combination of rocks from
Marilan, Lila and Itapecerica quarries (samples 2, 3, 4, 9 and 10, Fig. 4.6b) shows a robust regression
“errochron” age of 2,539 +750/-580 Ma and an initial 143Nd/144Nd of 0.5091 + 0.00023/-0.0025.
The combination of samples 2, 3, 4, 9, 10, 11, 14 and 15 (Fig. 4.6c) displays a robust
regression “errochron” age of 2,539 +580/-640 Ma and an initial 143Nd/144Nd of 0.50914 + 0.00041/-
0.00052. Fig. 4.6d represents the set of rocks of Fig.4.6c including the migmatites from Fazenda
Corumbá (samples 16, 17, 18 and 19). The combination of these “non-cogenetic” rocks shows a robust
regression “errochron” age of 2,572 +250/-470 Ma and an initial 143Nd/144Nd of 0.5091 + 0.00024/-
0.00030. The same result is obtained when a migmatite sample from Carmópolis de Minas quarry
(sample 20, Fig. 4.6e) is included. The robust regression “errochron” displays an age of 2,593 +340/-
280 Ma and an initial 143Nd/144Nd of 0.50910 + 0.00024/-0.00030. Fig. 4.6f shows a combination of
almost all rocks from the gneissic unit (Table 4.2). The result displays a robust regression errochron
age of 2,593 +280/-490 Ma and an initial 143Nd/144Nd of 0.50907 + 0.00024/-0.00030.
4.5 DISCUSSION
The São Francisco Craton has a polyphase Archaean history (Teixeira et al. 1998, Table 4.3)
and, based on some aspects of its evolution, we are going to focus the interpretations of our results on
previous existing results.
Oliveira, 2004 Evolução tectônica de um.........
0.5 107
0.5 108
0.5 109
0.5 110
0.5 111
0.5 112
0.5 113
0.096 0.100 0.1 04 0.108 0.112 0.116 0.1 20147Sm/144 Nd
143 N
d/14
4 Nd
Age = 3014 ± 290 MaInitial 143Nd/144Nd =0. 50882 ± 0.00021
MSWD = 2.0
data-point error e llipses are 2σ
0.5106
0.5108
0.5110
0.5112
0.5114
0.5116
0.5118
0.09 0.11 0.13 0.15147Sm/144Nd
143 N
d/14
4 Nd
Age = 2539 +750/-580 Ma
In itial 143Nd/14 4Nd =0.5091 ± 0.0014
data-point error ellipses are 2σ
0.5106
0.5108
0.5110
0.5112
0.5114
0.5116
0.5118
0.09 0 .11 0.13 0.15147Sm/144Nd
143 N
d/14
4Nd
Age = 2539 +580/-640 Ma
In itial 14 3Nd/144Nd =0.50914 ± 0.00046
data-point error ellipses are 2σ
0.510 6
0.510 8
0.5110
0.5112
0.5114
0.5116
0.5118
0.09 0 .11 0.1 3 0.15147Sm/14 4Nd
143 N
d/14
4Nd
Age = 2572 + 250/-470 Ma
Initial 14 3Nd/144Nd =0.50910 ± 0.00027
data-point error ellipses are 2σ
0.5106
0.5108
0.5110
0.5112
0.5114
0.5116
0.5118
0.09 0.11 0.13 0.15147 Sm/144Nd
143 N
d/14
4 Nd
Age = 2593 +340/-280 MaInitial 143Nd/144Nd =0.50908 ± 0.00027
data-point error e llipses are 2σ
0.5104
0.5106
0.5108
0.5110
0.5112
0.5114
0.5116
0.5118
0.08 0.10 0.12 0.1 4 0.16147Sm/144 Nd
14
3 Nd/
144 N
d
Age = 2593 +280/-490 Ma
Initia l 143Nd/144Nd =0.50907 ± 0.00029
data-point error ell ipses are 2σ
a b
c d
e f
Figure 4.6 Sm-Nd errochron diagram showing nearby correlation between the three gneisses units as demonstrated for Rb-Sr.
As reported by Machado & Carneiro (1992), Teixeira et al. (1996, 1998a), and Carneiro et al.
(1998 a, b), the period of 2,612-2,593 Ma is associated with a Neoarchaean pulse of granite plutonism,
representing the youngest generation of granites in the southern part of the São Francisco Craton.
An immediate consequence of these studies has some important implications in the
interpretation of our data.
1. What does the Rb-Sr and Sm-Nd method date in high-grade metamorphic terrains
means?
74
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Table 4.3 – Summary of the Archaean tectonomagmatic events in the southern part of the São Francisco Craton in the light of U/Pb and Sm-Nd data. This table was summarized by Teixeira et al. 2000.
Age (Ma)
2,612-2,593 Late-tectonic granitoid intrusions after amalgama and final crust stabilization [1,2]Data compiled from Carneiro et al. 1998a; Teixeira et al. 1996, 1998a and Noce et al. 1998).[1] Campo Belo gneissic-migmatitic terrain, [2] Bonfim dome and [3] Belo Horizonte dome
Formation of the Bonfim TTG suite and the Rio das Velhas greentone belt (includingCongonhas and Caeté) - the Rio das Velhas orogeny [2]. Crustal reworking andemplacement of mafic-ultramafic bodies and gabbro-noritic dikes [1]. Granitoidintrusions in the entire terrain [1,2] and regional migmatization [2]. Progressive terrainassembly starting at 2,780 Ma ago.
2,778 - 2,698
[1] Crystallization of the neossome material in migmatites, as well as TTG crustreworking [2,3]
2860 ± 10 to 2,839 ± 17
3205 ± 25 [1] Emergence of early continental crust, compatible with TDM ages on the countryrocks, up to 3.25 Ga. Formation of the Piumhi greentone belt and correlatives.
Characteristics of the crustal evolution in the Southern São Francisco Craton
[1] Major generation of the Campo Belo crust and progresswive magmatic accretion.The gneisses have Pb isotopic signatures consistent with mantle-like single-stageevolution, yielding comparable TDM ages between 3.00 and 2.90 Ga. Origin iof theBelo Horizonte gneiss protholiths [3], as well as part of the Bonfim gneiss protholiths.Development of granulitic facies metamorphism.
3047± 25
2. Are our results reflecting the reactivation of earlier discontinuities, as recorded at the
margins of Rio das Velhas Greenstone Belt (according to Endo & Machado 1998)?
3. Can the Rb-Sr and Sm-Nd data presented here be seen as evidence for a strong
metamorphic event that affected the area around ~2,650-2,550 Ma? The period of ca. 2.6 Ga is
associated with the Neoarchaean granitic pulse. For the same period, Teixeira et al. (1996) reported the
isotopic resetting of the Rb-Sr and Pb/Pb systems in the Campo Belo Complex, and defined a regional
thermal overprint most likely related to low-temperature fluids.
4. Thus, could the ca. 2.6-2.59 Ga granitoids be products of the migmatisation event and
record the same history as the migmatisation period?
5. How many migmatization events occurred in the Campo Belo Metamorphic Complex?
Our results stress out a tectonothermal event at about 2.6Ga in the Campo Belo Metamorphic
Complex, even if most of the results are from combinations of cogenetic and “non-cogenetic” rocks.
Some of the isochrons were determined with only three points, which is not consistent statistically.
They were used as an indication of the behavior of some cogenetic samples.
75
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76
Recently, Oliveira et al. (paper submitted to Lithos) reported that the rocks from the studied
area had lost some elements (i.e. Sr, Nb, and Ta) via fluid loss during the prograde metamorphic event.
This fact seems to be useful for our Rb-Sr and Sm-Nd interpretations once it implies that there were
losses of some trace and REE elements probably about 2.6Ga.
Some authors give some important elucidations about the behavior of the Sm-Nd and Rb-Sr
systems under high-grade metamorphic events. Numerous experimental studies have established that
Sm-Nd fractionation occurs during melt formation and their ratios are affected by crustal anatexis or
by late-stage fractional crystallization (i.e. Harrison & Watson 1983, Pimentel & Charley 1991,
Montel 1993).
Moorbath et al. (1997) identified in early-Archaean rocks suites affected by late
tectonothermal events the opening of the Sm-Nd system, leading to effective resetting accompanied by
complete or near complete Nd-isotope homogenization. Based on this information and taken with care,
it is fair to consider that the Sm-Nd isochrons can provide reliable information on the timing of the
tectonothermal processes (i.e. migmatization) that affected the studied area.
For Sato & Siga Junior (2000), the production of juvenile continental crust was larger in the
Proterozoic than in the Archaean. According to them: “there was a certain increase in the depletion
rate of Sm-Nd and Rb-Sr in the upper mantle at about 2.2Ga, as a reflex of the strong differentiation of
the upper mantle to continental crust”. As illustrated by Collerson (1983), the Rb-Sr systematics
obtained from a large population of multiply metamorphosed and deformed rocks (including
migmatization) has the ability to depict the age of the protolith (known from U/Pb zircon dating).
When smaller populations are used (e.g. neosomes, or biotite-rich paleosomes), the Rb-Sr method
often dates the time of migmatization and not of the protolith formation.
For the studied area, Teixeira (1993) interpreted the Rb-Sr ages of ca. 2.566±53 Ga for
enderbites in terms of reworking (complete isotope resetting), without a detailed explanation on how
the Sr homogenization could have occurred.
Using our new Rb-Sr and Sm-Nd data, including trace element geochemistry for a wide
variety of samples, we will further test this hypothesis. Large, homogenous, fresh samples were taken
from 8 different quarry fronts from Itapecerica, Cláudio and Oliveira Units.
The data presented here are, in general, compatible with the earlier results of Teixeira (1993),
but the isochrons for different rock types have distinct initial 87Sr/86Sr isotope ratios. A straightforward
interpretation of these isochrons, which define the time of protolith formation (and not of
migmatization), shows that the initial ratios correspond to a range of mantle- (amphibolites, 87Sr/86Sr =
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
0.7019), intermediate- (migmatites from Fazenda Corumbá quarry, 87Sr/86Sr = 0.7051), and crustal-
derived (Marilan quarry, 87Sr/86Sr = 0.7072) melt sources. Therefore, the best combination of cogenetic
and “non-cogenetic” samples displays initial ratios of the order of 0.707.
If the Rb-Sr and Sm-Nd regression lines (or robust regression lines) define the age of
migmatization, the maximum protolith age can be estimated from the difference between the initial Sr
and/or Nd-isotope ratios of the regression and of the coeval mantle. In order for complete resetting to
occur, it is necessary to homogenize Sr and/or Nd-isotopes at the quarry scale. An equilibration of this
magnitude would also seriously affect the Rb-Sr and Sm-Nd ratios of different parts of the quarry, as
already tested by many experimental studies (i.e. Collerson 1983; Hickman 1984, Tobisch et al. 1994,
Moorbath et al. 1997). Indeed, based on our geochemical data, we can point out that some elements
were lost during the metamorphic event that affected the Candeias, Cláudio and Itapecerica Units (Fig.
4.1).
In the Cs/Rb vs Cs/Th plot (Fig. 4.7) a strong positive trend is displayed, which requires
preferential element loss in the order Cs>Rb>Th. Hence, when compared with the Upper Continental
Crust average (MacLennan 2001), our rocks are depleted in Cs and Rb. The continental crust has a
Cs/Rb ratio of about 0.036 (MacDonough et al. 1991), whereas our rocks have lower ratios.
Apparently, Sr was not affected by element loss, as there is no obvious depletion in this element.
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.00 0.10 0.20 0.30 0.40Cs/Th
Cs/
Rb
Figure 4.7 - Positive trend correlation between Cs;Rb vs Cs;Th from Cláudio (open triangle), Itapecerica (solid square) and Candeias (solid diamond) Units.
The important conclusion from the trace element investigation is that the present Rb-Sr ratio
for the quarry is certainly lower than the pre-migmatization ratio. Hence, when calculating the crustal
residence time prior to 2.57 Ga from the present-day Rb-Sr, we will obtain a maximum estimate.
For the late Archaean mantle, we assume an initial 87Sr/86Sr of 0.7014 and use the difference
between the initial values recorded by the isochrons to estimate the maximum protolith age. 77
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For the Fazenda quarry we obtain a crustal prehistory of only ca. 70 Ma, while the samples
from Marilan allow estimating a prehistory of ca. 120 Ma. This means that the protoliths of these
migmatites formed between 2,670 and 2,730 Ma, the period which Machado & Carneiro (1992)
defined the Rio das Velhas orogeny (2,780¯2,700 Ma). According to Machado & Carneiro (1992) this
was the time when the study area was intruded by mafic magmas.
The neodymium isotope data display similar values at about 2.5 Ga but with larger errors
(table 2 and Fig. 4.4). The results by themselves are not conclusive statistically but in association with
the Rb-Sr method, they provide an interesting assumption. At least we can assume that around 2.6 Ga
the Rb-Sr and Sm-Nd systematics were disturbed.
The results of the Sm-Nd data allow us to put some constraints on the behavior of this system
for the high-grade metamorphic terrain. εNd values (chondritic uniform reservoir - CHUR) vary from
–20 to –38 and imply that at the time of “metamorphism”, Sm-Nd was disturbed. Or, alternatively, the
rocks were derived from different protoliths. A complex evolution is indicated by εNd values
calculated at 2.6 Ga, which vary from ~-2 to -5.3 for the migmatites, and -0.79 to -3.44 for the
amphibolites (table 4.2). This implies that a large amount of recycled older crust participated in the
early evolution of the Campo Belo Complex. All rocks show older model ages but with variable εNd
along their evolution (Fig. 4.8). It may indicate different rates of element losses during later processes.
T (Ga)
εNd
Amphibolites
Gneisses (migmatised)
Depleted Mantle
2.6 Ga
CHUR
20
10
0
-10
-20
-30
-40
-500 0.5 1 1.5 2 2.5 3.53 4 4.5
Figure 4.8 Nd versus time (DePaolo 1981) for the studied samples (see table 4.2 for reference)
78
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79
Campos et al. (2003), studying zircons from mesosome and leucosome of migmatites from the
Passa Tempo Metamorphic Complex, southeast of the Campo Belo Metamorphic Complex, defined
the minimum age for the granulite-facies metamorphism at 2,622 ± 18Ma (mesosome) and 2,599 ±
45Ma (leucosome). This possibility clearly shows that our new data support the premise that the Rb-Sr
and Sm-Nd systematics are a result of disturbances during prograde metamorphic event.
4.6 SUMMARY
In this study, calculations based on a combination of cogenetic and “non-cogenetic” rocks led
us estimate the age of disturbances in the Rb-Sr and Sm-Nd systems. Samples from different gneissic
units, including the amphibolites (boudins), display a similar evolution.
From the isotopic results discussed above, it is understandable that the crustal evolution of the
Campo Belo Metamorphic Complex had some contributions around 2.6 Ga. The Sr/Nd systematic by
itself is not conclusive and may yield equivocal misleading interpretations. However, some remarkable
conclusions and inferences can be drawn:
1 – Based on geochemical evidence, 87Sr/86Sr initial rations, εNd (CHUR), fSm-Nd, it is clear that
the Rb-Sr and Sm-Nd systematics were substantially affected during 2.6 Ga;
2 – The model proposed by Teixeira et al. (1996) with Rb-Sr and Pb/Pb isotopic resetting
associated with “low-temperature fluids” is realistic but it is difficult to envisage the complete
resetting at the quarry scale.
3 – Recently, Campos et al. (2003), dating migmatite zircons, defined the minimum age for
the granulite-facies metamorphism in the southeast part of the São Francisco Craton. Such a model has
been favored for the studied rocks are hypersthene-bearing gneisses under granulite metamorphic
facies. From this it may have the same correlation to our Sr/Nd results and the same implications for
the evolution of the Campo Belo Metamorphic Complex.
4 - Alternatively, the studied rocks were contaminated by unradiogenic (mantle) Sr during the
2.6 Ga migmatization event, as recorded by the amphibolite isochron initial ratio.
For the next paper (work in progress) the combined data (trace element geochemistry, Sr-
isotopes, Nd isotopes, Ar-Ar ages and U/Pb zircon ages) will yield a complete picture, not only for the
timing, but also for the element mobility associated with the migmatization event(s).
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CAPÍTULO 5
IMPLICATIONS FOR TRANSAMAZONIAN 40Ar/39Ar AGES FOR THE CAMPO BELO METAMORPHIC COMPLEX, SOUTHERN SAO
FRANCISCO CRATON, BRAZIL.
