Revista Brasileira de Geociências 17(4):473-480, dezembro de 1987

PETROLOGY AND ISOTOPE SYSTEMATICS OF MAGMA MUSHES:SOME PORPHYRITIC GRANITOIDS OF NORTHEASTERN BRAZIL

JUDE McMURRY*, LEON E. LONG** and ALCIDES N. SIAL***

ABSTRACT More than 80 coarsely porphyritic granitoid plutons with K-feldspar megacrysts(Itaporanga-type granites) intrude metarnorphic rocks of Northeastern Brazil. Textural evidence forfIlter pressing and flow foliation indicates that these bodies were emplaced as viscous, crystal-ladenmushes. A representative Itaporanga-type pluton, the Monte das Gameleiras intrusion, consists of avariety of rock types with Si02 ranging from 49.5 to 71.6 weight percent. Chemically andpetrographically, this hornblende-bearing pluton has !-type characteristics with some S-type affinities.Whole-rock oxygen isotope data are in keeping with the distribution of 5180 for unaltered graniticrocks. Similarly, REE data suggest a single magmatic process, at least for the more felsic rock types,without significant post-intrusive effects. A 5-point, whole-rock Rb-Sr isochron for the Monte dasGameleiras pluton gives t = 511 ± 26 Ma and an initial 87Sr/86Sr = 0.7099 ± 0.0004. This iscontrasted with disparate ages calculated from texturally similar bodies such as the Fazenda Novapluton, for which a 5-point whole-rock isochron gives t = 630 ± 24 Ma with initial 87Sr/86Sr = 0.7065± 0.0005, and the type locality Itaporanga pluton, for which a 6-point isochron gives t = 625 ± 22 Ma.

RESUMO Mais de 80 plutões graníticos grosseiramente porfuíticos, com megacristais de feldspatopotássico (granitos tipo Itaporanga), cortam rochas metamórficas do Nordeste do Brasil. Evidênciastexturais de diferenciação por fIltragem e foliação de fluxo indicam que esses corpos foram intrudidoscomo acumulações viscosas de cristais. Um plutão representativo do tipo Itaporanga, a intrusão deMonte das Gameleiras, consiste em uma variedade de litotipos com Si02 variando de 49,5% a 71,6%em peso. Química e petrograficamente, este plutão com hornblenda tem características do tipo-! ealgumas afinidades com o tipo-S. Dados de isótopos de oxigênio para rocha total estão de acordocom a distribuição de 5018 para rochas graníticas inalteradas. Similarmente, dados de REE suge­rem um processo magmático simples, pelo menos para os tipos de rochas mais félsicas, sem efeitospós-intrusivos significativos. Uma isócrona Rb-Sr de 5 pontos para rocha total, do plutão de Monte dasGameleiras, fornece t = 511 ± 26 Ma e uma razão inicial 87Sr/86Sr = 0,7099 ± 0,0004. Esta contrastacom as idades calculadas para corpos texturalmente semelhantes, tais como o plutão de Fazenda Nova,onde uma isócrona Rb-Sr de 5 pontos para rocha total fornece t = 630 ± 24 Ma com razão inicial87Sr/86Sr = 0,7065 ± 0,0005, e o plutão da localidade-tipo de Itaporanga, para o qual uma isócronade 6 pontos fornece t = 525 ± 22 Ma.

INTRODUCTION Numerous plutons and dikes of awide variety of rock types intrude the polymetamorphosedand deformed shield rocks of Northeastern Brazil.Conspicuous among these is a group o.f porphyriticgranitoids with coarse, tabular phenocrysts of alkalifeldspar. At least 80 of these so-called "Itaporanga-type"plutons are known; they are located where metamorphismattained at least amphibolite facies with associatedextensive migmatization (Almeida 1971, Brito-Neves &Pessoa 1974). Most have received little more thanreconnaissance study. A thorough knowledge of theirpetrology, however, is important to understanding thenature of the lower crust from which the magmaspresumably were derived during the last major orogeny tohave affected this portion of Gondwanaland.

We have analyzed samples from a large Itaporanga-typegranitoid located about 80 km from the Atlantic coast onthe boundary between Paraíba and Rio Grande do Nortestates (Fig. 1). The Monte das Gameleiras pluton is anenclave-rich, predominantly I-type granitoid (Chappell &White 1974). Like other Itaporanga-type granitoids, itappears to have been emplaced as a viscous, crystal-ladenmagma derived from near-solidus partial melting with

incorporated mafic phases inéluding hornblende and biotite.In addition, a Rb-Sr study was made of anotherItaporanga-type igneous body in the vicinity of FazendaNova, located 140 km west ofRecife (Fig. 1).

