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Fluids in high- to ultrahigh-temperature metamorphism along collisional

sutures: Record from fluid inclusions

Toshiaki Tsunogae a,b,⇑, M. Santosh c

a Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japanb Department of Geology, University of Johannesburg, Auckland Park 2006, South Africac Department of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan

a r t i c l e i n f o

Article history:

Available online 9 December 2010

Keywords:

Fluid inclusions

Microthermometry

Ultrahigh-temperature metamorphism

Suture zone

Clockwise and counterclockwise P–T path

Petrology tectonics

Metamorphic fluid

Palghat–Cauvery suture zone

Limpopo complex

Napier complex

a b s t r a c t

Petrographic studies and microthermometric investigations on fluid inclusions associated with high- to

ultrahigh-temperature metamorphic rocks in three major Precambrian suture zones on the globe demon-

strate the dominant occurrence of CO2-rich fluids. These rocks form part of hot orogens developed along

collisional plate boundaries. The sapphirine-quartz-bearing Mg–Al-rich rock from the Palghat-Cauvery

Suture Zone, a trace of the Cambrian Gondwana suture zone in southern India, preserves evidence for

a prograde high-pressure event and subsequent peak ultrahigh-temperature metamorphism along a

clockwise path, and contains abundant CO2-rich inclusions in corundum, garnet, and sapphirine. Most

of the fluid inclusions are either primary or secondary and preserve low-density CO2-rich fluids

(0.569–0.807 g/cm3). Similar low-density CO2-rich fluid inclusions (0.853–0.953 g/cm3) are also present

in pelitic granulites from the Limpopo Complex of southern Africa, a Neoarchean granulite-facies orogen

formed by continent–continent collision. In contrast, the garnet–orthopyroxene granulite from Tonagh

Island in the Neoarchean Napier Complex in East Antarctica contains very high-density primary

(1.095–1.129 g/cm3) and secondary (0.960–1.179 g/cm3) carbonic inclusions in garnet and quartz. The

calculated isochores for the fluid inclusions from the Palghat-Cauvery Suture Zone and the Limpopo

Complex yield significantly lower-pressure estimates than those predicted from peak metamorphicconditions. We interpret this as a result of significant density decrease due to rapid decompression along

a clockwise P–T trajectory. In contrast, the estimated isochores for primary inclusions in garnet–

orthopyroxene granulites from the Napier Complex are consistent with the peak P–T conditions

estimated from mineral phase equilibria for the Tonagh Island rocks, suggesting that most of the fluid

inclusions in these rocks did not undergo any marked effect of volume change and density decrease.

The contrasting fluid densities among the localities investigated in this study are probably related to

the nature of the P–T trajectory; the Tonagh Island rocks had a near-isochoric exhumation history

whereas the metamorphic orogens in the other two sutures witnessed rapid decompression. Our results

suggest that whereas the composition of the syn-metamorphic fluids are preserved in all cases, density

reversal occurs within inclusions as a function of the tectonic history and exhumation style.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

The P–T–t architecture and fluid regimes of metamorphic

orogens have received wide attention in evaluating the history of

evolution of continents and supercontinents, and have also been

central to some of the recent debates surrounding the plate tec-

tonic paradigm (e.g., Brown, 2010; Ernst, 2010; Maruyama et al.,

2010; Tirone and Ganguly, 2010; Santosh, 2010; Santosh et al.,

2009a). Chemical and physical characterization of the role of fluids

associated with high-grade metamorphism has been one of the fo-

cal themes in petrological research, particularly in understanding

the stability of mineral assemblages, fluid–rock interaction pro-

cesses, and degree of partial melting in lower crust (e.g., Santosh

and Omori, 2008; Omori et al., 2009; Touret, 2009). The nature of

fluids associated with metamorphic processes has been inferred

through a number of techniques including thermodynamic calcula-

tions of mineral assemblages, geochemical and isotopic studies and

investigation of fluids trapped within inclusions in various miner-

als. Among these, detailed investigation of the fluid inclusions in

various metamorphic minerals provides direct information on the

nature, composition, and density of fluids at various stages of the

metamorphic processes (e.g., Touret, 1985, 2001, 2009). Recent

investigations on the characterization of fluids associated with

1367-9120/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.jseaes.2010.11.016

⇑ Corresponding author at: Graduate School of Life and Environmental Sciences,

University of Tsukuba, Ibaraki 305-8572, Japan.