5.1 ABSTRACT
The gneisses, amphibolites, gabbronorite and gabbros from Cláudio, Supracrustal and Mafic
Fissural Units in the Campo Belo Metamorphic Complex display imperative information for the
evolution of the Transamazonian and post-Transamazonian event in the Archaean basement in the
southern part of the São Francisco Craton. By combining field, structural, petrographic and the 40Ar/39Ar data, the evolution of the Campo Belo Metamorphic Complex within the Transamazonian
and post-Transamazonian period is obvious. 40Ar/39Ar age plateaus of about 2.0-1.9 Ga have been
obtained for amphiboles and biotites from the (migmatized) Cláudio Unit gneisses and amphibolites
from the Cláudio and Supracrustal Units. These results indicate that during the Transamazonian
orogeny, the Campo Belo Metamorphic Complex reached at least the greenschist to amphibolite facies
once the closing temperature of amphiboles and biotites are 500°C and 300°C respectively. The
question that still remains is related to the deformational event, whether it is Archaean or
Transamazonian. Based on previous work in the nearby Quadrilátero Ferrífero and our fieldwork
information, we believe that there are great possibilities for the deformational event having taken place
some time about 2.2Ga, related to the Transamazonian orogeny but not in Archaean and the 2.0-1.9 Ga
representing the cooling ages of this event. The 40Ar/39Ar age plateaus of about 1.7Ga and 0.9Ga have
been obtained for biotites and amphiboles of undeformed and unmetamorphosed gabbronorites and
gabbros from the Mafic Fissural Unit. These results suggest that at 1.7Ga intense break up of the crust
(Staterian period, 1.8-1.6Ga, related to the continental rift scenario or Espinhaço rift) or a simple
reactivation of the pre-existent Archaean structures occurred. The 1.0-0.9Ga intervals can be
correlated with the reactivation of the Mesoproterozoic structures and might be correlated to the
Macaubas rift.
5.2 INTRODUCTION
The 40Ar/39Ar technique has been widely used with success for dating weathering,
sedimentary, metamorphic and tectonic processes. It has been used to infer the thermal histories of
many periods of the Earth’s early evolution (e.g. evolution of the Pilbara, São Francisco and Amazon
Cratons).
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However, there is no widely known information on how the 40Ar/39Ar system behaved under
high-grade metamorphic conditions at the basement of the southern São Francisco Craton (Fig. 5.1a).
Figura 5.1 Panel (a) -Geological map of the southern São Francisco Craton (modified by Campos Sales 2004). Key: 1- Neoproterozoic undivided cratonic cover; 2- Mesoproterozoic Espinhaço Supergroup; 3- Mesoproterozoic(?) São João Del Rei/Andrelândia Groups; 4-- Mesoproterozoic (?), Dom Silvério Group; 5- Paleoproterozoic granitoids; 6- Paleoproterozoic indiscriminate greenstone-type sequences; 7- Paleoproterozoic Minas Supergroup; 8- Neoarchean granitoids; 9- Neoarchean and Mesoproterozoic gabbroic and dioritic rocks (sills and dikes); 10- Neoarchean ultramafic plutonic suite; 11- Neoarchean Rio das Velhas Supergroup; 12- Archean metamorphic complexes partially reworked on Proterozoic time; 13- Faults and fractures (CSZ = 82
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83
Cláudio Shear Zone; JBSZ = Jeceaba-Bom Sucesso Shear Zone); 14- Fold axes; 15- Lithologic contacts; 16- Cities. Panel (b) Geological map of the study area (modified from Oliveira 1999, Oliveira & Carneiro 2001) showing the following features: 1) Fissure Mafic Unit; 2) Supracrustal Unit; 3) Candeias Gneissic Unit; 4) Itapecerica Gneissic Unit; 5) Cláudio Gneissic Unit; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Key Outcrops: A – Kinawa dimension stone quarry; B - Corumbá dimension stone quarry; C – Amphibolite outcrop, D – Noritic Gabbro outcrop and, E e F – Gabbros outcrops.
Until now, the foremost aim of geochronological research has been to settle the timing of the
crust formation in the Campo Belo Metamorphic Complex (CBC), with limited emphasis on its
tectonometamorphic evolution. Competing models for the evolution of the southern São Francisco
Craton have been reported along the years, to build a tectonic evolution of the Archaean basement
(Teixeira 1985, 1993, Machado et al. 1992, Teixeira & Silva 1993, Teixeira et al. 1996, 1998, Pinese
1997, Carneiro et al. 1998a, b). Most of the works mainly draw attention on U-Pb, Rb-Sr, Sm-Nd and
K-Ar techniques. The U-Pb method has been more widely used and yields more accurate information,
whereas the 40Ar/39Ar has not been used with the same frequency and the results, up to date, have not
been as precise. The Campo Belo Metamorphic Complex (southern São Francisco Craton, Fig 1) has
been the subject of much debate, primarily when the migmatization and the deformational event(s),
and the mafic swarm dykes that emplaced the region in large scale are concerned. In order to consider
and investigate the evolution and the potential problems of the CBC, we are now going to deal with
the 40Ar/39Ar results for minerals (biotites and amphiboles) from gneisses, amphibolites and
gabbros/noritic gabbros. In particular, the aim of this study is to understand the timing of some of the
latest tectonothermal event(s) that occurred in the CBC (e.g. intrusion of NW swarm mafic dikes, Fig
1b).
5.3 PROBLEMS TO BE ADDRESSED
The southern part of the São Francisco Craton has been extensively studied, raising many
questions that result in a wide range of geochronological data (Teixeira 1985, 1993, Machado et al.
1992, Teixeira & Silva 1993, Teixeira et al. 1996, 1998, Pinese et al. 1995, Carneiro et al. 1998a, b).
Nonetheless, many questions remain unanswered. Based on field relationship, petrologic and structural
analyses, Oliveira & Carneiro (2001) recognized three gneissic units (Cláudio, Candeias and
Itapecerica), one supracrustal unit (ultramafics, mafics, garnet-sillimanite-schist, garnet-sillimanite-
quartzite and banded iron formation), and one fissural mafic unit (gabbronorite dykes) in the CBC
(Fig. 1). According to these authors, the gneissic units are strongly deformed (including
migmatization) and metamorphosed up to the granulite facies. Amphibolite boudins are found within
the three gneissic units. The supracrustal unit is strongly deformed and metamorphosed at least up to
the amphibolite facies. The fissural mafic unit is not deformed and not metamorphosed. The dykes
crosscut the gneisses and supracrustal units.
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Based on this information and on some of the geochronological data available for the CBC,
Oliveira & Carneiro (2001) defined five tectonic events for the studied area, the most recent being
related to the positioning of some of the undeformed gabbronorite dykes (ca. 2,658Ma, Pinese 1997).
Subsequently, Oliveira & Carneiro (2001) defined the minimum age for the deformational event that
strongly affected the CBC.
However, there are some tricky situations when assessments are made using some of the
existing data (e.g. Pinese 1997), our field relationships and our new data, which will be shown in this
study. The Sm-Nd isochron age for gabbronorite dykes (ca. 2,658 Ma, Pinese 1997) was defined as the
time of the emplacement of this rock. This information together with other published data, our new 40Ar/39Ar data and field observations will be used to better understand the CBC evolution and to
answer the following questions:
• Is ~2,658 Ma the age of the swarm dyke emplacement?; Are the high-grade
metamorphic and/or deformational event(s) older than 2,658 Ma?; Is there more than one
migmatization event?; Which is the Transamazonian influence in the Archaean
basement of the São Francisco Craton?
Hence, our aim is to better evaluate the relation between our data and the existing data.
5.4 GEOLOGICAL SETTING
The Campo Belo Metamorphic Complex (CBC) records a polyphase crustal history ranging
from Mesoarchaean to Neoarchaean (Teixeira 1985, 1993, Machado et al. 1992, Teixeira & Silva
1993, Teixeira et al. 1996, 1998, Pinese 1997, Carneiro et al. 1998 a, b). The geochronological results
indicate that the evolution of this terrain was most intensive during the Rio das Velhas Orogeny at
2.78-2.7 Ga (Machado & Carneiro 1992). The only evidence for the Transamazonian Orogeny (2.16-
2.0 Ga) in the CBC are the K-Ar and 40Ar/39Ar data in the 2.0-1.7 Ga interval obtained for biotites and
amphiboles from some of the gneisses (Teixeira & Figueiredo 1991, Teixeira et al. 1996).
However, Archaean rocks from the nearby Quadrilátero Ferrífero show ample evidence of
Transamazonian deformation and metamorphism at 2.065-2.035 Ga (e.g. Machado et al. 1992, Alkmin
& Marshak 1998). The close proximity, lithological similarities, and possible analogous tectonic
histories of the two areas suggest that the CBC should also record stronger evidences for
Transamazonian processes.
We will present 40Ar/39Ar step-heating analyses for biotites and amphiboles from three
different units of the CBC: gneisses and amphibolites from the Cláudio Unit (Fig 1b - outcrop A and
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B); amphibolites from the Supracrustal Unit (Fig 1b - outcrop C), and gabbros and a gabbronorite
from the Fissural Mafic Unit (Fig 1b-D, E and F).
5.5. SAMPLE SELECTION AND ANALYTICAL PROCEDURES
Samples were selected from the Cláudio Unit (2 gneisses and 1 amphibolite, from Kinawa and
Corumbá stone quarry), from the Supracrustal Unit (1 amphibolite), and from the Fissural Mafic Unit
(2 gabbros and 1 noritic gabbro). From each unit, fresh samples were collected to avoid weathering
interferences.
The samples were analyzed by the laser incremental heating 40Ar/39Ar method at the Argon
Geochronology in Earth Science Laboratory (UQ-AGES) of the University of Queensland.
Each rock sample weighed 10-15 kg. From each sample 100 grams of fragments containing
amphiboles and biotites were crushed, sieved to 0.2-2mm, and washed in absolute ethanol in an
ultrasonic bath. Approximately 10 single crystals of biotite and amphibole from each sample were
hand-picked under a binocular microscope. The minerals were loaded into Al-irradiation disks, as
illustrated in Vasconcelos et al. (2002), and irradiated, together with Fish Canyon sanidine neutron
fluence monitors, for 14 hours at the CLIC-Facility at the Oregon State University Triga reactor. After
a two-month cooling period, the samples were loaded into a copper disk, placed in an ultra-high
vacuum extraction line, and incrementally heated by an Ar ion laser beam. Each fraction of gas
released was cleaned by a cold trap operated at –140ºC, two C-40 SAES getter pumps, and analyzed
by a MAP-215-50 mass spectrometer. Details of the analytical procedures and correction factors are
presented in Vasconcelos (1999a, b) and Vasconcelos et al. (2002).
5.6 RESULTS
5.6.1 Petrograph Features
The gneisses that were chosen are dark gray (probably the protoliths) and gray (locally pink)
gneisses. The rocks are fine- to medium-grained, and strongly migmatized. The dark gray gneiss is
predominantly composed of plagioclase followed by quartz, microcline, biotite, amphibole (rare),
zircon, apatite, titanite (not common), opaques, white mica, chlorite, and epidote. Some samples show
antiperthitic intergrowths.
The gray gneiss is constituted by plagioclase and microcline, followed by quartz, biotite,
apatite, zircon, opaques and epidote. The main difference is that the dark gneiss has some amphibole
in its composition and is most likely the protolith. Both gneisses have granodioritic composition.
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86
The amphibolite from the gneissic unit is fine-grained, melanocratic, dense, phaneritic, black
and strongly foliated. In most places, the amphibolites are found in the form of disrupted thin boudins,
representing disrupted and metamorphosed dykes emplaced in the gneissic units. Green hornblende,
sometimes altered to biotite, is the main constituent. Locally, biotite schist (or biotitite) results from
retrograde metamorphism and shearing. Plagioclase is the second constituent, followed by quartz and
sometimes chloritized biotite. Pale yellow clinopyroxene, present in some rocks, can reach 10% of the
volume and alters to amphibole. Accessory minerals are zircon, opaque minerals and apatite.
The amphibolite from the Supracrustal Unit is a fine- to medium-grained rock that is
melanocratic, dense, phaneritic, black and strongly foliated. In general, the amphibolite is found in
contact with ultramafic rocks, mainly in the surroundings of the Cláudio town (Fig. 5.1). Green
hornblende, sometimes altered to biotite and chlorite, is the main constituent, followed by plagioclase
and subordinated quartz (< 10%). The main accessories are zircon, opaque minerals and apatite.
The noritic gabbro and the gabbro are generally medium- and coarse-grained, possibly
representing variations of crustal emplacement during magmatic crystallization. These rocks are in
general dark-colored to greenish, and present massive structure and phaneritic texture. When very
fine-grained, they can be aphanitic.
The gabbronorite shows ophitic, subophitic to intergranular textures, with subhedral to
euhedral crystals, and is medium- to fine-grained. Plagioclase is the most abundant mineral followed
by clino- and orthopyroxene (augite and hyperstene). Hyperstene is light pink, pleochroic, medium- to
fine-grained. Hornblende with green pleochroism is the alteration product of pyroxenes (uralitization).
Quartz appears in small quantities as clean, anhedral, fine-grained crystals. Zircon and opaque
minerals are accessories. White mica, epidote, carbonate and chlorite are secondary minerals and
product of saussuritization.
The gabbro presents ophitic, subophitic or intergranular textures. Plagioclase is the most
abundant mineral. Augite is the second component in volume and hornblende is its uralitization
product. When present, microcline is rare, showing crosshatch twinning and is associated with
myrmekites. Quartz appears in small quantities. Biotite is also rare, reddish and pleochroic. Zircon,
opaque minerals and apatite are accessories. White mica, epidote and chlorite are secondary minerals
observed in some of the rocks.
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Run ID# Sample 40Ar/39A
5.6.2 40Ar/39Ar Sytem
The results shown in this study correspond to biotites and amphiboles from migmatites and
amphibolite boudins within the Gneissic Unit, amphibolite from Supracrustal unit and gabbros and
noritic gabbro from the Fissural Mafic Unit. 40Ar/39Ar isotopic data for the three different CBC units
are shown in Table 1 in separated groups.
From the first group, three biotite single crystals from the Cláudio Unit gneisses were
analyzed (samples OPU 1436, Runs 2709-01/02/03, Fig 5.2 and OPU 1197, Runs 2708-01/02/03, Fig
5.3, Table 1).