REGIONAL SETTING These bodies are emplaced intothe Borborema tectonic province, a complex structuralmosaic of linear metasedimentary fold belts separated bymassifs of much older, previously deformed gneissicbasement (Almeida et aI. 1981). The final major orogeny inthis area was the Brasiliano Cycle, the metamorphic andintrusive events of which date from about 700 to 500 Maand wich caused many of the older rocks to be thermallyoverprinted (Wernick 1981, Brito-Neves et aI. 1974). Atthis time, regional metamorphism created rocks ofgreenschist to amphibolite facies under low P/high Tconditions.

Isotopic ages cluster about several periods of thermalactivity within the Brasiliano Cycle (Brito-Neves et aI.1974). Ages of 700 ± 20 Ma may correspond to earlythermal metamorphism accompanied by folding. Peakmetamorphic conditions are well~documented by K-Ar agesof about 630 ± 30 Ma. A younger cluster of intrusive and

* Department of Geological Sciences, University of Texas, Austin, TX 78713, USA. Current address: Department of Geosciences, TexasTech. University, Lubbock, TX 79409, USA

** Department ofGeological Sciences, University ofTexas. Austin, TX 78713, USA*** Departamento de Geologia, Universidade Federal de Pernambuco. CEP 50000, Pernambuco, PE, Brasil

474

___ I"""

EXPLANATlON

(2] OUATERNARY SEOIMENTARY AOCKSo TERTlARY SEOIMENTAAY ROCK$

PRECAMBRIAN METAMORPHIC ROCKS:• Faz.,.,da No,,",o •..

R.elf'. (2J SERIDO GROUPo CAICÓ GROUP

Ei) IGNEOUS INTRUSION

~'" -- FAULT. DASHED WHEAE INFERREO

--'-STATE BOUNDARY

Figure 1 - Location of study area, Northeastem Braziland generalized geologic map of the Monte das Carne­leiras region (after Barbosa et alo1974)

metamorphic ages ranges between 570 and 540 MaBrasiliano activity ceased with post-tectonic intrusionsending about 410 Ma ago (Long & Brito-Neves 1977).

PETROGRAPHY OF THE MONTE DAS GAMELEIRAS

PLUTON The major rock types of the Monte dasGameleiras pluton can be classified as:1. medium-to-coarse-grained, porphyritic silicic rocks(Itaporanga-type lithology);2. fine-grained, equigranular granites sensu stricto;3. dark, fine-grained, equigranular rocks (monzonites andquartz monzodiorites).

ln addition, elongated mafic enclaves are common, as areaplitic and pegmatitic dikes. Angular, metasedimentaryxenoliths are less abundant.

Porphyritic granitoid Depending OFl the proportion ofmegacrysts and matrix (which may be extremely variableeven on outcrop scale), the porphyritic rocks are granites,granodiorites, quartz monzonites, or quartz monzodiorites;for simplicity, these Itaporanga-type rocks are referred to as"porphyritic granitoids".

Abundant tabular or (rarely) ellipsoidal megacrysts (1 to10 cm long) of perthitic K-feldspar confer a striking textureto the porphyritic granitoids. ln hand sample, these palepink or gray megacrysts have a faintly perceptibleconcentric zoning caused by oriented inclusions ofplagioclase near the center in addition to inclusions ofbiotite, sphene, hornblende, and magnetite in the outerlayers of the megacrysts. Modal abundances of megacrystsvary considerably throughout the pluton, but in general themegacrysts constitute 25% to 40% of a given sample. lnthin section, megacrysts commonly are bordered by a

Revista Brasileira de Geociências. Volume 17, 1987

narrow, 0.5 to 2 mm, anhedral rim af very-finc-grainedquartz, microcline, sericitized and turbid plagioclase, andlobate myrmekite. Ellipsoid megacrysts have "tails" ofmillimeter-sized anhedral quartz, plagioclase, andmicrocline which pinch out into more mafic matrix severalmillimeters from both ends of the megacryst.

The megacrysts are perthitic; many also have poorlydeveloped patches of microcline grid twinning. ln contrast,K-feldspar in the matrix is equant, fine-grained, anhedralmicrocline with sharply defined twinning.

Quartz, plagioclase, biotite, and hornblende dominatethe porphyritic granitoids. Quartz generally occurs in large,anhedral, composite pools. Plagioclase forms mostlyanhedral, equant to subhedral tabular crystals, the largercrystals (1 to 5 mm) of which are faintly twinned andconcentrically zoned. Both biotite and hornblende areintensely pleochroic: biotite Q = pale yellow-green and ~ = 'Y= very dark green, and homblende Q = pale yellow-green, ~= brown-green, and 'Y= intense blue-green. Slender crystalsof apatite are common inclusions in biotite and hornblende.Other accessory phases include slender, metamict prisms ofallanite rimmed by epidote, magnetite, ilmenite rimmed bysphene, and discrete, stubby prisms of apatite and zircon.All of these minor phases occur in aggregates with biotiteand hornblende crystals.