E-mail addresses: [email protected] (T. Tsunogae), santosh@kochi-u.

ac.jp (M. Santosh).

Journal of Asian Earth Sciences 42 (2011) 330–340

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s e a e s

http://dx.doi.org/10.1016/j.jseaes.2010.11.016mailto:[email protected]:santosh@kochi-u.%20ac.jpmailto:santosh@kochi-u.%20ac.jphttp://dx.doi.org/10.1016/j.jseaes.2010.11.016http://www.sciencedirect.com/science/journal/13679120http://www.elsevier.com/locate/jseaeshttp://www.elsevier.com/locate/jseaeshttp://www.sciencedirect.com/science/journal/13679120http://dx.doi.org/10.1016/j.jseaes.2010.11.016mailto:santosh@kochi-u.%20ac.jpmailto:santosh@kochi-u.%20ac.jpmailto:[email protected]://dx.doi.org/10.1016/j.jseaes.2010.11.016


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high- to ultrahigh-temperature metamorphism, particularly along

collisional suture zones, have provided critical information on

P–T evolution and exhumation history of hot orogens along colli-

sional plate boundaries (e.g., Ohyama et al., 2008; Santosh et al.,

2008, 2010; Tsunogae and Santosh, 2010). However, not many

studies have attempted to compare the fluid inclusion data from

ultrahigh-temperature rocks formed within Precambrian collision

sutures in understanding the nature and role of fluids.

In this study, we synthesize information on the occurrence,

composition, and density of fluid inclusions trapped in various

granulite-facies minerals from ultrahigh-temperature metamor-

phic rocks in three collisional orogenic belts: Neoproterozoic to

Cambrian Palghat-Cauvery Suture Zone (Southern India; Fig. 1a),

Neoarchean to Mesoproterozoic Limpopo Complex (Southern Afri-

ca; Fig. 1b), and Neoarchean Napier Complex (East Antarctica; Fig.

1c). Together with newly obtained data, we attempt a comparison

on the nature of the trapped fluids from these regions. The Palghat-

Cauvery Suture Zone and the Limpopo Complex are regarded as

collisional sutures that record the history of high-pressure and

ultrahigh-temperature metamorphism (e.g., van Reenen et al.,

2008; Santosh et al., 2009b) and should thus preserve fluids

associated with the collisional event. The tectonic evolution of

Fig. 1a. Geological maps of the study areas with sample localities (solid circles). Geological map of southern India showing various granulite blocks and major shear/suturezones (modified after Santosh and Sajeev, 2006). ACSZ: Achankovil Shear Zone, CSZ: Cauvery Suture Zone, PCSZ: Palghat-Cauvery Suture Zone.

T. Tsunogae, M. Santosh/ Journal of Asian Earth Sciences 42 (2011) 330–340 331


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the Napier Complex is not well known, but is also regarded as a

possible collisional orogen. Our results provide new evidence on

the role ofCO2-rich fluids along collisional sutures as well as impli-

cations on density reversal as a probable function of the tectonic

history.

2. Palghat-Cauvery Suture Zone, Southern India

The southern granulite terrane in India comprises several crus-

tal blocks which were welded together during the Late Neoprotero-

zoic to Early Cambrian collisional orogeny related to the finalamalgamation of the Gondwana supercontinent (e.g., Santosh

et al., 2009b). The Palghat-Cauvery Suture Zone (PCSZ) is an E-W

trending network of transpressional shears that mark the bound-

ary between the Late Neoproterozoic to Early Cambrian Madurai

Block to the south and the Archean Dharwar Craton to the north

(Fig. 1a) (Chetty and Bhaskar Rao, 2006; Collins et al., 2007a; San-

tosh et al., 2006, 2009b; Clark et al., 2009a; Yellapa et al., 2010;

Sato et al., 2011; Naganjaneyulu and Santosh, 2010). Available geo-

chronological studies on the high-grade metamorphic rocks from

this region have confirmed the widespread effect of a ca. 550–

530 Ma thermal event related to the collisional amalgamation of

the Gondwana supercontinent (e.g., Santosh et al., 2003, 2006,

2009b; Collins et al., 2007a). Previous petrological studies on this

region documented the evidence for a prograde high-pressure(HP) event and subsequent ultrahigh-temperature (UHT) meta-

morphism along a clockwise path (e.g., Shimpo et al., 2006; Collins

et al., 2007b; Clark et al., 2009b; Nishimiya et al., 2010 ).