Table 5.1 – Ar-Ar results for amphibolites and gabbros from the Campo Belo Metamorphic Complex.
r 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2180-01A OPU-1449 1747.838 3.413317 0.6653571 1555.228 88.8 3347.519 55.47862 0.003469 1.092725 9.46E-152180-01B OPU-1449 702.3066 5.344283 0.455155 570.3704 80.9 1969.075 32.50222 0.003469 0.8074775 7.00E-152180-01C OPU-1449 373.2233 4.913256 7.87E-02 351.554 93.9 1438.391 13.70003 0.003469 0.8779572 7.61E-152180-01D OPU-1449 387.5748 8.594833 2.28E-02 383.823 98.4 1527.159 7.685193 0.003469 1.841213 1.60E-142180-01E OPU-1449 396.5858 10.97339 3.68E-02 389.5734 97.5 1542.529 15.07735 0.003469 0.7594066 6.57E-152180-01F OPU-1449 457.3651 13.8996 3.23E-02 453.3518 98.2 1704.758 37.30393 0.003469 0.2634937 2.28E-152180-01G OPU-1449 485.8857 14.99253 5.75E-02 475.0664 96.7 1756.822 35.72347 0.003469 0.3027964 2.61E-152180-01H OPU-1449 561.7035 23.16973 3.58E-02 562.1008 98.4 1951.616 20.1375 0.003469 0.8053211 6.99E-152180-01I OPU-1449 629.0363 31.49738 0.1028571 614.7294 95.6 2059.95 52.7584 0.003469 0.2943828 2.54E-152180-01J OPU-1449 716.7209 61.16437 2.20E-03 753.2787 100.6 2317.468 31.40362 0.003469 0.5677816 4.91E-152180-01K OPU-1449 444.5306 55.87882 0.3020013 374.3751 80.9 1501.618 101.1494 0.003469 7.97E-02 6.95E-162180-01L OPU-1449 491.0943 23.63671 -0.1996837 561.2839 112.4 1949.882 71.70731 0.003469 0.1121377 9.77E-162180-02A OPU-1449 7802.523 55.62703 5.171618 6534.552 80.5 5708.352 658.4087 0.003469 0.3960506 3.42E-152180-02B OPU-1449 1377.848 0 0.99901 1082.64 78.6 2813.168 83.48084 0.003469 0.5656868 4.90E-152180-02C OPU-1449 477.4907 9.571774 5.71E-02 464.5012 96.6 1731.678 31.71706 0.003469 0.3450225 2.99E-152180-02D OPU-1449 478.6707 7.912679 4.68E-03 480.5828 99.8 1769.813 9.330807 0.003469 2.334085 2.03E-142180-02E OPU-1449 459.8378 6.366028 -2.72E-03 463.2119 100.3 1728.585 12.66193 0.003469 1.2817 1.11E-142180-02F OPU-1449 448.1851 16.35887 1.57E-02 449.9899 99.2 1696.561 38.2117 0.003469 0.2330477 2.00E-152180-02G OPU-1449 486.0663 12.54496 -2.57E-02 499.0556 101.8 1812.646 27.84972 0.003469 0.3732559 3.21E-152180-02H OPU-1449 601.5396 21.75173 -2.52E-02 620.1705 101.5 2070.788 31.96062 0.003469 0.4945986 4.27E-152180-02I OPU-1449 590.0297 27.88138 -7.86E-02 627.7667 104.3 2085.811 40.44253 0.003469 0.3171633 2.75E-152180-02J OPU-1449 631.8542 40.89171 3.34E-02 643.7113 98.9 2116.942 51.12788 0.003469 0.267714 2.32E-152180-02K OPU-1449 511.8784 46.31487 4.15E-02 520.2003 98.3 1860.458 66.19894 0.003469 0.1452477 1.26E-152180-02L OPU-1449 751.577 58.4231 -0.1631197 838.837 107 2459.892 123.7599 0.003469 0.1012284 8.81E-162180-03A OPU-1449 817.0952 35.17219 7.68E-02 817.3889 97.6 2425.231 401.0683 0.003469 3.66E-02 3.19E-162180-03B OPU-1449 981.1253 0 0.2590971 904.5613 92.2 2562.14 116.0211 0.003469 0.1949464 1.68E-152180-03C OPU-1449 476.7265 20.79379 0.0447432 472.0363 97.6 1749.647 38.50641 0.003469 0.2668675 2.31E-152180-03D OPU-1449 413.1431 11.27289 -5.31E-02 433.1374 104 1654.901 35.84023 0.003469 0.2210207 1.92E-152180-03E OPU-1449 436.9986 31.71816 4.54E-02 435.7823 97.5 1661.503 84.59877 0.003469 9.56E-02 8.38E-162180-03F OPU-1449 440.6527 40.15272 0.1593132 408.2474 90 1591.557 91.05933 0.003469 0.0792954 6.88E-162180-03G OPU-1449 789.0109 77.11564 0.1566265 791.7252 94.9 2382.862 130.8885 0.003469 0.1191047 1.04E-152180-03H OPU-1449 1280.738 135.3115 0.3312702 1319.055 93.2 3100.191 162.9218 0.003469 0.1770943 1.53E-152180-03I OPU-1449 763.0077 157.5188 0.3688003 749.4204 87.3 2310.773 184.138 0.003469 0.0747308 6.48E-162180-03J OPU-1449 349.509 162.737 0.9341264 97.31874 24.7 524.7626 485.6647 0.003469 2.05E-02 1.69E-162180-03K OPU-1449 815.3812 139.0875 0.4506273 768.3007 85 2343.302 231.2616 0.003469 5.90E-02 5.13E-162180-03L OPU-1449 727.5408 189.7613 0.5338377 674.7336 80.4 2176.016 289.3983 0.003469 4.20E-02 3.59E-16
Oliveira, 2004 Evolução tectônica de um.........
Continued
88
Run ID# Sample 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2181-01A OPU-1449 106.8412 0.9271115 3.64E-02 96.22137 90 519.6211 13.38902 0.003469 0.1751133 1.51E-152181-01B OPU-1449 264.7779 5.65E-02 2.87E-03 263.9453 99.7 1172.73 6.067581 0.003469 1.721638 1.49E-142181-01C OPU-1449 325.8661 0.443608 -2.99E-03 326.8855 100.3 1367.458 4.265551 0.003469 4.376956 3.79E-142181-01D OPU-1449 356.5383 0.2834306 -2.37E-03 357.3308 100.2 1454.607 5.788318 0.003469 4.249619 3.68E-142181-01E OPU-1449 350.8351 0.6484853 -1.67E-04 351.0947 100 1437.096 10.22687 0.003469 1.195985 1.04E-142181-01F OPU-1449 351.3958 9.65E-02 -2.48E-02 358.7486 102.1 1458.564 17.66338 0.003469 0.4859516 4.20E-152181-01G OPU-1449 377.8242 0 -3.05E-02 386.8334 102.4 1535.221 17.13318 0.003469 0.5280277 4.57E-152181-01H OPU-1449 309.8572 2.641925 -3.40E-02 320.6952 103.3 1349.212 39.65302 0.003469 0.1532382 1.34E-152181-01I OPU-1449 334.403 0.7657065 3.38E-02 324.6574 97 1360.912 20.93505 0.003469 0.3991233 3.44E-152181-01J OPU-1449 332.6162 0.4214739 -2.30E-02 339.5462 102.1 1404.212 19.16739 0.003469 0.4168005 3.61E-152181-01K OPU-1449 238.9554 0 -9.70E-04 239.2412 100.1 1090.162 9.766659 0.003469 0.6134046 5.31E-152181-01L OPU-1449 177.0969 0 -3.35E-02 186.9942 105.6 901.9965 20.79065 0.003469 0.1610508 1.40E-152181-02A OPU-1449 338.3415 0 4.82E-02 324.1108 95.8 1359.302 39.63087 0.003469 0.17568 1.52E-152181-02B OPU-1449 424.1656 0.1169202 -3.84E-03 425.3433 100.3 1635.303 7.017803 0.003469 2.965867 2.57E-142181-02C OPU-1449 406.7154 0.4190923 -4.23E-03 408.1188 100.3 1591.224 5.565933 0.003469 5.313475 4.60E-142181-02D OPU-1449 419.9988 0 -4.81E-03 421.4207 100.3 1625.36 6.862374 0.003469 3.653772 3.17E-142181-02E OPU-1449 397.5142 0 5.63E-03 395.8503 99.6 1559.158 19.14188 0.003469 0.5175781 4.48E-152181-02F OPU-1449 342.4591 22.44436 0.1823479 294.9865 84.8 1271.394 79.8735 0.003469 8.53E-02 7.41E-162181-02G OPU-1449 420.0019 0 -7.48E-02 442.1093 105.3 1677.199 46.29022 0.003469 0.1928405 1.66E-152181-02H OPU-1449 482.1943 0 -7.56E-02 504.5345 104.6 1825.157 32.50879 0.003469 0.3237331 2.80E-152181-02I OPU-1449 453.0268 0 -8.26E-03 455.4663 100.5 1709.894 37.55124 0.003469 0.264877 2.28E-152181-02J OPU-1449 454.5916 1.721516 -0.0169764 460.2995 101.1 1721.58 31.73915 0.003469 0.336589 2.91E-152181-02K OPU-1449 420.123 0 -3.43E-02 430.2443 102.4 1647.651 141.0108 0.003469 4.78E-02 4.09E-162181-02L OPU-1449 423.0378 0 -0.4817237 565.3863 133.6 1958.573 472.1283 0.003469 1.24E-02 9.78E-172181-02181-02181-02181-02181-02181-02181-02181-02181-02181-02181-02181-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-02183-03E OPU-1448 333.837 4.453163 -0.4843536 478.8112 143 1765.651 167.8844 0.003469 2.46E-02 2.14E-162183-03F OPU-1448 229.0444 9.999579 -1.25E-02 235.1764 102 1076.206 37.62711 0.003469 9.24E-02 8.05E-162183-03G OPU-1448 274.7886 0 -3.78E-02 285.9463 104.1 1243.213 257.8928 0.003469 0.0143556 1.22E-162183-03H OPU-1448 360.1054 40.43517 -0.5230469 532.9917 143.8 1888.778 211.9883 0.003469 2.13E-02 1.84E-162183-03I OPU-1448 322.2749 87.87887 0.1704026 297.1601 86.5 1278.105 283.7079 0.003469 1.83E-02 1.57E-162183-03J OPU-1448 333.9741 203.4607 0.1153953 368.5338 94.5 1485.644 326.4127 0.003469 0.01682 1.42E-162183-03K OPU-1448 412.8615 160.2661 0.2019222 412.2032 88.6 1601.774 232.0209 0.003469 3.09E-02 2.67E-162183-03L OPU-1448 694.9224 352.0703 -0.4346609 1131.208 122.4 2875.976 372.0831 0.003469 2.87E-02 2.48E-16
3A OPU-1449 212.9778 0 0.2628963 135.2912 63.5 694.2147 11.64202 0.003469 0.578805 5.03E-153B OPU-1449 313.628 0.2834591 2.07E-02 307.6077 98.1 1310.017 6.976649 0.003469 1.675673 1.45E-143C OPU-1449 339.3865 0.3168235 4.01E-03 338.3012 99.7 1400.63 6.27521 0.003469 3.475123 3.00E-143D OPU-1449 321.9626 3.25E-02 -3.16E-03 322.9057 100.3 1355.749 7.096637 0.003469 2.986041 2.59E-143E OPU-1449 296.5725 0.4888675 1.19E-02 293.182 98.8 1265.804 8.317242 0.003469 1.484219 1.28E-143F OPU-1449 289.6907 0 2.32E-03 289.0029 99.8 1252.791 16.73579 0.003469 0.4669849 4.02E-153G OPU-1449 374.0562 0 -2.69E-02 382.0166 102.1 1522.303 48.07734 0.003469 0.1625992 1.40E-153H OPU-1449 254.1665 2.185524 0.1484874 210.7825 82.8 990.1091 73.78488 0.003469 6.43E-02 5.56E-163I OPU-1449 366.5403 1.331553 2.91E-03 366.1273 99.8 1479.022 18.18604 0.003469 0.492826 4.26E-153J OPU-1449 353.2706 0 -1.88E-02 358.8209 101.6 1458.766 25.97333 0.003469 0.3139652 2.72E-153K OPU-1449 344.9902 0 -0.0236825 351.9876 102 1439.614 26.10149 0.003469 0.2845822 2.45E-153L OPU-1449 237.2128 0.4619912 3.26E-02 227.686 96 1050.203 179.2703 0.003469 2.19E-02 1.81E-161A OPU-1448 -14989.3 0 3.481819 -16018.2 106.9 1.00E-20 2797.274 0.003469 0.1315681 1.14E-151B OPU-1448 2591.687 6.326802 0.1119839 2570.537 98.7 4139.04 250.7975 0.003469 0.2866706 2.49E-151C OPU-1448 432.8749 2.146638 4.36E-02 420.7805 97.1 1623.731 62.0424 0.003469 0.1386549 1.20E-151D OPU-1448 317.5836 6.345264 -0.012582 323.2397 101.3 1356.734 58.6752 0.003469 0.1021599 8.94E-161E OPU-1448 201.0267 1.254422 -5.39E-02 217.239 108 1013.3 30.35419 0.003469 0.1174413 1.02E-151F OPU-1448 200.1956 6.936418 0.0416263 189.36 94.1 910.9547 21.40816 0.003469 0.1909983 1.65E-151G OPU-1448 412.4518 16.30751 5.82E-02 401.1179 96.1 1572.996 79.58252 0.003469 9.26E-02 8.13E-161H OPU-1448 456.693 18.86632 -9.60E-02 493.0852 106.5 1798.913 177.345 0.003469 4.05E-02 3.52E-161I OPU-1448 693.4927 13.71642 -0.1303426 740.2261 105.7 2294.717 165.0871 0.003469 7.02E-02 6.13E-161J OPU-1448 598.0215 0 0.3879206 483.3902 80.8 1776.389 426.0099 0.003469 2.77E-02 2.40E-161K OPU-1448 1354.688 19.56403 -5.30E-02 1391.048 101.3 3179.24 163.6344 0.003469 0.1940719 1.68E-151L OPU-1448 1067.166 0.2260272 0.2204755 1002.192 93.9 2704.067 133.4107 0.003469 0.1727848 1.50E-152A OPU-1448 501.9196 13.24415 0.4696964 367.5841 72.6 1483.034 436.8021 0.003469 2.46E-02 2.14E-162B OPU-1448 1237.954 0 0.9406496 959.9914 77.5 2644.084 684.3466 0.003469 4.86E-02 4.24E-162C OPU-1448 635.9517 0 -1.257028 1007.403 158.4 2711.337 414.5803 0.003469 2.18E-02 1.90E-162D OPU-1448 223.7789 0 -0.5544353 387.6138 173.2 1537.306 192.8202 0.003469 1.49E-02 1.30E-162E OPU-1448 223.0071 17.9459 -8.36E-02 252.3011 111.7 1134.282 68.39537 0.003469 4.86E-02 4.22E-162F OPU-1448 203.5284 5.657342 -3.60E-03 205.8535 100.7 972.2019 13.83351 0.003469 0.2903119 2.50E-152G OPU-1448 194.8299 0.020586 -0.0905931 221.6041 113.7 1028.811 25.56305 0.003469 0.1261844 1.10E-152H OPU-1448 244.0153 23.0683 -4.46E-02 263.2652 106.1 1170.507 50.01251 0.003469 7.08E-02 6.11E-162I OPU-1448 274.392 39.69995 1.86E-02 279.819 99.1 1223.858 177.5125 0.003469 2.38E-02 1.98E-162J OPU-1448 242.3187 0 -0.3746994 353.0415 145.7 1442.581 256.5719 0.003469 1.22E-02 9.40E-172K OPU-1448 202.2819 12.89667 0.2181246 140.1034 68.6 714.5959 134.2296 0.003469 2.76E-02 2.30E-162L OPU-1448 241.6169 15.80829 -0.2603228 323.3736 132.3 1357.129 94.74907 0.003469 3.36E-02 2.83E-163A OPU-1448 1051.271 0 -1.822158 1589.718 151.2 3380.959 357.3832 0.003469 3.98E-02 3.47E-163B OPU-1448 2126.553 0 -1.976382 2710.573 127.5 4225.314 424.8829 0.003469 0.0739194 6.48E-163C OPU-1448 687.4936 0 -2.053751 1294.376 188.3 3072.276 387.3412 0.003469 2.26E-02 1.98E-163D OPU-1448 1314.005 0 -5.115503 2825.635 215 4293.24 1505.941 0.003469 1.07E-02 9.25E-17
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