Fine-grained granites Many of the fine-grained granitesoccur as crosscutting dikes in the porphyritic granitoids.Other exposures, although massive and fairly extensive,contain angular fragments of the porphyritic rocks. Theiroverall texture is granitoid and equigranular, with anaverage grain size less than 0.5 mm for most samples.Plagioclase and quartz are subordinate to microcline inthese rocks. Scattered flakes of pleochroic green biotite(less than 5% modally) impart a slight foliation.

Minor mafic phases include sphene, elongate zircon,apatite (as íninute inclusions in plagioclase and as discreteprisms), allanite rimmed with epidote, and rare Fe-Tioxides.

Dark Intrusives The color index exceeds 40% for thesemore mafic, equigranular rocks. Complexly twinned,normally zoned plagioclase is the most abundant mineral.Biotite and less abundant homblende exhibit the same

intense green pleochroism as do their counterparts in theporphyritic granitoids. Some hornblende crystals havegrainy cores of relict clinopyroxene.

Quartz and microcline are present only in minorabundances. Quartz occurs as small, equant, individualgrains. Microcline is anhedral, equant, and seriate; somecrystals are rimmed by lobate myrmekite.

The accessory mineral suite of the dark intrusivesresembles that of the porphyritic granitoids. Sphene,apatite, and Fe-Ti oxides are common inclusions inhomblende and also occur as discrete crystals. Zircon andepidote-rimmed allanite are present in some samples.

Other rock types Dark, lens-shaped stringers arecoínrnon as enclaves in the porphyritic granitoids. Modesand mineralogy are similar to the dark intrusives, but someenclaves also have ellipsoidal crystals of perthitic K-feldsparwhich resemble more tabular megacrysts in the surroundinggranitoid. These large, cigar-shaped crystals are comparablyrimmed by very fine-grained quartz and feldspars, and they

Revista Brasileira de Geociências, Volume 17. 1987 475

contain oriented rows of inclusions of slightly alteredplagioclase, just as the megacrysts do.

Large, angular xenoliths are far less common than theelongated mafic enclaves. Some xenoliths are compositeblocks, including those in which amphibolite is in sharpcontact with pink, slightly porphyritic granite which in tumveins the amphibolite, ali of which is entrained inporphyritic granitoid.

Pegmatites with abundant K-feldspar, quartz, and biotiteare ubiquitous but are of minor importance volumetricaliy.Narrow, even-sided, 2-t0-6 cm aplitic dikes crosscut aliother lithologies.

MEGASCOPIC TEXTURES Tabular K-feldspar mega­crysts in the porphyritic granitoids weather in high relieffrom the matrix and accentuate the evidence that the Montedas Gameleiras pluton was intruded as a viscous, crystal-ladenmush. In places, the megacrysts are aligned in faint swirls orductile shears; elsewhere, they are subparaliel or are soconsistently aligned that the rock has a gneissic appearance.Concentrations of megacrysts form cumulate-like layerswhich can be traced along strike for tens of meters. Filterpressing appears to have operated on a smali scale toproduce some extremely megacryst-rich varieties of theporphyritic granitoid where interstitial magma was injectedinto narrow fissures in wall rock.

Contacts between the porphyritic granitoid and the darkintrusive are complex, but the two rock types appear tohave been intruded over a brief time interval during whichboth magmas "mingled" (Vemon 1983) and deformedplasticaliy against each other. Some contacts are sharp andplanar; others grade into undulating, swirls of felsic andmafic rock on the scale of a single outcrop. Elsewhere,lobes of dark intrusive have intruded convex1y into theporphyritic granitoid. Flow foliation and a curvedconcentrated band of megacrysts at the contact signify thatthe granitoid was plastically deformed by the intrusion.

MIC ROPROBE ANAl YSES Chemical compositions of

selected minerals were determined in Monte das Gameleiras

rock types at the University of Texas with athree-spectometer Applied Research Laboratories EMXelectron microprobe equipped with an energy-dispersivedetector. Computer programs appropriate for 15 kvaccelerating potential were used to calculate matrixcorrections (Bence & Albee 1968, Albee & Ray 1970) andto convert the corrected data to weight percent oxides andatomic proportions. Natural and synthetic mineral samRleswere used as primary and secondary standards.

Plagioclase Large, zoned plagioclase crystals in thematrix of the porphyritic granitoid (Table 1) have coresof calcic oligoclase (An24 to An27) which grademonotonically into rims of sodic oligoclase (An17 toAn2o). Plagioclase inclusions in alkali feldspar megacrystsare AnIs on the average, but their outer parts are likewisemore sodic, and most are mantled by optically distinctcollars of albite.