Fluid inclusion studies of metamorphic rocks within the PCSZ

include those on Mg–Al-rich granulites (Ohyama et al., 2008) and

mafic granulites (Nishimiya et al., 2008; Santosh et al., 2010).

The results from these studies show the occurrence of abundant

CO2-rich inclusions in Mg–Al-rich rocks and retrogressed eclogites

(high-pressure granulites). Most of the reported inclusions are sec-

ondary in origin, probably trapped during the post-peak decom-

pression stage, and generally contain low-density (0.59–0.95 g/

cm3) CO2. However, primary and pseudosecondary high-density

(>1.06 g/cm3) CO2-rich fluid inclusions have been found in matrix

staurolite and plagioclase from the Mg–Al-rich rocks (Ohyamaet al., 2008). Primary fluid inclusions within garnet in the mafic

granulites from Kanja Malai and Sittampundi within the PCSZ are

also high-density CO2 (up to 1.15 g/cm3; Santosh et al., 2010).

However, recent geochronological studies suggest that some of

these rocks are relicts of an earlier high-grade metamorphic event

in the Neoarchean (e.g., Sato et al., in press; Saitoh et al., 2011 ).

This is also consistent with the occurrence of extremely high-den-

sity carbonic fluid inclusions in an Archean charnockite from the

Salem Block north of the PCSZ (Santosh and Tsunogae, 2003; San-

tosh et al., 2004).

In order to evaluate the nature of fluids associated with the

high-grade metamorphism within the PCSZ, we obtained new

petrographic and microthermometric data of fluid inclusions in asapphirine-bearing Mg–Al-rich rock from Panangad in the central

part of the PCSZ. The analyzed rock (sample MD19-2D) contains

Mg-rich staurolite inclusions in garnet and the staurolite is partly

replaced by sapphirine + quartz corona (Nishimiya et al., 2010;

Tsunogae and Santosh, 2010), suggesting a prograde HP metamor-

phism and subsequent peak UHT metamorphism. The rock is com-

posed of porphyroblastic garnet in the matrix of gedrite,

sapphirine, corundum, and spinel, with accessory staurolite and

rutile (Fig. 2a). Biotite and högbomite are present in the matrix

as retrograde minerals. Poikiloblastic coarse-grained (up to 3 cm)

garnet in the sample contains inclusions of sapphirine, staurolite,

biotite, and rutile, which are all subidioblastic to xenoblastic (Fig.

2b). The inclusion staurolite is fine-grained (0.7 mm in length)

and yellowish in color. It is commonly surrounded by sapphirineor sapphirine + quartz corona (Fig. 2b), suggesting the formation

of sapphirine + quartz after staurolite. As the host garnet displays

an irregular grain boundary with sapphirine, Nishimiya et al.

(2010) argued that garnet also reacted to form sapphirine + quartz

from staurolite through the progress of the following FMASH con-

tinuous reaction which probably took place during the prograde

metamorphism.

Grtþ St ! Spr þ QtzþH2O ð1Þ

Matrix sapphirine is coarser in grain size (up to 7 mm) than that

in garnet, and often intergrows with corundum(up to 2.5 mm) and

spinel (up to 3 mm). As the minerals are present as coarse-grained

subidioblastic phases together with sillimanite (pseudomorph

after kyanite) and gedrite, they were probably formed around thepeak of metamorphism.

Fig. 1b. Geological maps of the study areas with sample localities (solid circles). Geological map of the central part of the Limpopo Complex (after Brandl, 1992).

332 T. Tsunogae, M. Santosh / Journal of Asian Earth Sciences 42 (2011) 330–340


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Fluid inclusions were found in corundum, sapphirine, and gar-

net, which are all regarded as peak metamorphic minerals. The ob-served inclusions are classified into two groups according to the

classification of Roedder (1984): primary and secondary inclusions.