continued Run ID# Sample 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2184-01A OPU-1446 817.942 0 0.1117804 784.9101 96 2371.442 214.2008 0.003469 0.061539 52184-01B OPU-1446 1593.002 0 1.043347 1284.692 80.6 3061.204 369.3299 0.003469 9.61E-02 82184-01C OPU-1446 1780.376 47.09152 0.6985208 1631.732 88.6 3420.875 318.2798 0.003469 0.1227217 12184-01D OPU-1446 815.1066 89.11835 0.301074 782.1631 89.9 2366.818 524.2797 0.003469 2.67E-02 22184-01E OPU-1446 295.6558 56.66198 0.3580562 202.3411 65.7 959.332 231.6686 0.003469 2.32E-02 22184-01F OPU-1446 232.7525 6.166049 2.39E-03 233.5391 99.9 1070.554 16.46174 0.003469 0.3093892 22184-01G OPU-1446 195.7881 8.115561 6.23E-03 195.6989 99.4 934.7404 7.060019 0.003469 0.7245493 62184-01H OPU-1446 216.0508 8.948283 -9.25E-02 245.6265 113 1111.868 28.07577 0.003469 0.142517 12184-01I OPU-1446 220.8952 44.23801 0.1810237 176.3449 77.3 861.1125 48.20153 0.003469 0.0817516 72184-01J OPU-1446 249.346 109.8593 0.3065722 181.3491 67.1 880.4397 98.7686 0.003469 4.76E-02 42184-01K OPU-1446 390.5477 226.8535 -2.84E-02 495.8109 106.7 1805.196 67.12064 0.003469 0.1033536 82184-01L OPU-1446 974.2678 747.1708 0.4067008 1924.876 93.6 3677.214 351.847 0.003469 5.25E-02 42184-02A OPU-1446 1936.25 0 -5.42E-02 1952.276 100.8 3699.413 211.4438 0.003469 0.1977057 12184-02B OPU-1446 2140.737 0 0.1665368 2091.524 97.7 3808.189 392.7798 0.003469 0.1304559 12184-02C OPU-1446 950.1853 54.08004 -0.2099347 1056.669 107 2778.663 234.2353 0.003469 7.27E-02 62184-02D OPU-1446 283.7227 24.32528 0.3277985 192.0439 66.5 921.0638 148.8412 0.003469 3.62E-02 32184-02E OPU-1446 241.303 0 -3.01E-03 242.1907 100.4 1100.22 40.37247 0.003469 0.110881 92184-02F OPU-1446 200.4554 6.175158 1.96E-03 201.2307 100 955.2444 5.428173 0.003469 0.9983303 82184-02G OPU-1446 194.83 7.261765 -7.13E-03 198.5174 101.4 945.2166 7.067428 0.003469 0.7988963 62184-02H OPU-1446 180.2689 17.77555 0.0392063 172.2245 94.3 845.0417 39.47394 0.003469 7.51E-02 62184-02I OPU-1446 198.907 12.95873 -1.71E-02 206.858 103 975.866 18.73667 0.003469 0.2169482 12184-02J OPU-1446 184.0721 53.44958 -1.08E-02 198.9082 104 946.6642 31.98316 0.003469 8.97E-02 72184-02K OPU-1446 195.3922 209.7168 -6.93E-03 250.8326 109.4 1129.375 101.4957 0.003469 0.0321899 22184-02L OPU-1446 140.3121 726.8198 0.4212003 148.5654 51.7 749.8862 438.6124 0.003469 1.13E-02 92184-03A OPU-1446 573.8021 18.30331 -6.44E-02 602.0048 103.6 2034.347 126.4224 0.003469 7.10E-02 62184-03B OPU-1446 439.2595 0 6.96E-02 418.6917 95.3 1618.409 65.17932 0.003469 0.1343139 12184-03C OPU-1446 214.8636 6.460583 0.1801415 162.8758 75.5 808.0394 44.20745 0.003469 0.1093635 92184-03D OPU-1446 150.8062 10.00895 -2.55E-02 160.2518 105.5 797.5157 39.67867 0.003469 6.17E-02 52184-03E OPU-1446 149.9444 10.77722 7.92E-04 151.7013 100.4 762.791 32.70909 0.003469 7.84E-02 62184-03F OPU-1446 181.8389 5.857783 7.01E-03 180.9705 99.1 878.9846 8.091221 0.003469 0.5593799 42184-03G OPU-1446 181.6922 5.317775 2.62E-02 175.0287 96 855.9942 7.745055 0.003469 0.6295535 52184-03H OPU-1446 166.2782 2.092875 2.44E-02 159.4601 95.8 794.3284 28.16837 0.003469 0.1153242 12184-03I OPU-1446 128.2674 0.8417313 1.96E-03 127.8279 99.6 662.1426 6.30289 0.003469 0.4785461 42184-03J OPU-1446 138.9073 3.30032 -1.73E-02 144.6089 103.9 733.4719 17.29961 0.003469 0.1499647 12184-03K OPU-1446 140.3522 1.787206 -9.10E-03 143.3597 102 728.2581 15.06858 0.003469 0.1777856 12184-03L OPU-1446 248.6851 49.68697 2.47E-02 254.1668 98.6 1140.498 277.9777 0.003469 1.61E-02 12194-01A OPU-1201 281.1536 0 9.59E-02 252.8265 89.9 1136.035 41.15543 0.003469 0.1270595 12194-01B OPU-1201 534.9059 9.62E-02 1.21E-02 531.3865 99.3 1885.249 7.454326 0.003469 3.114593 22194-01C OPU-1201 560.8096 0.2179675 -4.15E-03 562.1383 100.2 1951.695 6.810146 0.003469 6.170714 52194-01D OPU-1201 572.2424 0.100056 -4.77E-03 573.6996 100.2 1976.057 6.893978 0.003469 8.650077 72194-01E OPU-1201 567.0492 0.3675642 -3.25E-03 568.1837 100.2 1964.475 5.511804 0.003469 9.663971 82194-01F OPU-1201 578.8595 6.85E-02 -9.92E-04 579.1852 100.1 1987.502 4.324224 0.003469 31.87206 22194-01G OPU-1201 536.6616 0.796846 1.21E-02 533.4534 99.3 1889.792 12.87829 0.003469 2.100658 12194-01H OPU-1201 574.3566 0.3754547 -3.63E-03 575.6092 100.2 1980.049 5.499069 0.003469 11.34757 92194-01I OPU-1201 568.4706 0.2751079 -8.82E-03 571.2073 100.5 1970.833 8.059273 0.003469 4.279093 32194-01J OPU-1201 588.0862 1.424331 -1.42E-03 589.2057 100.1 2008.222 16.61049 0.003469 1.361636 12194-01K OPU-1201 529.1992 0 0.1325582 490.0274 92.6 1791.839 77.56839 0.003469 0.2121937 12194-01L OPU-1201 504.768 13.83029 0.1332461 471.0584 92.4 1747.325 120.9061 0.003469 0.1014526 82194-02A OPU-1201 343.3807 0 0.3869409 229.0388 66.7 1054.927 42.19444 0.003469 0.2334845 22194-02B OPU-1201 565.1915 0.1869084 2.55E-03 564.5262 99.9 1956.754 6.600706 0.003469 5.082433 42194-02C OPU-1201 572.8978 0.4233072 -6.03E-03 574.8829 100.3 1978.531 7.102812 0.003469 7.737461 62194-02D OPU-1201 576.4192 0.2775048 -4.45E-03 577.8671 100.2 1984.758 6.113786 0.003469 8.693728 72194-02E OPU-1201 575.4453 0.1319339 -4.46E-03 576.825 100.2 1982.586 5.028979 0.003469 9.178036 72194-02F OPU-1201 572.7666 9.04E-02 -4.40E-03 574.1089 100.2 1976.913 6.847305 0.003469 8.359644 72194-02G OPU-1201 578.7531 0.354073 -3.30E-03 579.8992 100.2 1988.985 5.458435 0.003469 12.04834 12194-02H OPU-1201 584.0718 9.09E-02 -1.95E-03 584.6927 100.1 1998.919 4.714739 0.003469 20.43789 12194-02I OPU-1201 586.6818 0.4402634 -4.63E-03 588.2648 100.2 2006.286 5.828993 0.003469 11.21447 92194-02J OPU-1201 585.1809 0.3802012 -1.06E-02 588.494 100.5 2006.758 9.691809 0.003469 4.049575 32194-02K OPU-1201 580.2636 0.3789895 -2.46E-02 587.7247 101.3 2005.175 19.39525 0.003469 1.181823 12194-02L OPU-1201 601.5041 0 -1.42E-02 605.7036 100.7 2041.827 20.931 0.003469 1.096475 92194-03A OPU-1201 196.8448 0 -7.29E-02 218.3739 110.9 1017.345 38.75059 0.003469 0.104178 92194-03B OPU-1201 550.3241 0.2464153 -4.48E-03 551.7612 100.2 1929.546 6.0049 0.003469 7.223352 62194-03C OPU-1201 567.0687 0 -6.03E-03 568.8507 100.3 1965.879 5.646704 0.003469 9.456384 82194-03D OPU-1201 571.4658 7.03E-02 -4.49E-03 572.8264 100.2 1974.228 5.592074 0.003469 11.37252 92194-03E OPU-1201 566.6289 0.2883143 -3.04E-03 567.6629 100.2 1963.378 5.33504 0.003469 11.30298 92194-03F OPU-1201 568.3456 0 -4.93E-03 569.8009 100.3 1967.878 6.140637 0.003469 10.02246 82194-03G OPU-1201 569.3312 7.52E-02 -4.16E-03 570.597 100.2 1969.551 4.447071 0.003469 16.70033 12194-03H OPU-1201 573.3402 0.2214342 -3.05E-03 574.3464 100.2 1977.41 5.13146 0.003469 18.72183 12194-03I OPU-1201 547.2694 0.6229975 1.83E-03 547.0178 99.9 1919.33 16.41572 0.003469 1.22425 12194-03J OPU-1201 587.5007 0 -7.65E-03 589.7593 100.4 2009.36 20.02703 0.003469 1.087149 92194-03K OPU-1201 604.2464 2.719483 -2.20E-02 612.1199 101.1 2054.729 49.53078 0.003469 0.3005671 22194-03L OPU-1201 631.7556 0 -0.1617863 679.5626 107.6 2185.04 78.74072 0.003469 0.1728907 1
.39E-16
.33E-16
.06E-15
.31E-16
.03E-16
.68E-15
.27E-15
.23E-15
.06E-16
.15E-16
.95E-16
.52E-16
.71E-15
.13E-15
.29E-16
.12E-16
.59E-16
.65E-15
.92E-15
.57E-16
.88E-15
.71E-16
.76E-16
.61E-17
.21E-16
.16E-15
.45E-16
.32E-16
.80E-16
.84E-15
.44E-15
.00E-15
.13E-15
.30E-15
.53E-15
.32E-16
.10E-15
.70E-14
.34E-14
.50E-14
.36E-14
.76E-13
.82E-14
.82E-14
.72E-14
.18E-14
.84E-15
.95E-16
.01E-15
.40E-14
.70E-14
.52E-14
.95E-14
.24E-14
.04E-13
.77E-13
.69E-14
.51E-14
.02E-14
.49E-15
.11E-16
.26E-14
.19E-14
.86E-14
.77E-14
.68E-14
.45E-13
.62E-13
.06E-14
.41E-15
.60E-15
.50E-15
89
Oliveira, 2004 Evolução tectônica de um.........
continued
90
Run ID# Sample 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 40*Ar/39Ar %Rad Age ± (Ma) J Ar40 (nA) Ar40 Moles2195-01A2195-01B2195-01C2195-01D2195-01E2195-01F2195-01G2195-01H2195-01I2195-01J2195-01K2195-01L2195-02A2195-02B2195-02C2195-02D2195-02E2195-02F2195-02G2195-02H2195-02I2195-02J2195-02K2195-02L2195-03A2195-03B2195-03C2195-03D2195-03E2195-03F2195-03G2195-03H2195-03I2195-03J2195-03K2195-03L2197-01A2197-01B2197-01C2197-01D2197-01E2197-01F2197-01G2197-01H2197-01I2197-01J2197-01K2197-01L2197-02A2197-02B2197-02C2197-02D2197-02E2197-02F2197-02G2197-02H2197-02I2197-02J2197-02K2197-02L2197-03A2197-03B2197-03C2197-03D2197-03E2197-03F2197-03G2197-03H2197-03I2197-03J OPU-1447 597.3459 0 -0.175215 649.1212 108.7 2127.384 128.9442 0.003469 7.14E-02 6.24E-162197-03K OPU-1447 596.6439 0 0.4018914 477.8842 80.1 1763.47 391.6108 0.003469 2.73E-02 2.34E-162197-03L OPU-1447 690.8331 10.24771 -0.1548909 742.7606 106.7 2299.157 63.43309 0.003469 0.2084706 1.80E-15
OPU-1201 -5328.76 725.993 19.23759 -22411.1 205.6 1.00E-20 5553.918 0.003469 1.91E-02 1.65E-16OPU-1201 43850.93 0 -55.02038 60109.45 137.1 9642.463 10037.95 0.003469 8.46E-02 7.28E-16OPU-1201 962.2963 2.425494 -0.8438889 1213.927 125.9 2978.158 214.6449 0.003469 8.61E-02 7.50E-16OPU-1201 569.3311 56.6344 -0.4703434 742.3322 125.2 2298.407 157.9089 0.003469 6.35E-02 5.50E-16OPU-1201 711.1115 37.10556 -0.2494512 808.8471 110.8 2411.239 125.2695 0.003469 0.12258 1.06E-15OPU-1201 623.163 4.223846 -1.74E-02 630.5072 100.9 2091.2 11.01023 0.003469 2.882039 2.49E-14OPU-1201 596.6716 4.715508 -4.10E-03 600.2427 100.3 2030.773 5.480608 0.003469 13.18604 1.14E-13OPU-1201 582.7369 4.76492 -6.34E-03 586.9515 100.4 2003.582 5.857551 0.003469 8.519933 7.36E-14OPU-1201 582.5627 5.518765 -1.76E-03 585.7874 100.2 2001.18 17.05419 0.003469 1.284807 1.11E-14OPU-1201 613.9022 6.052143 -1.23E-02 620.6393 100.7 2071.719 17.35033 0.003469 1.11639 9.68E-15OPU-1201 648.8312 8.297447 -7.98E-03 655.6652 100.5 2139.935 26.84655 0.003469 0.6630435 5.74E-15OPU-1201 623.4151 1.3654 -0.1208442 659.8646 105.7 2147.943 49.3133 0.003469 0.2859618 2.47E-15OPU-1201 -1923.32 58.0513 2.288506 -2705.62 134.9 1.00E-20 829.4608 0.003469 4.03E-02 3.51E-16OPU-1201 -11665.4 51.18307 9.417664 -14984.2 123.8 1.00E-20 3832.591 0.003469 0.0609974 5.30E-16OPU-1201 810.7805 24.46477 -1.037533 1138.895 138 2885.719 263.0095 0.003469 5.52E-02 4.79E-16OPU-1201 936.7759 33.40083 -0.2124923 1026.305 107 2737.467 200.3528 0.003469 0.1088833 9.42E-16OPU-1201 642.5646 5.842078 -9.72E-02 674.5142 104.5 2175.605 34.55242 0.003469 0.583611 5.06E-15OPU-1201 581.0839 4.575725 -1.27E-02 587.094 100.7 2003.875 9.630903 0.003469 5.479017 4.75E-14OPU-1201 583.0057 5.571941 -2.40E-04 585.8085 100.1 2001.224 13.34458 0.003469 1.650195 1.43E-14OPU-1201 664.1889 0 -0.3029191 753.7007 113.5 2318.199 134.1489 0.003469 0.112248 9.77E-16OPU-1201 576.9186 0 -1.27E-02 580.6563 100.6 1990.558 52.17223 0.003469 0.2669014 2.31E-15OPU-1201 703.1042 0 -6.98E-02 723.7422 102.9 2265.568 77.98853 0.003469 0.2338408 2.02E-15OPU-1201 726.3989 0 -0.1651388 775.1967 106.7 2355.039 63.3992 0.003469 0.2716582 2.35E-15OPU-1201 637.4862 0 -0.6917983 841.9118 132.1 2464.807 203.9507 0.003469 5.28E-02 4.55E-16OPU-1201 -2391.24 0 -5.976665 -625.138 26.1 1.00E-20 62646.13 0.003469 7.21E-03 6.06E-17OPU-1201 3753.459 0 -5.468123 5369.289 143 5370.482 2918.77 0.003469 2.01E-02 1.74E-16OPU-1201 1332.556 0 -1.579539 1799.309 135 3571.839 541.2145 0.003469 3.79E-02 3.27E-16OPU-1201 31949.27 1303.24 -73.33874 651033.2 168.1 13932.65 29342.96 0.003469 1.80E-02 1.57E-16OPU-1201 706.4421 0 -2.53E-04 706.5161 100 2234.595 201.1554 0.003469 0.0614913 5.32E-16OPU-1201 588.9595 4.931784 -1.91E-02 597.0615 101 2024.302 9.389779 0.003469 2.449361 2.12E-14OPU-1201 585.7548 4.480125 -2.11E-03 588.5847 100.2 2006.945 8.071585 0.003469 4.19276 3.62E-14OPU-1201 588.5042 5.372116 -2.83E-03 591.9984 100.2 2013.955 11.676 0.003469 2.308088 2.00E-14OPU-1201 587.9821 2.991231 0.0059618 587.6904 99.7 2005.104 65.11457 0.003469 0.1885169 1.63E-15OPU-1201 577.3757 0 -6.55E-02 596.7195 103.4 2023.605 99.41925 0.003469 0.114819 1.00E-15OPU-1201 608.6054 3.028832 -1.39E-02 614.2638 100.7 2059.02 24.61705 0.003469 0.6842729 5.91E-15OPU-1201 664.8311 3.083371 -6.27E-02 685.084 102.8 2195.303 63.15903 0.003469 0.2300299 1.98E-15OPU-1447 -159.007 0 0.1291249 -197.164 124 -2078.1 3680.576 0.003469 3.56E-03 2.53E-17OPU-1447 5082.311 214.1152 -2.426878 6848.475 114.4 5789.505 2122.276 0.003469 4.46E-02 3.92E-16OPU-1447 -3164.32 558.51 11.06965 -10533.5 202 1.00E-20 3741.209 0.003469 1.45E-02 1.27E-16OPU-1447 142452.1 0 -301.2719 231477.9 162.5 12068.55 251196 0.003469 9.95E-03 8.73E-17OPU-1447 -444.21 212.6103 4.472642 -2057.21 393.8 1.00E-20 1359.998 0.003469 7.65E-03 6.69E-17OPU-1447 -361.648 88.02013 2.357926 -1121.02 290.8 1.00E-20 861.9825 0.003469 1.11E-02 9.76E-17OPU-1447 758.6059 9.488237 -6.77E-02 784.5869 102.7 2370.899 72.71104 0.003469 0.2362239 2.04E-15OPU-1447 595.9044 12.23734 -1.92E-02 607.7738 101.1 2046 10.81844 0.003469 2.293041 1.99E-14OPU-1447 589.2591 12.55377 -1.28E-03 595.8831 100.2 2021.899 8.212396 0.003469 5.660081 4.90E-14OPU-1447 571.2375 11.82533 2.08E-03 576.3431 100.1 1981.581 7.984511 0.003469 3.013153 2.60E-14OPU-1447 585.8601 9.207178 -1.61E-02 595.1824 100.9 2020.469 22.45288 0.003469 0.948452 8.24E-15OPU-1447 660.3475 0 -0.2368203 730.3271 110.6 2277.269 119.3234 0.003469 0.1366989 1.19E-15OPU-1447 -970.164 48.79684 3.696507 -2131.91 212.2 1.00E-20 897.2115 0.003469 1.92E-02 