Plagioclase in a dark-colored intrusive, GA-17, is zonedfrom andesine cores (An34 to An36) to oligoclase rims(An24 to An2S)' Note that there is mínimal compositionaloverlap between plagioclase cores in the granitoid andplagioclase rims in the dark intrusive (Table 1).

Alkali feldspar The proportion of KAlSi3 Oscomponent in alkali feldspar is consistently high in thematrix microcline and in the megacrysts of the porphyriticgranitoid (Table 1). Microcline in the dark intrusive is alsocomparably high.

Megacrysts are unzoned with respect to K, Na, or Ca,but significant variation in Ba was detected in microprobetraverses (Fig. 2). The Ba variations in the megacrystscorrespond to opticaliy distinguishable zones. For example,the major portion of a typical megacrist in sample GA-5contains concentric bands of oriented plagioclase inclusionsand has a high, consistent Ba concentration of about 5,000pprn. The outer millimeter of the megacryst isinclusion-free, and Ba concentration drops to about 3,000

Table 1 - Typical plagioclase and alkali te/dspar rnicroprobe analyses (oxides - W t %) Monte das Carneleiras pluton

Rock Porphyritic GranitoidDark Intrusive

Sample

GA-l1 GA-15aGA-17

Mineral

Plagio-Plagio-Plagio-Plagio-Plagio-Plagio-K·feldsparK·feldsparPlagio-Plagio-K-feldsparclase

claseclaseclaseclaseclase claseclase

Remarks

Core ofRim ofCore ofRim of"Mande"MatrixMegacrystMatrixCore ofRim ofMatrixzoned

zonedinclusioninclusionrimmingunzoned(perthitic)(microcline)zonedzoned(microcline) .matrix

matrixin mega-inmega-inchsion matrixmatrix

crystalcrystalcrystcryst crystalcrystal

Si02

62.866.668.068.571.266.065.064.861.864.764.0

Al203

23.822.723.323.220.422318.318.626.324.418.9CaO

5.24.23.62,90.22.90.00.07.65.20.2

Na208.69.79.79.911.99.50.90.87.48.80.9

K20

0.20.10.20.60.10315.215.50.10.114.6BaO

n.d.n.d.n.d.n.d.nd.n.d.0.50.7llihn.d.1.9-Total

100.6103.3104.8105.1103.8101.099.9100.4103.2103.2100.5--- Mol.% An24.919.116.813.71.014.00.00.035.924.41.1

Mol.%Ab74.380.482.383.298.584.68.37.463.475.18.4

Mol.% Or0.80.50.93.50.51.590.791.40.80.587.0

Mol.%Cn

------1.01.2--3.5

476 Revista Brasüeira de Geociêru:ias. Volume 17, 1987

°0L--~--~4--~-----'8'-----'--+Troverse (mm)

inclusion---..t

4 8Troverse (mm)

mafic enclaves, and metasedimentary xenoliths. Chemicalbehaviour is more disparate for Al203, in which the contentfor enclaves of ali types is markedly aberrant. Na2O dataare scattered, and MgO values are erratic.

Strontium in the dark-<:olored intrusive rocks exceeds1,000 ppm (Table 6). The more felsic rocks contain from600 to barely 100 ppm Sr, diminishing with increasingSi02. Rubidium concentrations are low overali (107 to 172ppm), with less variability and no distinct variation withSi02 •

Field evidence indicates that the porphyritic granitoidsand the dark-<:olored intrusives were intruded

contemporaneous1y and that they have similar mineralassemblages. However, divergent Al2 03, MgO, and Rb-Srisotopic data do not support the conclusion that the twomagma types are cogenetic. The connection between them,if any, is uncertain and requires more detailed chemical andisotopic study.

matri.•••

••

b.

o.

matri.•

+

12

12

+

+ core++ ++1'+

+

shell

GA- 05

GA-15o

7,500 .

I.t"""rlm +I"" + + + +~

[5,000 + +.. + #+ +a. + ++

Figure 2 - Electron microprobe traverses of alkali feldsparmegacrysts in porphyritic granitoid of the Monte dasGameleiras pluton. Variations in Ba concentration cor­respond to optically discemible zones in the megacryst.Ba concentration in matrix microcline plotted alongsidetraverse for comparison

ppm. The outerrnost edge of the megacryst is a 'narrow,anhedral rim characterized by very high, erratic Ba valuesand by intricate, bulbous myrrnekite; both features havebeen ascribed in other studies to late-stage hydrothermaleffects (Mehnert & Busch 1981, Phillips 1980). Microclinein the matrix has variable Ba but is generaliy comparable tomegacryst rim and/or core values.

SYMIIOL8+ GEOCHEMICAL SAMPLE

o RB-SR SAMPLE• VILLAGE

Hornblende and biotite Amphibole in ali of theanalyzed rock types is magnesian hastingsitic hornblende(terrninology after Leake 1978). Amphiboles in one sampleof porphyritic granitoid have edenitic cores, however(Tab.2).