Primary fluid inclusions are present in medium-grained (0.2–

1.2 mm) corundum (Fig. 3a) and sapphirine (Fig. 3b) in the matrix.

The trapped inclusions are ovoidto irregular in shape andrelatively

coarse-grained as compared to the secondary inclusions discussed

later. They occur as aggregates of a few to ten inclusions within

the core or mantle of host corundum and sapphirine. Such isolated

inclusions or inclusion clusters, which are totally contained within

a grain and not focused along identifiable cracks, are regarded to

have been trapped during the growth of the host minerals (e.g.,

Touret, 2001). They potentially preserve the information on

prograde to peak metamorphic fluids. Secondary inclusions are, in

contrast, distributed along trails in corundum (Fig. 3c) and garnet(Fig. 3d). The inclusion arrays continue to the edge of the host min-

erals. If host mineral is fractured in the presence of a fluid subse-

quent to its growth, the fluid enters the fracture and rehealing of the fracture would result in such inclusions aligned along linear ar-

rays that cut across the crystal. Therefore, the secondary inclusions

were probably trapped after the crystallization of host minerals,

thereby possibly preserving retrograde metamorphic fluids. The

secondary inclusions in the sampleexamined in this study are com-

monly rectangular in shape (Fig. 3c) and verythin (5lm), although

some tubular and vermicular fluid inclusions, which have been of-

ten reported from garnets in granulite-facies rocks (cf., Touret,

1985; Ohyama et al., 2008), are present in some garnet grains

(Fig. 3d). A solid phase (possibly carbonate) is also commonly pres-

ent in the cavity of the secondary fluid inclusions in garnet.

Microthermometric measurements of the inclusions were per-

formed with a Linkam heating/freezing system at the University

of Tsukuba. In the heating/cooling experiment, melting tempera-ture (T m) and homogenization temperature (T h) of two-phase

Fig. 1c. Geological maps of the study areas with sample localities (solid circles). Geological map of Tonagh Island in the Napier Complex, East Antarctica (after Osanai et al.,

1999, 2001).



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(gas–vapor) inclusions were measured. Heating rates of the sam-ples are 1 C/min for T m and 5 C/min for T h. Repeated microther-

mometric measurements indicate that the precision of

microthermometric results reported in this study is within ±0.1 C

for T m and ±0.2 C for T h. The results of microthermometric

measurements are shown in histograms (Fig. 4) and summarized

in Table 1. Fluid densities and isochores were calculated using the

computer program ‘‘MacFlinCor’’ developed by Brown and Hage-

mann (1994) based on the equation and thermodynamic data of

Brown and Lamb (1989).

The analyzed primary and secondary fluid inclusions in sample

MD19-2D from the PCSZ display T m between 57.0 and 56.6 C

(Fig. 4 and Table 1). The temperature range obtained is very close

to the triple point of CO2 (56.6 C), indicating that the dominant

fluid present during the prograde and retrograde evolution of theserocks is CO2. No H2O-bearing fluid inclusion was identified in the

examined rock, although some retrograde hydrous minerals such

as biotite and högbomite are present. All the primary and second-

ary inclusions homogenize into the liquid phase at relatively high

temperature (T h = + 16.7–30.5 C, which corresponds to low CO2densities of 0.569–0.807 g/cm3) (Fig. 4 and Table 1). Among them,

secondary fluid inclusions in corundum show relatively lower T h(+16.7–18.3 C, 0.792–0.807 g/cm3), but the densities of these

inclusions are lower than those from the Napier Complex in East

Antarctica as discussed in a later section.