1.63E-16OPU-1447 -640.908 0 0.9942626 -934.713 145.8 1.00E-20 2045.651 0.003469 8.00E-03 7.00E-17OPU-1447 776.3328 0 -7.805427 3082.835 397.1 4436.418 3202.18 0.003469 4.86E-03 4.49E-17OPU-1447 595.5114 15.06713 -1.44024 1033.237 171.7 2746.955 586.2128 0.003469 1.32E-02 1.15E-16OPU-1447 1206.048 0 -1.590609 1676.072 139 3462.065 502.5746 0.003469 3.59E-02 3.13E-16OPU-1447 615.0359 12.18084 -2.65E-02 629.2064 101.4 2088.644 20.78431 0.003469 0.8972141 7.78E-15OPU-1447 588.3161 13.18823 -5.03E-04 595.0173 100.2 2020.131 13.62359 0.003469 1.366829 1.18E-14OPU-1447 594.0254 13.69345 8.01E-03 598.4963 99.8 2027.223 18.57128 0.003469 0.913064 7.91E-15OPU-1447 382.0926 16.08467 0.4460657 254.4151 65.8 1141.324 202.7553 0.003469 4.09E-02 3.51E-16OPU-1447 378.5005 41.14386 0.2348748 321.6198 82.5 1351.949 246.2478 0.003469 2.85E-02 2.44E-16OPU-1447 513.6598 4.256368 3.24E-02 505.9281 98.2 1828.325 135.862 0.003469 6.48E-02 5.65E-16OPU-1447 716.144 20.47163 0.1287131 689.6448 94.9 2203.736 183.2776 0.003469 7.52E-02 6.54E-16OPU-1447 -5877.21 166.899 6.521143 -8828.52 132.6 1.00E-20 1793.013 0.003469 4.82E-02 4.21E-16OPU-1447 1651.524 15.8452 -2.727981 2486.616 148.9 4085.288 749.4877 0.003469 2.68E-02 2.32E-16OPU-1447 1074.102 0 -1.195211 1427.286 132.9 3217.756 310.939 0.003469 4.89E-02 4.24E-16OPU-1447 665.4 17.95465 -4.90E-02 689.9936 102.4 2204.38 43.55493 0.003469 0.2800759 2.42E-15OPU-1447 586.7699 12.56349 -2.65E-02 600.9017 101.5 2032.11 15.94522 0.003469 1.168782 1.01E-14OPU-1447 594.2444 12.35462 -1.54E-03 600.8903 100.2 2032.087 10.16749 0.003469 3.602395 3.12E-14OPU-1447 567.1243 12.24068 -1.13E-02 576.3905 100.8 1981.68 11.97652 0.003469 1.580963 1.37E-14OPU-1447 567.5506 8.349698 -4.27E-03 572.8292 100.3 1974.234 30.94914 0.003469 0.4473234 3.86E-15OPU-1447 497.0765 16.65657 0.0808818 480.1043 95.5 1768.69 128.0252 0.003469 6.53E-02 5.73E-16
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
91
Run 2709-01 100
50 0
10 20 30 40 50 60 70 80 90 100
2200
2100
2000
1900
1800
1700
1600 0
Integrated Age = 1920 ± 2 Ma
10^%-10
0.01 10^-6
100
A
BC D
F G HJ
K L NM
O
P
QR
IE
Run 2709-02
0
100 50
10 20 30 40 50 60 70 80 90 100
2200
2100
2000
1900
1800
1700
1600 0
Integrated Age = 1920 ± 2 Ma
10^%-10
0.01 10^-6
100
A
B C D F GH J K L NM O
PQ
R
EI
Run 2709-03
0
100 50
10 20 30 40 50 60 70 80 90 100
2200
2100
2000
1900
1800
1700
1600 0
Integrated Age = 1864 ± 2 Ma
10^%-10
0.01 10^-6
100
A
B
C D F GH
J K LN
MO
PQ
R
Cumulative %39Ar Released
Figure 5.2 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1436 (Fig. 5.1b-point A)
Oliveira, 2004 Evolução tectônica de um.........
Run 2708-01
0
100 50
10 20 30 40 50 60 70 80 90 100
2200
2100
2000
1900
1800
1700
1600 0
Integrated Age = 1898 ± 3 Ma
10^%-10
0.01 10^-6
100
AB
C DE F G H I
J
K
L
N
MOP
QR
Run 2708-02
0
100 50
10 20 30 40 50 60 70 80 90 100
2200
2100
2000
1900
1800
1700
1600 0
Integrated Age = 1886 ± 3 Ma
10^%-10
0.01 10^-6
100
AB C D E F G H I J K L NM O
PQ
R 1881 ± 4 Ma
Run 2708-03
0
100 50
10 20 30 40 50 60 70 80 90 100
2200
2100
2000
1900
1800
1700
1600 0
1931 ± 5 Ma
Integrated Age = 1937 ± 4 Ma
10^%-10
0.01 10^-6
100
A B C D E FG H I J K L N
M
O
P
Q
R
Cumulative %39Ar Released Figure 5.3 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles from samples OPU 1197 (Fig. 5.1b-point B).
92
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Fig. 5.2 displays 40Ar/39Ar isotopic data for three biotite single crystals from the dark gneiss of
the Corumbá quarry (Fig 1b-A). It shows an inhomogeneous cumulative % of 39Ar release, but well-
correlated integrated ages of 1,920±2Ma, 1,864±2Ma, and 1,838±3Ma. The analogous information
was obtained for the three-biotite single crystals from grey gneiss of Kinawa quarry, which yields an
integrated age of 1,937±4Ma, 1,898±3Ma, and 1,886±3Ma. The substantial information is that both
samples display well-defined steps with similar integrated ages. In addition, the samples show similar
spectral behaviors with loss of radiogenic argon by diffusion. The Ca/K rations show a similar
disturbance as well, probably related to impurities. Both samples are from different quarries but of
similar tectonic evolution, as observed by Oliveira & Carneiro (2001), Oliveira et al. (submitted to
Lithos). From the same Gneissic Unit, three biotite single crystals (Runs 2194-01/02/03, Fig 5.4) and
three-hornblende single crystals (Runs 2195-01/02/03, Fig 5.5) from amphibolites boudins were
analyzed. The three biotite single crystals (Fig. 5.4) yields similar, well-defined plateaus and
integrated ages differing not more than 10Ma. The Run 2194-01 displays a precise plateau of
1,975±10Ma. The same is screening for the Run 2194-02 that are evidence for two plateaus of
1987±4Ma and 1999±5Ma. The Run 2194-03 illustrates another accurate plateau of 1,970±3Ma.
With exception of Run 2194-02, the Ca/K rations are very low and the plateaus from the three
single crystals are homogeneous.
The three-amphibole single crystals (Fig 5.5) show a good link with the biotite single crystals
displayed in Fig. 5.4. Runs 2195-01, 02, and 03 exhibit ages of 2,027±14 Ma, 2,003±8 Ma and
2,016±7 Ma respectively (Fig 5.5). As expected, the ages are older than those for the biotites. It is
related to the closing of the temperature system.
Assuming total equilibration at about 1.9-2.0Ga, the plateaus can be fit in a broadly similar
evolution that may be correlated with the coolest prograde episode, discussed later on.
The second group consists of three biotite single crystals from the amphibolite from the
Supracrustal Unit (Runs 2197-01/02/03, Fig 5.6). The analyses of these three amphibole single crystals
(Runs 2197-01, 02 and 03, Fig 6) yield precise plateaus, despite a younger, but not significant
disturbance (Fig 5.6-2197-02 and 03). The plateaus occur at 2,031±13 Ma, 2,040±20 Ma and 2,032±9
Ma (Fig 5.6).
Dykes represent the third distinctive group of rocks. They are undeformed and
unmetamorphosed and they can be subdivided into two generations: gabbronorite and gabbros. These
rocks display alteration, such as saussuritization and uralitization, as attested by Oliveira & Carneiro
(2001).
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A
B C D E F G H I J KL
A
B C D E F G H I J
L
1975 ± 10 Ma
Integrated Age = 1969 ± 3 Ma
Run 2194-01
0 10 20
0
100 40
10 20 30 40 50 60 70 80 90 100
4000
30
3500 3000 2500 2000 1500 1000
500 0 0
0
100 40
4000 3500 3000 2500 2000 1500 1000
500 0 10 20 30 40 50 60 70 80 90 100 0
0.0 0.4 0.8
Integrated Age = 1987 ± 2 Ma
1999 ± 5 Ma 1987 ± 4 Ma
Run 2194-02
0
100 40
4000 3500 3000 2500 2000 1500 1000
500 0 10 20 0
2 4 6
Int
Run 2194-03
Figure 5.4 - Plot 40Ar/39Ar apparent age vs quarry (Fig 5.1b, point A)
Cumulative %39Ar released
94
30 40 50 60 70 80 90 100
0
egrated Age = 1965 ± 2 Ma
1970 ± 3 Ma
cumulative argon released for biotites of amphibolites from Corumbá
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Run 2195-01
0
100 40
10 20 30 40 50 60 70 80 90
4000 3500 3000 2500 2000 1500 1000
500 0 0 100
Run 2195-0
0
100 40
10 20 30 40 50 60 70 80 90
4000 3500 3000 2500 2000 1500 1000
500 0 0
2
100
Run 2195-0
0
100 40
10 20 30 40 50 60 70 80 90
4000 3500 3000 2500 2000 1500 1000
500 0 0
1
100
2027 ± 14 Ma
Integrated Age = 2042 ± 4 Ma
2003 ± 8 Ma
Integrated Age = 2051 ± 8 Ma
2016 ± 7 Ma
Integrated Age = 2032 ± 12 Ma
0
1000 500
1500
0
100 50
150
0 1000
3000 2000
Cumulative %39Ar Released
Figure 5.5 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Corumbá quarry (Fig 5.1b, point A)
95
Oliveira, 2004 Evolução tectônica de um.........
Cumulative %39Ar Released
0
100 40
4000 3500 3000 2500 2000 1500 1000
500
0 10 20 30 40 50 60 70 80 90 100
Run 2197-01
2131 ± 13 Ma
Integrated Age = 2035 ± 7 Ma
0
1500 1000 500
10 20 30 40 50 60 70 80 90 100
0
100 40
4000 3500 3000 2500 2000 1500 1000
500
0
Run 2197-02
2140 ± 20 Ma
Integrated Age = 2046 ± 13 Ma
0
120 80 40
10 20 30 40 50 60 70 80 90 100
0
100 40
4000 3500 3000 2500 2000 1500 1000
500
0
Run 2197-03
2032 ± 9 Ma
Integrated Age = 2040 ± 7 Ma
0
400 200
Figure 5.6 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of amphibolites from Supracrustal Unit (Fig 1b, point C).
96
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
This group belongs to the Fissural Mafic Unit of Oliveira & Carneiro (2001). Three amphibole
crystals (Runs 2180-01/02/03, Fig 5.7), and three biotite single crystals (Runs 2181-01/02/03, Fig 5.8)
were taken from the gabbronorite
40Ar/39Ar isotopic data for Runs 2180-01, 02 and 03 yield good plateaus, but not of the same
high quality as for the previous two units. The amphibole single crystals exhibit plateaus at 1,530±9
Ma, 1,752±15 Ma and 1,690±40 Ma. The three biotite single crystals of Runs 2181-01/02/03 yield
three disturbed plateaus at 1,332±3Ma, 1,611±13Ma and 1,340±30Ma respectively (Fig 5.8).
Most likely, the high thermal gradient produced by the intrusion of dykes in the “cold gneissic
unit” caused alterations such as saussuritization, uralitization and chloritization, as attested by Oliveira
& Carneiro (2001). Having this in mind, the age variation for amphiboles and biotites from the same
rock is attributed to contamination by another mineral phase. This is clearly shown by Ca/K rations
illustrated in Figs. 5.7 and 5.8, lacking uniform behavior mainly at the end of the spectrum.
Two more gabbros, representing the “second” dyke swarm generation, were analyzed. Three
amphibole crystals from each gabbro were chosen and analyzed (Runs 2183-01/02/03, Fig 5.9 and
Runs 2184-01/02/03, Fig. 5.10). The first one shows good plateaus at 940±50 Ma, 990±30 Ma and
1080± 40Ma. The second one (Fig. 5.10) also shows good correlations and displays accurate plateaus
at 960±50Ma, 952±7Ma and 864±14Ma.
The gabbros give an idea of the behavior of two dissimilar intrusions that occurred in different
periods of the Earth’s evolution, but share analogous parameters (hot intrusions in contact with cold
host rocks). This may explain the disturbance at the beginning and at the end of the spectrum.
More details will be discussed on the next section.
5.7 DISCUSSION
The combined 40Ar/39Ar are interpreted as part of two distinct events that affected the southern
part of São Francisco Craton. The first one evidences the tectonothermal event that affected the
Gneissic Unit and the Supracrustal Unit. The second one is related to the emplacement of mafic dykes.
The 40Ar/39Ar results for both gneisses from the Cláudio Unit give an integrated age (Figs. 5.2
and 5.3). Note that both gray and dark-gray gneisses indicate ages of about 1,900Ma, and the
uncertainties from one step to another are very low. The variation from one step to another can be
attributed to alteration processes (as chloritization surrounding biotite).
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98
Cumulative %39Ar Released
Run 2180-01
0
100 50
10 20 30 40 50 60 70 80 90 100
4000 3500 3000 2500 2000 1500 1000
500 0 0
Run 2180-02
0
100 50
10 20 30 40 50 60 70 80 90 100
4000 3500 3000 2500 2000 1500 1000
500 0 0
Run 2180-03
0
100 50
10 20 30 40 50 60 70 80 90 100
4000 3500 3000 2500 2000 1500 1000
500 0 0
1530 ± 9 Ma
Integrated Age = 1802 ± 7 Ma
1752 ± 15 Ma
Integrated Age = 1916 ± 8 Ma
1690 ± 40 Ma
Integrated Age = 1990 ± 30 Ma
0
100 50
150
0
100 50
150
0
400 200
A
B
C D E F GH I
J
K
L
A
B
C D E F GH I J
K
A
B
C D E F
G
H
I
J
KL
Figure 5.7 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbronorite from fissural mafic Unit (Fig 5.1b – point D).
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
Cumulative %39Ar Released
0
100 50
10 20 30 40 50 60 70 80 90 100
4000 3500 3000 2500 2000 1500 1000
500
0
Run 2181-01
Integrated Age = 1332 ± 3 Ma
0
6 4 2
0
100 50
10 20 30 40 50 60 70 80 90 100
4000 3500 3000 2500 2000 1500 1000
500
0
Run 2181-02
1611 ± 13 Ma
Integrated Age = 1615 ± 4 Ma
0
60 40 20
0
100 40
10 20 30 40 50 60 70 80 90 100
4000 3500 3000 2500 2000 1500 1000
500
0
Run 2181-03
1340 ± 30 Ma
Integrated Age = 1314 ± 3 Ma
0
4 2
A
BC D E FG
HI JK
L
0
AB C D E
F
GHI JKL
A
B C D E FG
H
I JK
L
0
0
Figure 5.8 Plot 40Ar/39Ar apparent age vs cumulative argon released for biotites of gabbronorite from fissural mafic Unit (Fig 5.1b – point D).
99
Oliveira, 2004 Evolução tectônica de um.........