Biotite compositions are homogeneous throughout thepluton (Tab. 2). It would be difficult to distinguishbetween biotite from a porphyritic granitoid and from adark intrusive on the basis of chemical or optical properties.

WHOLE·ROCK GEOCHEMISTRY Seventeen samplesfrom the Monte das Gameleiras pluton, selected for broadlithologic and geographic coverage (Fig. 3), were analyzedat Sudene laboratory at the Universidade Federal dePernambuco. Na and K were analyzed by flamephotometry, ali other major elements by atomic absorptionspectrophotometry, and Rb and Sr were determined byisotope dilution. Selected analyses are presented in tables 3and 6.

As expected from the assortrnent of rock types, thevariation of Si02 in the Monte das Gameleiras pluton isbroad, ranging from 49.5 weight percent for a dark-coloredintrusive, to 64 to 69 weight percent for porphyriticgranitoids, to between 69 and 71.6 weight percent forfine-grained, nonporphyritic granites (Table 3). Thedistribution. of CaO, total FeO, and K20 versus Si02 issystematic for ali samples, including dark-colored intrusives,

Figure 3 - Sample location map, Monte dos Gameleiraspluton

OXYGEN ISOTOPE GEOCHEMISTRY Oxygenextractions were performed by reaction with flu'orine at theUniversity of Georgia, Athens. Isotopic analyses were madeusing a VG Micromass 602C double-collector massspectometer. A rose quartz standard was analyzed routinelyduring the analyses as a monitor of perforrnance. Fourwhole-rock samples of porphyritic granitoid and six offine-grained granite from the Monte das Gameleiras plutonwere selected for analysis. Most samples have ratherhomogeneous 0180 values of about +8.3 to +8.6%0 butseveral have anomalously low or high values to give a totalrange of 0180 of +7.0 to +9.2%0 (tab. 4). The isotopicsignatures of theses anomalous samples do not appear tobe systematically correlated with whole-rock Si02,geographic distribution, or rock type. The total range ofoxygen isotope values is similar to data from several otherporphyritic granitoids in Northeastern Brazil such asItaporanga, Conceição, and Bodocó (Sial & Longunpublished data).

Values of 018 O in quartz are higher than in feldspar andwhole rock, as expected if the oxygen isotope compositionsare essemtially at equilibriun (Tab. 4). Values of 6qz-bi for

Revista Brasikirade Geociências, Volume 17, 1987

Table 2 - Averaged values of hornblende and biotitemicroprobe analyses (oxides - Wt%) Monte das Gameleiraspluton (Total Fe reported as FeO)

Rod< Porphyritic:: GraniloidsDaril-colored intnuive

Slmplt

GA·lS. GA.oSCÁ-I'Mineral

Homblende8KllileHomble,*Hombltnde8iotiteHomblendeBiolite

RemaraCo ••Rim

NO.of,322.wysa:

•69

Si01

41.835.343.641.73>.9 41.035.1TiO:

0..6l.70..'0..6U 1.'1.8AltO)

9.314.68.'8.914.6 10.614.9F,O

20..0.18.918.119.4J8.8 18.319.4MnO

0..50..50..50..60..5 0..40..2MgO

9.312.49.59.812.6 9.912.3C.O

11.50..0.11.2lU0..0. 11.70..0.NatO

l.S0..0.1.'1.'0..0. U0..1K,O

1.29.2l.ll.l9.' 1.49.4CrIO,

0..0.0.0.0.0.0..0.0..0. 0..10..0.F

0.30..70.30.30..8 0..20..'

~96.0.9339'.295.193.9 96393.6

Table 3 - Representative chemical analyses (oxided - Wt%)and nonn calculations (CIPW - mol%), Monte das Game­leiras pluton

Rock Porphyritic:: .,.,uloidFint-paincd pniteD.rk<Olored•••r~

intl'Uliwc::ndn~.

Som'"

GA-IbGA.{)SGA-IS.GA.olGA .•••GA.()llCÁ-12.CÁ·17GA-lSb

Si01

68.866.165.869.011.667.750.249.S56.7TiO)

0..'0..60..30..'0..0.0..5U1.30..7Al,O,

15.216315.015.215.015.618.016.814.0Fe,OJ

1.0.1.21.4U0..71.42.53.42.4F<O

1.72.22.11.90..6l.77.26.''.9MnO

0..0.0..0.0..0.0.0.0..0.0..0.0..10..10..1MgO

0..50..72.0DA1.0.0..43.65.15.9C.O

2.53.32.82.2l.72.'7.'8.15.0.NatO

3.'3.83.83.54.0.3.7'.23.43.4K,O

5.3'.85.75.15.0.- 5.43.'4.24.7PIO,

0..20..30..0.0..10..0.0..20..0.0..0.0..8

Totah

99.199.398.998.999.699.298398598.6

q

20.216.512.621.922.218.4--1.2, ---0..2-----'" 31.628.534.t30.629.632.621.425.327.9.b

33.134.'34.232.435.833.422.014.730.8'"9.813.57.110.38310.219.818.29.1

di1.4U53-0..11.614.418.08.'