3. Limpopo Complex, Southern Africa

The Limpopo Complex is a classic example of granulite-faciesorogen formed during the collision of Kaapvaal and Zimbabwe

Cratons (e.g., van Reenen et al., 1987, 2008). The Central Zoneof the complex is composed mainly of various supracrustal lithol-

ogies such as metasedimentary and metavolcanic rocks distrib-

uted along a dominant N–S trending foliation (Fig. 1b) (e.g.,

Brandl, 1983). The peak metamorphic age from the Central Zone

is still a matter of controversy as both Neoarchean (2.6–2.7 Ga)

(e.g., Boshoff et al., 2006; van Reenen et al., 2008) and Palaeopro-

terozoic (2.0 Ga) (e.g., Jaeckel et al., 1997; Zeh et al., 2004) ages

have been reported from the same metamorphic complex. In a

recent work, van Reenen et al. (2008) argued based on detailed

geochronological studies that the major collisional event took

place at 2.6–2.7 Ga. Previous petrological investigation of the

Central Zone demonstrated that the zone underwent peak granu-

lite-facies metamorphism at about 800–900 C and 8–10 kbar fol-

lowed by near isothermal decompression to 4–5 kbar along aclockwise P–T path (e.g., Horrocks, 1983; Windley et al., 1984;

Tsunogae and Miyano, 1989; Droop, 1989; Tsunogae et al.,

1992; Zeh et al., 2004). However, recent petrological investiga-

tions of sapphirine-bearing Mg–Al-rich rocks from the Central

Zone demonstrated that the peak metamorphic condition of the

granulites from the Central Zone is higher, 890–930 C at 9–10

kbar (e.g., Tsunogae and van Reenen, 2006). Tsunogae and van

Reenen (2007, 2011) recently reported Mg-rich ( X Mg 0.58) stau-

rolite inclusions within garnet in garnet–orthopyroxene granulite

and its replacement by sapphirine + quartz, suggesting prograde

HP (P > 14 kbar) metamorphism and peak UHT (T 1000 C)

metamorphism along a clockwise P–T path. However, the fluid

composition associated with the HP-UHT metamorphism is not

known due to the lack of systematic fluid inclusion study on

the zone.

Fig. 2. Photomicrographs of granulites discussed in this study. (a) Intergrowth of corundum (Crn), gedrite (Ged), and sapphirine (Spr) around porphyroblastic garnet-rich in

Mg–Al-rich rock (sample MD19-2D) from Panangad in the PCSZ. Retrograde biotite (Bt) occurs along thegrain boundaries of the minerals. Staurolite and rutile are not shown

in the figure as they are minor minerals. (b) Sapphirine + quartz corona around yellowish staurolite (St) inclusion in poikiloblastic garnet in sample MD19-2D. Rt; rutile. (c)

Porphyroblastic garnet in the matrix of cordierite (Crd), biotite, sillimanite (Sil), and quartz in pelitic granulite (sample 3–3L) from the Limpopo Complex. (d) Subidioblastic

garnet, orthopyroxene (Opx), and quartz in a dry granulite from the Napier Complex (sample B98021001C from Tonagh Island). Opaque minerals are ilmenite and magnetite.

(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



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In this study, we obtained new fluid inclusion data on a pelitic

granulite (sample 3–3L) collected from a road cut along Messina-

Tshipise road, about 2 km northwest of Tshipise in South Africa

(Fig. 1b). The pelitic granulite contains poikiloblastic garnet in

the matrix of quartz, sillimanite, plagioclase, cordierite, biotite,

and rutile. Garnet occurs as subidioblastic porphyroblasts (up to

3 mm in diameter). The rock typically contains biotite, sillimanite,

quartz, and rutile (Fig. 2c). Garnet occurs commonly in direct con-

tact with sillimanite and quartz, but it is partially replaced by cor-

dierite, suggesting the progress of retrograde decompression

reaction (2) (Fig. 2c):

Grtþ SilþQtz ! Crd ð2Þ

Sillimanite is the only Al2SiO5 mineral in the sample, although

Tsunogae and Miyano (1989) reported relict kyanite in sillimanite

from this locality as an evidence of prograde HP metamorphism.

Sillimanite often occurs as fibers and prisms in the matrix and also

as inclusions within garnet. Cordierite occurs as sub- to xenoblastic

minerals (up to 0.5 mm) around garnet as a retrograde product.