940 ± 50 Ma
Integrated Age = 1740 ± 20 Ma
0 20
0
100 50
10 20 30 40 50 60 70 80 90 100
4000
40
3500 3000 2500 2000 1500 1000
500 0 0
0
100 50
4000 3500 3000 2500 2000 1500 1000
500 0 10 20 30 40 50 60 70 80 90 100 0
0
40 80
Integrated Age = 1121 ± 19 Ma
990 ± 30 Ma
Run 2183-02
0
100 50
4000 3500 3000 2500 2000 1500 1000
500 0 10 20 30 40 50 60 70 80 90 100 0
0 400 800
Integrated Age = 1920 ± 50 Ma
1080 ± 40 Ma
Run 2183-03
A
B
CD
E F
G H
I
J
KL
Run 2183-01
A
B
C
D E F G HI J
K
L
A
B
C
D EF
G
H
I JK
L
Cumulative %39Ar Released
Figure 5.9 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point E).
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Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
940 ± 20 Ma
Integrated Age = 1150 ± 10 Ma
0 10
0
100 50
10 20 30 40 50 60 70 80 90 100
4000
20
3500 3000 2500 2000 1500 1000
500 0 0
0000
0
100 50
4000 3500 3000 2500 2000 1500 1000
500 0
0
1500
Integrated Age = 1063 ± 6 Ma
952 ± 7 Ma
Run 2184-02
0
100 50
4000 3500 3000 2500 2000 1500 1000
500 0 10 20 30 40 50 60 70 80 90 100 0
0
80 120
Integrated Age = 821 ± 5 Ma
864 ± 14 Ma
Run 2184-03
Run 2184-01
Cumulative %39Ar Released
AB
C
D
E
F G HI J
K
L
AB
C
DE F G H I JK
L
10 50
10 20 30 40 50 60 70 80 90 100 0
000
40
Figure 5.10 - Plot 40Ar/39Ar apparent age vs cumulative argon released for amphiboles of gabbro from fissural mafic Unit (Fig 5.1b – point F).
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The radiometric data for amphibolites from the Cláudio and the Supracrustal Units show good
correlation, with very low errors. In addition, the plateaus are very smooth lines with no disturbances
along the spectrum (Figs 5.4 and 5.5). The same behavior is attested by the amphiboles of the
Supracrustal Unit with a small disturbance at the end of the spectrum (Fig 5.6-2197-02 and 03). The
accuracy of the obtained data is compatible with the plateau definition, which comprises at least three
contiguous steps (e.g. Berger & York 1981, McDougall & Harrison 1988).
As defined by Oliveira & Carneiro (2001) the Gneissic Unit (Cláudio, Itapecerica and
Candeias) was affected by a high-grade metamorphic event (up to the granulite facies), and the
Supracrustal Unit by at least an upper amphibolite facies event. Both units experienced a strong
deformational event, which Oliveira & Carneiro (2001) characterized as the Cláudio shear zone (Fig
5.1), the most evident structure in the studied area. Recently, Oliveira et al. (2003, submitted to Anais
da Academia Brasileira de Ciências and commented in chapter 4) characterized the age of 2.6 Ga as of
a migmatization event. The adjacent structures are folds; shear bands, S/C foliations, and rotated
boudins. Recently, Oliveira et al. (2003, in preparation and commented in the following chapter)
recognized a granulite facies event at about 2.1 Ga. Hence, it remains unclear whether this event can
be seen as a part of a second prograde event followed by deformation related to the Cláudio shear zone
or it is related to an isolated charnockitic intrusion, as better explained in the following chapter.
From the exposed above, reliable information exists on the times that the migmatization and
deformational events occurred in the Campo Belo Metamorphic Complex (CBC). It is possible that
more than one deformational and migmatization events affected the CBC.
In the literature, CBC records indicate TTG crust reworking (migmatization event) at ca.
2,860 Ma (Teixeira et al. 1996) and at ca. 2,600 Ma (Oliveira et al. 2003). In contrast, there remains
one question: Is the deformational event that affected the gneissic unit and supracrustal unit late
Archaean or Proterozoic, or both?
Field, structural, petrographic and the new geochronological data for the gneisses and the two
amphibolite generations help make some considerations:
1 – the isotope concordance of samples from two dissimilar units suggests how significant the 40Ar/39Ar data is for tracing the evolution of the high-grade metamorphic terrains,
2 – are these ages related to the Transamazonian metamorphic event (Transamazonian
orogeny), followed by a deformational event, as attested in the Quadrilátero Ferrífero (Machado et al.
1992, Teixeira 1993)?
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103
3 – do these data only represent uplift and cooling at the time of the tectonomagmatic
evolution of the Mineira Belt (Teixeira 1992, Teixeira et al. 1997)?
4 – do these data show the low-grade metamorphic history that overprinted all rocks of the
Campo Belo Metamorphic Complex, as recognized by Oliveira & Carneiro (2001) in the studied area
and south of it, around the Campo Belo town (e.g. Carneiro et al. 1988 a, b, Fernandes & Carneiro
2000, Campos et al. 2003)?
Based on our study, we believe that the data presented here are correlated to the evolution of
the Mineira Belt (Teixeira et al. 1997). They represent not only continent exhumation, but also the
response to the NW-verging contraction proposed by Alkmin & Marshak (1998). These authors
correlated the 2.5-2.0 Ga intervals with various stages of the Paleoproterozoic Wilson Cycle.
From the considerations above, three hypotheses arises:
(i) Taking into account that the argon block temperature occurs around 500ºC for amphibole
and 300ºC for biotite (Teixeira at al. 1997), it is plausible to consider that the 40Ar/39Ar age for our
rocks underwent temperatures of at least upper greenschist to amphibolite facies;
(ii) The Supracrustal Unit of the studied area is a small remnant of the Mineira Belt, as
defined by Teixeira et al. (1997), around the Lavras Town;
(iii) The tectono-deformational event attested by Oliveira & Carneiro (2001) in CBC can be
correlated with the second stage of the Mineira Belt evolution, resulting in the development of a
dome-and-keel structure, as described by Alkmin & Marshak (1998) for the Quadrilátero Ferrífero.
The following results presented in our work correspond to two generations of mafic dykes that
crosscut the crystalline crust of the Campo Belo Complex (Fig 5.1). As previously discussed, the
gabbros and gabbronorite are neither deformed nor metamorphosed. The 40Ar/39Ar mineral plateau
ages indicate that the dykes are post-Archaean and they were emplaced between two different periods
of the CBC evolution: the gabbronorite between 1,752±15 and 1,530±9 Ma, and the gabbros between
1080±40 and 864±14 Ma (Figs. 5.7, 5.8, 5.9 and 5.10).
The plateaus in Fig 5.7 for the gabbronorite hornblendes show some disturbance along the
steps. This disturbance can be interpreted as alteration or inclusion of another mineral phase in the
hornblendes. The biotites (Fig. 5.8) show the same disturbed spectrum, with ages varying from
1,632±13 and 1,332±3 Ma. This can be explained by chloritization, as seen in the thin-section.
Another explanation is hydrothermalism. As discussed previously, intense saussuritization affected the
gabbronorite and gabbros.
Oliveira, 2004 Evolução tectônica de um.........
104
The gabbro amphiboles show either plateaus with some disturbance, as for the sample shown
in Fig. 5.9, or good plateaus, as for the sample of Fig. 5.10. The latter shows good agreement between
plateau and integrated ages.
Two important geochronological records exist for the NW-SE dykes of the southern São
Francisco Craton: one is related to the emplacement of the basic gabbronorite (Sm-Nd isochron age of
2,697± 65 Ma; Pinese et al. 1995, Pinese 1997), and the other, to the emplacement of tholeiitic dykes
at ca. 1.9 Ga (Pinese et al. 1995).
Our results are geologically significant and provide imperative information for the evolution
of the southern São Francisco Craton. Some hypotheses arise when comparing our data with previous
data for the same area:
(i) The NW-SE structures into which the mafic dykes were emplaced can be correlated
with the Staterian continental rift scenario (1.8-1.6Ga, Brito Neves et al. 1995). A similar episode, the
Uruguayan dyke swarm, occurred in the Rio de la Plata Craton between 1,727-1,700Ma (Teixeira et al.
1999). Another record, at ca. 1.7Ga, is related to the Espinhaço rift, affecting the São Francisco
Craton;
(ii) The emplacement of the noritic gabbros and gabbros cannot be earlier than ~2.0Ga. This
assertion is based on geochronological records for the gneisses and amphibolites from the Gneissic
and Supracrustal Units (Figs 5.2, 5.3, 5.4 and 5.5), which are strongly deformed and metamorphosed.
Hence, the NW-SE dykes are neither deformed or metamorphosed, and crosscut the gneissic and
supracrustal units, as be evidenced from field relations (Oliveira & Carneiro 2001);
(iii) The emplacement of the mafic dykes can be correlated with the extensional orogenic
phase, conditioned to the NW-SE, N-NE extensional fractures and E-W Riedel crustal fractures (Endo
& Machado 1998);
(iv) The latest thermal evidence in the CBC is the emplacement of the ~960Ma gabbros,
emplaced in preexisting NW-SE structures.
5.8 SUMMARY
Our 40Ar/39Ar results depict two of the latest events of the CBC evolution: the first one shows
that the Transamazonian orogeny intensely affected the Campo Belo Metamorphic Complex, and the
second one evidences the intense breaking-up of the sialic crust after post-Transamazonian orogeny.
Contribuições às Ciências da Terra – Série D, vol. 7, 134p., 2004
105
From the considerations above, the gneisses, amphibolites, gabbronorite and gabbros of the
Campo Belo Metamorphic Complex have undergone wide disturbances since the earliest event that
affected the surrounding areas. The new data-set presented in our study yielded the following results
and conclusions:
(i) All gneisses and amphibolites from both units record the same peculiar closing temperature
at about 2.0Ga. It can be correlated with the age of exhumation and retrograde metamorphic processes
from the high-grade metamorphic event at 2.05 Ga, as attested by U-Pb data for charnockites (Oliveira
et al. 2003, in preparation).
(ii) The noritic gabbro was emplaced at about 1.7 Ga.
(iii) The two gabbros display the same age at about 1.0 Ga.
(iv) The Sm-Nd age for the undeformed gabbronorite of ca. 2,676 Ma presented by Pinese
(1997) is uncertain, once our 40Ar/39Ar results for deformed and metamorphosed gneisses and
amphiboles fall about 2.0 Ga and these rocks are crosscut by the noritic gabbro and gabbros. The latter
are post-Transamazonian, rather than Archaean.
(iv) Finally, our study demonstrates the need for comprehensive Ar-Ar datasets. Only limited 40Ar/39Ar data for high-grade metamorphic terrains is available in the literature.
Oliveira, 2004 Evolução tectônica de um.........
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CAPÍTULO 6 GEOCRONOLOGIA U-Pb
6.1 INTRODUÇÃO
Conforme apresentado no Capítulo 2 trata-se, a região em estudo, de uma crosta siálica
polideformada com eventos metamórficos que vão da fácies granulito/anfibolito superior à fácies
anfibolito inferior/xisto verde, além de variados eventos magmáticos intrusivos. Adicionalmente, esse
substrato siálico apresenta-se variavelmente migmatizado.
Em termos geocronológicos, os resultados Rb-Sr e Sm-Nd (em rocha total) apresentados no
Capítulo 4 mostraram que as rochas das Unidades Gnáissicas (Cláudio, Candeias e Itapecerica)
sofreram distúrbio isotópico por volta de 2,6 Ga.
Já os resultados geocronológicos Ar-Ar (Capítulo 5), obtidos a partir de anfibólios e biotitas
dessas unidades gnáissicas; de boudins de anfibolitos inseridos nessas unidades; e de anfibolitos da
Unidade Supracrustal mostraram um resfriamento situado entre 2,0-1,9 Ga.
Por sua vez, os resultados Ar-Ar obtidos a partir de anfibólios e biotita extraídos dos gabros e
gabronoritos da Unidade Máfica Fissural indicaram duas épocas distintas de resfriamento: a primeira
situada por volta de 1,7 Ga e a segunda situada por volta de 1,0 Ga. Como essas rochas exibem
características ígneas, não estão deformadas, tem apenas um metamorfismo incipiente de fácies xisto
verde, tais idades são, também, idades de cristalização. E, conseqüentemente, o reequilíbrio para fácies
xisto verde pode ser posterior a 1,0 Ga.
Apesar desse avanço, em termos da contribuição geocronológica apresentada nesta Tese, para
compreensão da evolução tectônica regional, algumas questões fundamentais continuam sem resposta.
Por exemplo: quais seriam as relações temporais entre os eventos de migmatização e granulitização?
seria, por acaso, o distúrbio isotópico manifestado pelos sistemas Rb-Sr e Sm-Nd o reflexo da
granulitização ou migmatização?
Para tentar responder essas questões, foram selecionados quatro afloramentos das unidades
gnáissicas e cinco rochas. A escolha dos afloramentos, e das amostras especificamente, foi
determinada pelas seguintes razões: a) determinar a idade de cristalização dessas rochas; b) determinar
a idade dos eventos de migmatização e granulitização.
As amostras selecionadas para esse estudo têm a seguinte proveniência (Figuras 6.1, 6.2): a
primeira é um charnockito da pedreira Alemão (amostra AH 08); a segunda é um enderbito da pedreira
Oliveira (amostra AH 07), a terceira é um gnaisse migmatítico de composição tonatítica da pedreira
Oliveira, 2004 Evolução tectônica de um.........
108
Kinawa (amostra AH 11); a quarta é um gnaisse migmatítico de composição granodiorítica (amostra
AH 14) e, a quinta, um gnaisse, também migmatítico, de composição granítica (amostra AH 15).
Figura 6.1 – Mapa geológico modificado por Oliveira & Carneiro (2001), mostrando as diferentes unidades na área mapeada. Simbologia: 1 – Unidade Máfica Fissural; 2 – Unidade Supracrustal; 3 – Unidade Gnáissica Candeias; 4 - Unidade Gnáissica Itapecerica; 5 - Unidade Gnáissica Cláudio; 6 – Contato inferido; 7 – Foliação; 8 – Zona de Cisalhamento Cláudio e, 9 – Pontos dos zircões estudados.
6.2 RESULTADOS U-Pb
A preparação das amostras está descrita no Capítulo 1. Para datação dos zircões utilizou-se do
equipamento Ion Microprobe CAMECA IMS270 do Museu de História Natural da Suécia em
Estocolmo. Para calibração da razão Pb/U foi usado o padrão 915000 com idade de 1065 ± 0,3 Ma
(1σ). As concentrações de Pb e U são de 15 e 80ppm respectivamente (Wiedenbeck et al. 1995). Os
procedimentos analíticos em detalhe podem ser obtidos em Whitehouse & Russel (1997) e
Whitehouse et al. (1999).
De cada um das amostras escolhidas, para datação U-Pb, selecionou-se, em média, 35 zircões.
Esses zircões foram submetidos a catodoluminescência para seleção dos melhores grãos. Os resultados
analíticos são apresentados na Tabela 6.1.