01

------8.18.6-hy

2.0.3.14.82.'3.4U--173

ml1.0.1.21.51.20..7l.S2.73.62.5• 0..60..90..40..,0..0.0..81.81.80..9..0..40..50..10..30..0.0..30..10..0.1..

"'------9.89.7-

Tol.h100.1100.1100.1100.1100.1100.1100.199.999.9

samples GA-l and CB-38 provide apparent equilibriumtemperatures of 550°C and 500°C, respectively, indicatingredistribution of 180during slow cooling. Minor changes inbiotite composition might be due to weathering, facilitatedby circulation of water along rock fractures, or to apervasive post-intrusive hydrothennal effect. Whole-rockÓ180 less than +100100are consistent with an I-tipe magma(O'Neil et aI. 1977), in agreement with the other evidencefor the character of the source of Monte das Gameleiras

magma.

RARE EARTH ELEMENT GEOCHEMISTRV Trace

element and REE analyses were perfonned by a modifiedversion (Fryer 1977) of the thin-film X-ray fluorescenceprocedure of Eby (1972) at the Memorial University ofNewfoundland, St. Johns. Four samples of porphyriticgranitoid and one of fme-grained granite for which there areoxygen-isotope data were selected for analysis (Tab. 5). Theanalytical erro r is estimated to be less than 10% for ali theelements tabulated.

477

Table 4 - Oxygen isotope data (0100SMOW)

Sample Whole·rockQuartzFeldsparBiotite6qz-fs6qz-bi

Porphyritic granitoidsGA-Ib+ 8.6+.9.6+ 8.8+4.80.84.8

CG·38+ 9.2+ 9.7+8.3+4.61.45.1

GA-05+ 7.0-----

GA-7b + 8.0-----Fine-grained granitesGA·la

+ 8.3-----GA-{)9 + 7.9-----GA-{)6

+ 8.3-----GA-{)8 + 8.3-----GA-{)2 + 8.3-----GA-14 + 8.6-----

Trace element concentrations among the porphyriticgranitoid samples are similar to each other, with noconspicuous depletions or enrichments (Tab. 5). Relative tothe granitoids, the fine-grained granite is enriched in Th andBa and depleted in Ni and V.

REE data from the felsic Monte das Gameleiras rocks

point to a single magmatic process without evidence forsignificant post-intrusive events. There also does notappear to be much chemical evidence for the observedapparent "mingling" of felsic magma and more maficmagma. Chondrite-nonnalized abundance patterns for theporphyritic granitoids are all similar (Fig. 4). Each sampledisplays a steep, fractionated pattern with depleted heavyrare earths. Such a depletion of HREE suggests thelikelihood of hornblende fractionation and/or residualhornblende or garnet in the magma source (Arth & Baker1976, Frey et ai. 1978). It does not support the argumentthat hornblende was added to a silicic magma by magmamingling; unless the felsic magma was already so depleted inHREE that later additions are obscured, adding"extraneous" hornblende should enrich the rocks in HREE,contrary to observation. A slight LREE enrichment amongthe samples correlates with increasing silica contentoNegative Eu anomalies are modest, Eu/Eu * varying from0.61 to 0.87, and are consistent with the "bufferingeffects" of hornblende and plagioclase in the granitoids(Noyes et aI. 1983). Whereas the porphyritic granitoidsamples exhibit a near-parallel trend, the more silicic,rme-grained granite is substantially more enriched in LREEand depleted in HREE relative to the most silicicporphyritic granitoid. Apparently the LREE continued tobe concentrated in the evolving melt.

Rb-Sr GEOCHEMISTRV OF THE MONTE DAS GAME­lEIRAS PlUTON Rubidium and strontium were

analyzed in a variety of whole-rock samples by isotopedilution at the University of Texas. Fifty to 80 sweeps tothe mass spectrum were recorded for each analysis (Tab. 6).The coefficient of variation was less than ± 0.050/00 (20)for ali of the measured isotope ratios.

A 5·point Rb-Sr isochron (Fig. 5) based on samples ofgrafite and porphyritic granitoid from across the plutongives t = 511 ± 26 Ma, and initial 87Sr/86Sr) s,n 0.7099 ±0.0004 (20 errors). Note that data points for dark-coloredintrusives fall distinctly below the isochron defined by thedata from the felsic rocks. The aberrant points were notused in the isochron caIculation; if included, they producean age estimate of about 750 Ma, which is geologicallyunreasonable given the timeframe of the Brasiliano orogeny.