Amostras 207Pb/206Pb 206Pb/238U 208Pb/232ThAH08-2-1 110 120 50 0,93 1,1 0,2 0.1264 ± 0.0012 0.3033 ± 0.0043 0.0934 ± 0.0029 2049 ± 17 1707 ± 21 1656 ± 25 1805 ± 54
0.1026 ± 0.0031 2046 ± 12 1911 ± 23 1885 ± 27 1974 ± 57AH08-18-1 170 180 84 1 1,1 0,08 0.1262 ± 0.0009 0.3450 ± 0.0048AH08-18-1@1 140 120 61 0,81 0,83 0,45 0.1229 ± 0.0009 0.3049 ± 0.0043AH08-6-2 130 80 53 0,52 0,62 0,37 0.1252 ± 0.0010 0.3152 ± 0.0044AH08-14-1 310 290 140 0,89 0,95 0,11 0.1246 ± 0.0005 0.3333 ± 0.0047AH08-14-2 150 67 61 0,43 0,46 0,1 0.1262 ± 0.0008 0.3334 ± 0.0047AH08-5-1 84 73 38 0,84 0,87 0,22 0.1244 ± 0.0012 0.3261 ± 0.0047AH08-5-2 110 110 48 0,87 0,93 0,24 0.1242 ± 0.0015 0.2921 ± 0.0042AH07-1-1 780 480 220 0,39 0,62 8,88 0.1446 ± 0.0017 0.2062 ± 0.0029AH07-7-1 1300 790 440 0,38 0,62 2,98 0.1722 ± 0.0007 0.2502 ± 0.0035AH07-11-1x 710 300 290 0,43 0,42 6,8 0.1711 ± 0.0031 0.2969 ± 0.0042AH07-11-2 1300 53 260 0,49 0,042 16,18 0.0969 ± 0.0018 0.1542 ± 0.0022AH07-14-2 1300 310 250 0,17 0,23 10,92 0.1184 ± 0.0013 0.1501 ± 0.0021AH11-2-1 190 350 120 1,4 1,8 0,78 0.1823 ± 0.0013 0.3506 ± 0.0052AH11-2-2 930 150 160 0,073 0,16 3,35 0.1135 ± 0.0014 0.1453 ± 0.0023AH11-2-3 390 51 120 0,083 0,13 1,26 0.1393 ± 0.0008 0.2711 ± 0.0038AH11-4-1 450 260 230 0,4 0,58 0,31 0.1808 ± 0.0008 0.3920 ± 0.0055AH11-19-1 470 260 300 0,45 0,55 0,15 0.1874 ± 0.0006 0.4787 ± 0.0067AH11-19-2 520 130 250 0,17 0,24 0,36 0.1758 ± 0.0014 0.3892 ± 0.0055AH14-1-1 460 530 210 0,81 1,2 0,38 0.1638 ± 0.0011 0.3014 ± 0.0045AH14-1-2 410 130 190 0,26 0,31 0,15 0.1744 ± 0.0007 0.3718 ± 0.0052AH14-6-2 280 260 170 0,81 0,9 1,11 0.1759 ± 0.0010 0.4090 ± 0.0057AH14-7-1 330 330 160 0,67 1 2,67 0.1759 ± 0.0009 0.3278 ± 0.0047AH14-7-2x 730 360 150 0,22 0,49 1,97 0.1427 ± 0.0008 0.1602 ± 0.0023AH14-12-1 100 79 48 0,57 0,79 0,84 0.1815 ± 0.0023 0.3363 ± 0.0047AH14-12-2 820 82 130 0,096 0,1 2,05 0.1195 ± 0.0014 0.1350 ± 0.0019AH15-1-1 65 37 42 0,58 0,57 0,69 0.1999 ± 0.0018 0.4719 ± 0.0067AH15-1-2 1200 78 190 0,36 0,066 6,25 0.0873 ± 0.0023 0.1257 ± 0.0018AH15-11-1 540 160 190 0,19 0,31 0,84 0.1594 ± 0.0012 0.2852 ± 0.0040
0.1018 ± 0.0031 1999 ± 14 1716 ± 21 1674 ± 25 1960 ± 570.0892 ± 0.0028 2032 ± 13 1766 ± 22 1724 ± 25 1726 ± 510.0994 ± 0.0028 2023 ± 7 1854 ± 23 1825 ± 27 1915 ± 520.0990 ± 0.0030 2045 ± 11 1855 ± 23 1822 ± 26 1907 ± 550.1021 ± 0.0031 2020 ± 17 1819 ± 23 1786 ± 27 1964 ± 570.0984 ± 0.0034 2017 ± 22 1652 ± 21 1601 ± 24 1896 ± 630.0760 ± 0.0026 2284 ± 20 1209 ± 16 1107 ± 33 1481 ± 490.0830 ± 0.0024 2579 ± 7 1440 ± 18 1285 ± 33 1611 ± 440.1406 ± 0.0087 2568 ± 30 1676 ± 21 1520 ± 46 2660 ± 1540.9260 ± 0.0318 1564 ± 35 925 ± 12 891 ± 30 13248 ± 3370.0758 ± 0.0032 1932 ± 20 902 ± 12 844 ± 25 1477 ± 600.1060 ± 0.0031 2674 ± 12 1938 ± 25 1762 ± 40 2037 ± 560.0440 ± 0.0033 1856 ± 21 874 ± 13 824 ± 17 871 ± 640.0728 ± 0.0037 2219 ± 10 1546 ± 19 1456 ± 26 1421 ± 700.0961 ± 0.0028 2660 ± 7 2132 ± 26 1978 ± 40 1854 ± 510.1191 ± 0.0034 2719 ± 5 2521 ± 29 2428 ± 44 2274 ± 610.0989 ± 0.0031 2613 ± 13 2119 ± 26 1980 ± 38 1906 ± 570.0916 ± 0.0029 2495 ± 11 1698 ± 22 1560 ± 32 1771 ± 540.1144 ± 0.0033 2600 ± 7 2038 ± 25 1893 ± 37 2189 ± 590.1237 ± 0.0037 2614 ± 9 2210 ± 26 2083 ± 40 2357 ± 670.0899 ± 0.0026 2615 ± 8 1828 ± 23 1664 ± 40 1741 ± 490.0538 ± 0.0016 2260 ± 9 958 ± 13 870 ± 19 1059 ± 310.1021 ± 0.0033 2667 ± 21 1869 ± 23 1692 ± 39 1965 ± 610.0973 ± 0.0053 1948 ± 20 817 ± 11 761 ± 14 1877 ± 980.1535 ± 0.0055 2826 ± 15 2492 ± 29 2335 ± 51 2887 ± 960.3807 ± 0.0173 1367 ± 51 763 ± 10 741 ± 14 6520 ± 2550.0814 ± 0.0038 2450 ± 13 1618 ± 20 1487 ± 30 1583 ± 71
valores corrigidos idade 207 corrigida
idade 206Pb/238U
idade 208Pb/232Th
idade 207Pb/206Pb
Th-U (medido) f206 (%)U
(ppm)Th
(ppm)Pb
(ppm)Th-U
(calculado)
Table 6.1 – Resultados analíticos
Figura 6.2 – Fotografias das amostras onde foram coletados os zircões datados: a) Gnaisse da pedreira Kinawa; b,c) Gnaisse da pedreira Corumbá; d) Gnaisse da pedreira Oliveira e, e) Pedreira de rocha ornamental Alemão.
Os cristais de zircão dessa rocha apresentam coloração castanha-avermelhada translúcida e são
de uma mesma população. Após o seu imageamento por catodoluminescência, três cristais de zircão
foram selecionados para datações U-Pb obtidas a partir de seis spots (Tabela 1, Figura 6.3). Em termos
morfológicos (Figura 6.3), os cristais escolhidos são prismáticos com faces e arestas um pouco
arredondadas. O zoneamento interno desses zircões é simples (não complexo) e, também, sugere um
zoneamento de derivação magmática com algumas bordas de sobrecrescimento metamórfico (Figura
6.3).
Trata-se de um gnaisse migmatítico de composição tonalítica que pertence à Unidade
gnáissica Cláudio e foi coletado na pedreira de rocha ornamental Kinawa (Figura 6.1C). É uma rocha
fortemente migmatizada e deformada e, de coloração cinza mostrando, localmente, inúmeros
mobilizados félsicos róseo e branco (Figura 6.2a).
6.2.1 Amostra AH 11
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Figura 6.3 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH11. Os zircões pertencem a um gnaisse migmatítico da pedreira Kinawa (Figura 6.1 C)
O conjunto das razões U/Pb dessas análises, quando tratados no diagrama Concórdia (Figura
6.4), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2749 ± 6
Ma e, para o intercepto inferior, uma idade de U-Pb de 593 ± 14 Ma. O MSWD dessa discordia é de
0,89 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a partir do
alinhamento das três razões destacadas em negrito da Figura 6.4. Todas as análises refletem perda de
Pb (ou ganho de U), agumas das quais muito acentuado. Um dos pontos que determinam o melhor
alinhamento encontra-se próximo ao intercepto superior e, um outro encontra-se próximo ao extremo
inferior (Figura 6.4).
A idade encontrada pelo intercepto superior é interpretada como época de cristalização do
protólito dessa rocha. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico no
neoproterozóico.
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Figura 6.4 – Diagrama Concórdia U-Pb para os zircões da mostra AH 11 da Unidade Gnáissica Cláudio.
6.2.2 Amostra AH 14
Trata-se de um gnaisse migmatítico de composição granodiorítica e pertence à Unidade
gnáissica Cláudio e foi coletado na pedreira de rocha ornamental Corumbá (Figura 6.1D). É uma rocha
fortemente migmatizada e deformada e, de coloração cinza escuro, localmente, apresenta mobilizados
félsicos branco (Figura 6.2b).
Os cristais de zircão dessa rocha apresentam coloração castanha-avermelhada translúcida e são
de uma mesma população. Após o seu imageamento por catodoluminescência, quatro cristais de zircão
foram selecionados para datações U-Pb obtidas a partir de sete spots (Tabela 1, Figura 6.5). Em termos
morfológicos (Figura 6.5), os cristais escolhidos são prismáticos com faces e arestas um pouco
arredondados. O zoneamento interno desses zircões é simples (não complexo), excetuando-se o cristal
d que mostra pelo menos duas zonas bem distintas. Já o cristal a mostra inúmeras inclusões de outra
fase mineral. Entretanto todos os cristais escolhidos também sugerem um zoneamento de derivação
magmática com algumas bordas de sobrecrescimento metamórfico (Figura 6.5).
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Figura 6.5 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)
O conjunto das razões U/Pb dessas análises, quando tratada no diagrama Concórdia (Figura
6.6), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2658 ± 7,2
Ma e, para o intercepto inferior, uma idade de U-Pb de 385 ± 12 Ma. O MSWD dessa discordia é de
1.1 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a partir do
alinhamento das três razões destacadas em negrito da Figura 6.6. Todas as análises refletem perda de
Pb (ou ganho de U). Um dos pontos que determinam o melhor alinhamento encontra-se próximo ao
intercepto superior e, um outro se encontra próximo ao intercepto inferior (Figura 6.6).
A idade encontrada pelo intercepto superior é interpretada como época do protólito dessa
rocha. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico no paleozóico.
Entretanto, as idades aparentes 207Pb/206Pb exibidas na Figura 6.9 exibem resultados, que
variam de 2614 ± 9 Ma, 2260 ± 9 Ma e 1948 ± 20 Ma que sãocorrelacionáveis a pelo menos 3
períodos já registrados na porção meridional do Cráton São Francisco.
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Figura 6.6 – Diagrama Concórdia U-Pb para os zircões da amostra AH 14 da Unidade Gnáissica Cláudio.
6.2.3 Amostra AH 15
Trata-se de um gnaisse migmatítico de composição granítica. Essa rocha é pertencente a
Unidade gnáissica Cláudio e foi coletada na pedreira Corumbá (Figuras 6.1D). Essa rocha se
assemelha às rochas pertencentes à Unidade Gnáissica Itapecerica, principalmente, em termos de
coloração. É uma rocha fortemente migmatizada e deformada e, de coloração rósea (Figura 6.2c).
Os cristais de zircão dessa rocha apresentam coloração castanha-avermelhada translúcida e são
de uma mesma população. Após o seu imageamento por catodoluminescência, apenas dois cristais de
zircão foram selecionados para datações U-Pb obtidas a partir de três spots (Tabela 1, Figura 6.7). Em
termos morfológicos (Figura 6.7), os cristais escolhidos são prismáticos com faces e arestas um pouco
arredondados. O zoneamento interno desses zircões é simples (não complexo). Os cristais escolhidos
também sugerem um zoneamento de derivação magmática com algumas bordas de sobrecrescimento
metamórfico (Figura 6.7).
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Figura 6.7 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH14. Os zircões pertencem a um gnaisse migmatítico da pedreira Corumbá (Figura 6.1 D)
O conjunto das razões U/Pb dessas análises, quando tratada no diagrama Concórdia (Figura
6.8a), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2818 ±
1200 Ma e, para o intercepto inferior, uma idade de U-Pb de 688 ± 1200 Ma que são idades sem
significado geológicos. O MSWD dessa discordia é de 150. A reta foi traçada a partir do alinhamento
das três razões obtidas da Figura 6.8a. Um outro diagrama Concórdia (Figura 6.8b) fornecem uma
discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2910 +24/-23 Ma e, para o
intercepto inferior, uma idade de U-Pb de 911 +38/-37 Ma.
Por outro lado, os resultados individuais mostram para o ponto 1 do zircão a, uma idade
aparente de 2826 ± 15 Ma que pode estar próxima a idade do protólito dessa rochas. Esse pode
representar um cristal herdado. Já o ponto 2 desse mesmo zircão sugere um sobrecrescimento com
idade de 1367 ± 51 Ma que não tem um significado geológico, ao não ser de perda de chumbo. O
zircão (b) mostra uma idade de 2450 ± 13 Ma.
Foi construído um diagrama Concórdia (Figura 6.8c) com o conjunto das razões U/Pb obtidas
nos zircões das rochas da Figura 6.2b e 6.2c que são pertencentes ao mesmo domínio e foram
submetidas ao mesmo evento de deformação e migmatização. O conjunto dessas razões fornece uma
discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2715 ± 86 [±87] Ma e, para o
intercepto inferior, uma idade de U-Pb de 503 ± 153 Ma. O MSWD dessa discordia é de 153.
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A idade encontrada pelo intercepto superior para essa combinção é interpretada como época de
cristalização desse migmatítico. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico
no neoproterozóico.
Figura 6.8 – Diagramas Concórdia U-Pb para os zircões da amostra AH 15 da Unidade Gnáissica Cláudio (a, b) e para as amostras AH 14 e AH 15 da Unidade Gnáissica Cláudio.
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6.2.4 Amostra AH 07
Trata-se de um hiperstênio-gnaisse de composição tonalítica ou enderbítica que pertence à
Unidade gnáissica Candeias e foi coletado na pedreira de rocha ornamental Oliveira (Figura 6.1B). É
uma rocha esverdeada e, apesar de pertencer à mesma unidade que a amostra anterior (AH 08)
apresenta-se fortemente migmatizada e deformada (Figura 6.2d). Os cristais de zircão dessa rocha
apresentam coloração castanha-avermelhada translúcida e são de uma mesma população. Após o seu
imageamento por catodoluminescência, quatro cristais de zircão foram selecionados para datações U-
Pb obtidas a partir de cinco spots (Tabela 1, Figura 6.9). Em termos morfológicos (Figura 6.9), os
cristais escolhidos são prismáticos com bi-terminações bem desenvolvidas e apresentam zoneamentos
similares. Não foi possível determinar, com clareza, zonas de sobrecrescimento (Figura 6.9). O zircão
d apresenta características de um cristal prismático multifacetado. O zoneamento interno desses
zircões é simples e sugere a derivação de um zoneamento magmático.
Figura 6.9 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH07. Todos os cristais são de um enderbito da pedreira Oliveira (Figura 6.1 B)
O conjunto das razões U/Pb dessas análises, quando tratada no diagrama Concórdia (Figura
6.10), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2765
+39/-38 Ma e, para o intercepto inferior, uma idade de U-Pb de 693 +21/ -21 Ma. O MSWD dessa
discordia é de 3.7 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a
partir do alinhamento das três razões destacadas em negrito da Figura 6.10. Todas as análises refletem
perda de Pb (ou ganho de U) e elas estão posicionadas nas imediações do intercepto inferior.
A idade encontrada pelo intercepto superior é interpretada como época do protólito dessa
rocha. A idade encontrada pelo intercepto inferior mostra distúrbio isotópico no neoproterozóico.
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As idades aparentes 207Pb/206Pb exibidas na Figura 6.9 variam de 2579 ± 7 Ma, 2284 ± 20 Ma
e, 1932 ± 20 Ma. Esses resultados serão discutidos no próximo capítulo.
Figura 6.10 – Diagrama Concórdia U-Pb para os zircões da amostra AH 07 da Unidade Gnáissica Candeias.
6.2.5 Amostra AH 08
Trata-se de um charnockito de coloração esverdeada (Figura 6.2e) da Unidade Gnáissica
Candeias, coletado na pedreira de rocha ornamental Alemão (Figura 6.1A). Os zircões desse
charnoquito apresentam coloração castanha-avermelhada translúcida e são de uma mesma população.
Após o seu imageamento por catodoluminescência cinco cristais de zircão foram selecionados para
datações U-Pb, obtidas a partir de nove pontos (spots). Os resultados estão listados na Tabela 1. Em
termos morfológicos (Figura 6.11), os cristais escolhidos são prismáticos, com bi-terminações bem
desenvolvidas à exceção do cristal d. Os cristais apresentam zoneamento simples (não complexo) mas
os cristais d e e apresentam zoneamento de borda.
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Figura 6.11 – Imagens de catodoluminescência (CL) em uma mesma população de zircões da amostra AH08 e pertencentes ao charnockito da pedreira Alemão (Figura 6.1 A)
As idades aparentes 207Pb/206Pb exibidas na Figura 6.11, independentes de terem sido obtidas
nas bordas ou nas posições centrais dos cristais analisados, exibem uns padrões muito coerentes de
resultados, que variam de 2049 ± 17 Ma a 1999 ± 14 Ma.
O conjunto das razões U/Pb dessas análises, quando tratada no diagrama Concórdia (Figura
6.12), fornecem uma discórdia que apresenta, para o intercepto superior, uma idade U-Pb de 2066
+24/-18 Ma e, para o intercepto inferior, uma idade de U-Pb de 531 +150/-160 Ma. O MSWD dessa
discordia é de 0,93 e ela representa o melhor ajuste para o conjunto das razões obtidas e foi traçada a
partir do alinhamento das quatro razões destacadas em negrito na Figura 6.12. É importante salientar
que, apesar de todas as análises refletirem perda de Pb (ou ganho de U), elas estão posicionadas nas
imediações do intercepto superior.
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A idade encontrada pelo intercepto superior é, então, interpretada como época de cristalização
do charnockito. Mas, a idade encontrada pelo intercepto inferior apresenta uma margem de erro
excessiva e pode não ter significado aparente.
Figura 6.12 – Diagrama Concórdia U-Pb para os zircões da amostra AH 08 da Unidade Gnáissica Candeias.
6.3 SUMÁRIO
Os resultados mais evidentes que os resultados U-Pb mostraram são os seguintes:
1 – O protólito das rochas gnáissicas foi formado no intervalo de 3,0 e 2,75 Ga; 2 – O segundo
evento metamórfico ocorreu por volta de 2,05Ga. Esse evento atingiu a fácies granulito e foi caracterizado
através de datação U-Pb em zircões dos charnockitos; 3 - Um evento metamórfico, de menor intensidade, foi o
responsável pelo distúrbio das rochas durante o Evento Brasiliano, por volta de 0,5Ga. 4 – As rochas mostraram
perdas acentuadas de Pb (ganho de U) desde o Arqueano até o Paleozóico inferior. Discussões mais detalhadas
serão apresentadas no próximo capítulo.
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CAPÍTULO 7
CONSIDERAÇÕES FINAIS
7.1 GENERALIDADES
O Cráton São Francisco pode ser subdividido em duas porções sendo uma meridional e uma
setentrional. O acervo geocronológico permite subdividi-las em Províncias arqueanas e proterozóicas.
Na porção meridional do Craton São Francisco, de acordo com Teixeira et al. (2000), as
grandes etapas de formação de rochas estão situadas à volta de 3,2 Ga; 3,0 Ga; 2,8 Ga; 2,6-2,5 Ga; 2,2-
1,9 Ga e 2,0-1,7 Ga. Dessas, destacam-se as etapas de migmatização (2,8 Ga), granitogênese (2,6-2,5
Ga), formação do Cinturão Mineiro (2,20-1,90 Ga), soerguimento e resfriamento crustal (2,0–1,7Ga).
O cinturão Mineiro (Teixeira & Figueiredo 1991), relacionada à Orogenia Transamazônica, é
uma faixa móvel paleoproterozóica, parcialmente instalada sobre rochas arqueanas, intrudida por
granitóides com idades variando de 2,2-2,1 Ga (Teixeira et al. 1997, Noce et al. 1998, Valença et al.
2000, Ávila 2000). Esse cinturão está localizado nas bordas sul e sudeste do Craton São Francisco
(Figura.1.1), e caminha em direção ao norte.
Os pulsos metamórficos desse cinturão, de acordo com Ávila (2000), estão situados a volta de
2,13-2,12 Ga e 2,06-2,03. Idades dessa ordem de grandeza, 2,06-2,03 Ga foram encontradas por
Machado et al. (1989) nas rochas do Complexo Metamórfico Bação, que seria o substrato arqueano
retrabalhado na geração do Cinturão Mineiro.
Em linhas gerais, a porção setentrional do Craton São Francisco é constituída por uma crosta
siálica (e.g. charnockitos, enderbitos, gnaisses e migmatitos) e por seqüências vulcano-sedimentares.
Em termos geocronológicos, essas rochas podem ser enquadradas em duas províncias: a)
província de idade arqueana (3,0-2,7 Ga), onde se situa, por exemplo, o Complexo Jequié; b) província
de idade proterozóica inferior (2,25-1,9 Ga), onde se situam, por exemplo, os cinturões Itabuna e
Correntina-Guanabi à idades proterozóicas média a superior (compilados por Teixeira 1992)
No caso da província arqueana, Silva et al. (2002) confirmou a presença de um domínio
mesoarqueano, situado a volta de 3,0 Ga, caracterizado por intensa atividade tectônica. Além disso,
identificou um período de acresção crustal, situado entre 2,87 – 2,63 Ga, quando foram gerados os
protólitos dos ortognaisses granulíticos do domínio Itabuna – Salvador – Curaçá.
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Quanto à província de idade proterozóica inferior, o magmatismo pré-colisional do cinturão
Bahia Oriental foi datado em 2,20 - 2,13 Ga e o magmatismo sin-colisional em 2,09 Ga. Por sua vez, o
pico metamórfico de fácies granulito, relacionado a essa colisão, foi datado em 2,08 – 2,05 Ga (Silva
et al. 2002). Na província de idade neoproterozóica, Silva et al. (2002) reconheceu distúrbios
isotópicos situados à volta de 0,7 Ga (e.g. idades aparentes Pb-Pb e intercepto inferior U-Pb), nos
charnoquitos de Ihéus.
7.2 QUESTÕES ACERCA DA EVOLUÇÃO TECTÔNICA DO CRATON SÃO
FRANCISCO MERIDIONAL
Apesar do avanço, no conhecimento geológico da porção meridional do Craton São Francisco,
ocorrido nos últimos anos, algumas questões ainda continuam em aberto. Dentre elas destacamos as
seguintes:
1 Quantos eventos de migmatização tiveram lugar nessa região? Sabe-se que um deles pode
estar situado à volta de 2,86 Ga (Noce 1995, Teixeira et al. 1996)
2 Seria o período de 2,6 Ga o responsável pelo evento de migmatização, seguido de
deformação, sob fácies metamórfica granulito? Ou esse período marcaria apenas um distúrbio
isotópico, em um evento de menor intensidade, não necessariamente sob fácies granulito e sim,
reativação desse complexo com intrusões de corpos graníticos como já relatado anteriormente (Noce
1995, Teixeira et al. 2000) situados no Evento Rio das Velhas III de Endo (1997)?
3 Quantas etapas de formação de rochas tiveram lugar na evolução tectônica do Cinturão
Mineiro? Sabe-se que corpos granitóides foram colocados por volta de 2,2-2,1 Ga e que esse cinturão
apresenta retrometamorfismo para a Fácies Xisto-Verde
4 As rochas do Complexo Metamórfico Campo Belo teriam sido afetadas pelo Evento
Deformacional Transamazônico, como registrado nas rochas do Quadrilátero Ferrífero (e.g. Alkmim
& Marshak 1998)?
5 Estaria à geração da Zona de Cisalhamento Cláudio relacionada à primeira fase do Evento
Transamazônico, de componente N/S destral transpressivo/transtrativo (Endo 1997), registrado nas
rochas do Quadrilátero Ferrífero e adjacências?
6 Seria o período de 2,0-1,8 Ga a época de exumação do Complexo Metamórfico Campo
Belo?
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7 Seriam realmente arqueanos os diques de gabronoritos indeformados, de direção NW-SE,
conforme datação isocrônica Sm-Nd (Pinese 1997)?
Baseando-se nas questões, acima mencionadas, e nos dados apresentados nesta tese, traçar-se-
á, no próximo item, um painel evolutivo para o Complexo Metamórfico Campo Belo.
7.3 DISCUSSÃO
Oliveira (1999) e Oliveira & Carneiro (2001) subdividiram o Complexo Metamórfico Campo
Belo em seis unidades litodêmicas, sendo três delas gnáissicas, uma anfibolítica, uma supracrustal
(vulcâno-sedimentar) e uma máfica fissural.
De acordo com esses autores, essas unidades foram geradas no decorrer de cinco eventos
tectonotermais. O primeiro evento está relacionado ao período de formação do protólito das unidades
gnáissicas. O segundo está relacionado a um evento extensional com posterior deposição da unidade
vulcâno-sedimentar. O terceiro evento, de alto grau metamórfico e considerado o mais importante, foi
o responsável pelo rigoroso processo de migmatização que afetou a região. O mesmo evento foi o
responsável pela geração da Zona de Cisalhamento Cláudio com cinemática transpressiva destral. Em
termos geocronológicos, esse evento deveria ser anterior a 2658 Ma, pois essa foi a idade encontrada
por Pinese (1997), para os diques de gabronorito indeformados da região de Lavras. O quarto evento
foi caracterizado como fraturamento crustal e colocação da unidade máfica fissural que corta as
unidades arqueanas. Esses diques são indeformados, apresentam grau metamórfico incipiente e são
correlatos aos diques estudados por Pinese (1997). O quinto evento foi o retrometamorfismo regional
para xisto verde.
Com os novos dados geoquímicos apresentados nesta tese, o padrão geoquímico das rochas
félsicas do Complexo Metamórfico Campo Belo, principalmente em se tratando dos gnaisses de fácies
granulito (enderbitos) e charnoquitos, mostra particularidades distintas se comparados com
metagranitóides arqueanos típicos ou com a média das crostas superior e inferior e crosta total.
Observam-se também uma grande mobilidade para alguns dos elementos traços incompatíveis (e.g.
Cs, Rb, U, Nb, Ta e W). Essa mobilidade seria causada por desidratação devido ao aumento de
temperatura, atingindo a fácies granulito.
Os elementos Nb e Ta, por exemplo, são considerados imóveis no decorrer dos processos de
desidratação crustal por aumento de temperatura e perda de fluídos. Entretanto, as rochas do
Complexo Metamórfico Campo Belo são empobrecidas nesses elementos e as razões Nb/Ta são
altíssimas sendo comparadas com alguns eclogitos (ver Capítulo 3). Disso conclui-se que os
enderbitos e charnockitos, em questão, não são formados, exclusivamente, de componentes oriundos
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da crosta inferior, como mencionado por vários autores (e.g. Harris & Bickle 1989, Santosh et al.
1991a, b, Rudnick & Fountain 1995, Kramers & Tolstikhin 1997, Satish-Kumar & Santosh 1998).
Mesmo porque, se assim fosse, as rochas estudadas deveriam ser empobrecidas nos heat production
elements (U, K e Th) o que não é o caso, a excessão do U. Sendo assim, possivelmente, formação dos
enderbitos e charnockitos ocorreu a partir da desidratação de granitóides, durante um processo
metamórfico de alto grau.
Os resultados U-Pb dos zircões das rochas gnáissicas (enderbitos inclusive), mostraram idades
para o intercepto superior situadas entre 2,9-2,7 Ga. Todavia, as idades aparentes 207Pb-206Pb, obtidas
em diferentes posições nos cristais de zircão, mostraram distúrbios isotópicos situados em 2,6 Ga, 2,2
Ga e 1,9 Ga. Isso sugere, a princípio, que se tratam da época de geração dos protólitos dos gnaisses e
de eventos tectônicos posteriores, superimpostos às rochas da região em estudo.
Idades situadas entre 2,60-2,55 Ga foram obtidas pelos métodos Rb-Sr e Sm-Nd nessas rochas,
por meio de isócronas e errócronas, numa combinação de rochas cogenéticas e não cogenéticas. No
entanto, todas as amostras tinham as mesmas características: eram deformadas e migmatizadas. Idade
semelhante foi encontrada por Teixeira (1985) no enderbito da Pedreira Oliveira, que é o mesmo
enderbito estudado nesse trabalho. Trata de uma idade isocrônica Rb-Sr de 2566 ± 53 Ma (Ri= 0,706).
Uma idade U-Pb dessa mesma ordem foi encontrada por Campos et al. (2003) em zircões do
leucossoma e mesossoma de um migmatito do Complexo Metamórfico Passa Tempo (sudeste da área
estudada). Esse conjunto de características está apontando, então, para um evento de migmatização do
substrato gnáissico situado por volta de 2,60-2,55 Ga. Então, aparentemente, sendo um do(s) evento(s)
de migmatização.
Aparentemente, então, o evento de migmatização antecede o evento de fácies granulito, pois o
resultado mais conciso e relevante foi obtido no charnockito da Pedreira Alemão. O conjunto das
razões U/Pb dessa rocha, quando tratada no diagrama Concórdia, forneceram uma discórdia que
apresenta, para o intercepto superior, uma idade U-Pb de 2066 +24/-18 Ma. Adicionalmente, as idades
aparentes 207Pb-206Pb mostram um padrão muito coerente e situado à volta de 2049 a 1999 Ma,
independentemente, se forem obtidas nas bordas ou porções mais centrais dos cristais. As idades Ar-
Ar, em anfibólios e biotitas dos gnaisses e anfibolitos, situadas entre 2027 - 1950 Ma, corroboram essa
interpretação, marcando o final de um evento de alto grau que teve lugar ao término do
Paleoproterozóico, uma vez que, a temperatura de fechamento do sistema Ar em anfibólio é em torno
de 500oC.
Idades U-Pb semelhantes a essa, em rochas similares, foram obtidas por Silva et al. (2002),
para a porção setentrional do Cráton São Francisco (cinturões Itabuna e Salvador-Curaçá e no Bloco
Jequié). Tais idades foram interpretadas como retrabalhamento crustal, com recristalização
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metamórfica sob altas temperaturas e pressões, no decorrer da fase colisional do Cinturão Bahia
Oriental.
Situação semelhante também foi encontrada por Kamber et al. (1995 a, b) para o cinturão
Limpopo, na África do Sul. Esses autores caracterizaram uma sutura proterozóica de 2,2-2,0 Ga,
gerada em condições metamórficas de alto grau (fácies granulito), que foi o maior evento tectôno-
metamórfico que atingiu a região. As técnicas utilizadas por eles foram Pb-Pb e Sm-Nd (em granadas)
e Ar-Ar (em anfibólios).
Já os resultados Ar-Ar, principalmente em anfibólios, dos gabronoritos e gabros, mostraram
que eles se posicionaram na crosta em dois períodos distintos. O primeiro período ocorreu por volta de
1,7 Ga e pode ser correlacionado ao Rifte Espinhaço (ver Capítulo 5). O segundo período ocorreu por
volta de 1,0 Ga e podem ser correlacionadas ao Rifte Macaúbas. Sendo assim, fica evidente que os
diques de gabronorito, anteriormente datados por Pinese (1997), não são neoarqueanos, mas do final
do Paleoproterozóico – início Mesoproterozóico e final do Mesoproterozóico.
O último evento termal que afetou a área foi aquele responsável pelo distúrbio isotópico
identificado nas rochas estudadas. Isso fica evidente através dos interceptos inferiores nos diagramas
Concórdia que mostraram de idades U-Pb correlacionáveis ao Evento Brasiliano. As rochas félsicas, à
exceção do charnockito, mostraram perdas intensas de chumbo desde o Arqueano, como pode ser visto
no diagrama das Figuras 7.1 e 7.2.
Figura 7.1 – Diagrama U versus razão 207Pb/206Pb mostrando a perda de Pb com relativo ganho de U.
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Figura 7.2 – Diagrama das razões 207Pb/235U versus 206Pb/238U mostrando o distúrbio isotópico a que foi submetidas as rochas estudadas do Arqueano ao Paleozóico.
7.4 SUMÁRIO
A evolução tectônica da região pode ser sumarizada em sete eventos. No primeiro evento foram formados os protólitos das rochas gnáissicas estudadas, no intervalo de tempo situado entre 2,9 e 2,75 Ga. O segundo evento, ocorrido por volta de 2,6 Ga e caracterizado como um distúrbio isotópico nos sistemas Rb-Sr e Sm-Nd, estaria relacionado a um possível evento de migmatização regional. O terceiro evento ocorreu por volta de 2,05 Ga, sob condições metamórficas de fácies granulito, foi responsável pela formação de charnockitos a partir da desidratação de granitóides. O quarto evento ocorreu por volta de 2,0 Ga, e trata-se do resfriamento crustal ocorrido logo após o evento da fácies granulito. O quinto evento está caracterizado pelo magmatismo máfico fissural, ocorrido por volta de 1,7 Ga, e pelo intenso fraturamento da crosta continental, ou mesmo reativação de falhas pré-existentes, de direção NW/SE. O sexto evento, ocorrido por volta de 1,0 Ga foi o responsável pela colocação dos gabros que aproveitaram as falhas pré-existentes de direção NW/SE e, finalmente, o sétimo evento, foi o responsável pelo distúrbio isotópico presente nas rochas da região estudada. Tal evento, situado a volta de 0,5 Ga pode ser correlacionado a Orogenia Brasiliana.
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