478 Revista Brasi/eirade Geociências. Volume 17, 1987

~OOO

Figure 4 - Chondrite-normalized rare-earth datá, Monte dasGameleiras pluton. Abundances used for REE normalizationare Leedey chondrite data (Masuda et al 1973) dividedby 1.20 (Sun &Hanson 1977, Taylor & Gorton 1977)to make them comparable to average chondrite data

MONTE DAS GAMELEIRAS

O GA -IaO GA -Ib6 GA - 5O CG - 38O GA -7b

YbErOySm Eu GdLo C. p, Nd

100

•.~~uWwa:.....~uoo::wwa:

10

SampleGA-laGA-lbGA-Q5CG-38GA-7b

Rocktype

12222

Pb

3823322827Th

4226172419U

O4O26Rb

136150120127120Sr

316622603489524Y

1517152018Zr

292289278315262Nb

1727262724Zn

5945425949Cu

8910107Ni

O9555Ba

1,7891,5271,4841,3211,314V

2334313030Cr

O14O2Ga

1717141619La

11878736766Ce

223150137134130Pr

2214141313Nd

6849464543Sm

11837.8837.5Eu

1.81.91.82.01.5Gd

5.45.04.95.64.5Tb

1.20.80.91.40.7Dy

3.02.52.83.22.7Ho

0.30.20.00.00.0Er

0.91.11.31.50.9Yb

0.50.60.61.20.5

LREE

456360291283270LaN/SmN

6.55.95.95.15.5CeN/YbN

12060572867EujEu*

0.60.80.90.90.8

Table 5 - Whole rock trace element abundances (ppm)

Monte das Gameleiraspluton

Rock type: 1 = frne-grained granite; 2 = porphyritic granitoid Table 6 - Rb-Sr isotope data

Rb-Sr GEOCHEMISTRY OF THE FAZENDA NOVA

PLUTON As part of a reconnaissance study, Rb-Sr datawere also obtained from another classic Itaporanga-typepluton between 6 and 19 km east of Fazenda Nova,Pernambuco, about 200 km SSW of Monte das Gameleiras(Fig. 1). Unaltered porphyritie granitoid, consisting ofextremely coarse (up to 15 em) megaerysts of a1kalifeldspar set in a much fine-grained, generally mafie J:latrix,was sampled from five roadside exposures. Unlike theMonte das Gameleiras samples, the rocks from FazendaNova are strongly sheared. A major lineament marked by amylonite belt as mueh as 1 km wide trends eastward aerossPernambuco, and a splay of this main shear zone extendsinto the region of Fazenda Nova.

A whole-rock Rb-Sr isochron (Fig. 6, rabo 6)eorresponds to t = 630 ± 24 Ma (20) and initial 8 7Sr/8 6 Sr =

0.7065 ± 0.0005 (x). Analytieal errors of less than ± 0.3%were attributed to Rb/Sr ratios of the three samplesanalyzed by isotope dilution. Much larger errors of ± 3%were applied to Rb/Sr in samples PEAL-3 and -5, in whichRb and Sr were analyzed by X-ray fluoreseence at theUniversidade de São Paulo. Nevertheless, the linearity ofdata-point distribution is adequate and the best-fit line is atrue isoehron within the assigned errors (MSWD = 0.65).

Both this and the Monte das Gameleiras isoehron are

SampleRb (ppm)Sr (ppm)87Rb/86Sr87Sr/86Sr

(*) GA-02

1403351.1970.7191GA-Q5

1206030.5720.7141GA-08

1694031_2080.7187GA-12a2

1071,02603010.7100GA-13a

1633321.4180_7202GA-13b3

1725000.9910.7172GA-13p3

1654800.9930.7165GA-172

1091,0720.2940.7086

(**)PEAL-1

1722821.7610.7220PEAL-2

2094180433430.7362PEAL-3

1841493.5600.7394PEAL-4

153475340.5860.7126PEAL-5

1226100.5800.7104

Corrected for mass discrimination assuming that 86Sr/88Sr= 0.1194, Ã. = 1.42x10-11/year

2 Samp1e of "dark-colored intrusive", not included in isochroncalcu1ation

3 Samp1es of porphyritic granitoid from same outcrop; values aver­aged to a single point for isochron

4 Concentrations analyzed by XRF(*) Monte das-Gameleiras

(**) Fazenda Nova

well-behaved in spite of the presenee of shearing (FazendaNova) and megaseopie textures suggesting the "kneadingtogether" of a felsie, megaeryst-bearing magma and a moremafie, hornblende- and biotite-bearing magma (Monte das

Revista Brasikirade Geociências, Volume 17,1987

CONCLUSIONS The porphyritic granitoids and darkintrusives share similar mineral suítes and textures. If theyare genetically related, the porphyritic granitoid could haveresulted from mingling a silicic, partiy crystallized meltwith a more mafic, nearly liquid melt (Vemon 1983), orthe porphyritic rocks could result from an "unrnixing" ofsilicic partial melt and refractory material from the sourceregion (restite: White & Chappell 1977). Chemical andisotopic data at this point in our study support neitherhypothesis, however, and instead suggest that the darkintrusives are quite distinct from the more felsic rocksdespite field evidence that the two were intrudedcontemporaneously.

The Monte das Gameleiras pluton has some featurestypical of I-type granitoids, including a wide range of SiOzin varied rock types, b180 < +10%0, and a characteristicmineral assemblage of hornblende, biotite, sphene,magnetíte, and allanite. Nevertheless, the moderately highinitial 87Sr/86Sr of about 0.709, 0180 typically > +8%0,and low CaO suggest that source rocks may have had ametasedimentary component. The regularity of REEabundance pattems and of Rb-Sr isotopic behaviorindicates a rather uniform history of magma differentiationwith no substantial post-intrusive alteration. Oxygenisotope equilibrium compositions for mineral separatessupport the conclusion that post-intrusive effects wereslighi. Intrusion accompanied regional metamorphismduring the late stages of the Brasiliano Cycle.

The Monte das Gameleiras and Fazenda Nova intrusions

are examples of a large group of Itaporanga-type granitoids .Intringuingly, the various bodies are characterized by awide range of compositions (metaluminous, peralkalic,potassic), yet they share many of the same texturalfeatures, including alkali feldspar megacrysts and evidencefor "kneaded" magmas. Despíte the composítionalvariability, all appear to have been influenced by similardifferentiation processes and intrusive conditions. Theregional extent and good exposure of these plutonscommends them for further study, with implications forthe origin of similar felsic bodies in Northeastern Brazil andin "mountain roots" worldwide.

1.5

•

10

•

05

Oork-colo'ed inl,usiwes

••

1=630!. 24 Mo

815,/865,)." 0.7065' 0.0005

FAZENDA NOVA0.74

0725

Figure 5 - Five-point whole-rock isochron, Monte dasCarneleiras pluton. Data points co"esponding to a sarnpleof dark intrusive and to a mafic enclave were not usedin the isochron calculation but are indicated on the figureto illustrate that they are not colinear with data pointsrepresenting the more felsic rocks

0.73

MONTE DAS GAMELEIRAS

I a 511 ! 25 Mo

875r/865r l. '"0_7099! 0.0004

0720

cii'"..

.....

r--W 072CD

0710

li;'".."­

~0115

4

Figure 6 - Five-point whole-rock isochronfor the FazendaNova suite. Squares indica te data obtained by XRF

Gameleiras). It is noteworthy that the 5-point FazendaNova isochron, the 5-point Monte das Gameleiras isochron,and a 6-point isochron from the type Itaporanga pluton(625 ± 24 Ma, cf. Sial & Long unpublished data)correspond to somewhat diverse ages even though ali of theplutons consist of distinctive Itaporanga-type lithology.

Acknowledgements Earlier version of this manuscriptbenefited from comments by Daniel S. Baker, Calvin G.Barnes, Fred W. MacDowell, and Douglas Smith. We alsowish to thank Antonio Carlos Galindo, who providedvaluable guidance in the fiefd and supplied us with severalwhole-rock samples for analysis. This research wassupported in part by National Science Foundation (NSF)TraveI Grant 80LA/BOl to L.E. Long and by the GeologyFoundation of the University of Texas. A.N. Sial wishes tothank Conselho Nacional do Desenvolvimento Científico e

Tecnológico (CNPq) for an one-year fellowship at theUniversities of Georgia, USA, and Memorial University ofNewfoundland, Canada, where the oxygen isotope and REEanalyses were carried out.

REFERENCES

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EBY, G.N. - 1972 - Determination of rare-earth, yttrium andscandium abundances in rocks and minerals by an ion exchangeX·ny fluorescence procedure. Anal. Chem., 44:2137-2143.

FREY, FA., CHAPPELL, B.W., ROY, sn. - 1978 - Fractionationof rare-earth elements in the Tuoloumne Intrusive Series, SierraNevada batholith, California. Geology, 6:239-242 ..

FREYER, BJ. - 1977 - Rare-earth evidence in iron-formations forchanging Precambrian oxidation states. Geoch. Cosmoch. Acta,41 :361-367.

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Revista Brasileira de Geociências, Volume 17,1987

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MANUSCRITO 426Recebido em 12 de março de 1987

Revisão aceita em 04 de junho de 1987

Na Terra não devem existir muitos lugares acessíveis, onde ainda seja possível examinar áreas nunea exploradas pelo homem.

Joseph Wood Krutch, 1958, Grand Canyon, today and a11itsyesterdays. William Sloan Assoe.