Biotite occurs as laths or flakes in the matrix, forming the foliation

with sillimanite. It is also present as inclusions within garnet. Bio-

tite locally replaces garnet, indicating that some biotite is of retro-

grade origin. Plagioclase and K-feldspar are minor and present as

subidioblastic minerals in the matrix. Tsunogae and Miyano

(1989) calculated peak P–T condition of T = 800 C and P > 10 kbar

for the rocks on the basis of geothermobarometry and the occur-rence of relict kyanite.

Fig. 3. Photomicrographs of representative fluid inclusions discussed in this study. Numbers indicate homogenization temperatures as discussed in the text. Scale bars

indicate 20 microns. (a) Large primary inclusions in corundum in sample MD19-2D from Panangad. (b) Primary inclusions with irregular to ovoid cavities in sapphirine in

sample MD19-2D. (c) Trails of secondary rectangular inclusions in corundum in sample MD19-2D. (d) Irregular inclusions with a number of solid phases in garnet in sample

MD19-2D. (e) A trail of secondary fluid inclusions in plagioclase in sample 3–3L fromTshipise. (f) Rounded to ovoid primary inclusions in garnet in sample B98021001C from

Tonagh Island. (g) Ovoid primary inclusions in quartz in sample B98021001C. (h) Aggregates of fine-grained rectangular and rounded inclusions aligned along fractured

secondary planes in garnet in sample B98021001C. (i) Secondary fluid inclusions in quartz in sample B98021001C.



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Fluid inclusions are very rare in the pelitic granulite sample ex-

cept some minor secondary inclusions in plagioclase. Although

garnet in the rock contains numerous ovoid and rectangular inclu-

sion cavities as apparently primary phases, they are all fluid-free

and possibly filled with secondary minerals such as carbonates.

This is probably due to the extremely rapid exhumation history

of the Limpopo Complex wherein many of the inclusions under-went explosion and leakage of the trapped fluids, as discussed in

a later section. The secondary inclusions in plagioclase are fine-

grained (2–6lm) and ovoid in shape, and aligned along linear

arrays that cut across the host mineral (Fig. 3e). The inclusions dis-

play T m between 59.9 and 58.7 C (Fig. 4 and Table 1), which is

about 2 C lower than the T m of the Panangad sample from south-

ern India. Such a slight depression in T m of the carbonic inclusions

might indicate the presence of additional fluid components such as

N2 and CH4 with the dominantly CO2-rich fluid. The secondary car-

bonic inclusions homogenize into liquid phase at a wide T h range of

7.2 to +11.2 C (Fig. 4), which corresponds to CO2 densities of

0.853–0.953 g/cm3. Similar low-density CO2 has also reported from

sapphirine-bearing granulite from the Central Zone of the Limpopo

Complex with densities of 0.72–0.91 g/cm3 (Tsunogae and van

Reenen, 2007).

4. Napier Complex, East Antarctica

TheNapier Complexof Enderby Land, East Antarctica, is a typical

example of Neoarchean regional metamorphic complex that under-

went T > 1100 C UHT metamorphism (e.g., Ellis, 1980; Harley and

Hensen, 1990; Harley, 1998; Harley and Motoyoshi, 2000; Hokada,

2001). Amundsen Bay area in the western part of the Napier Com-

plex corresponds to the highest-grade region of the complex fromwhere equilibrium sapphirine + quartz assemblage has been widely

reported. Orthopyroxene + sillimanite + garnet assemblage, osumi-

lite, and inverted pigeonite in meta-ironstone in this area provide

additional evidence for regional UHT metamorphism (e.g., Harley,

1998, and references therein). Although the tectonic evolution of

the Napier Complex is not clearly known, the widespread occur-

rence of UHTmetamorphic rocks in thisregion suggeststheir forma-

tion associated with collisional orogeny during Neoarchean. Ellis

(1987) argued that the Napier Complex corresponds to the lower

part of a doubly thickened crustal segment as the complex records

decompression and subsequent isobaric cooling metamorphic his-

tory. Such a doubly thickened crust represents collisional orogeny.

We carried out fluid inclusion study of UHT granulites from

Tonagh Island in the Napier Complex for a comparison with the

data from the collisional sutures in southern India and South Africa.

Previous investigations on sapphirine-bearing UHT granulites from

Tonagh Island in the Napier Complex suggested a counterclockwise

P–T evolution for the rocks (e.g., Tsunogae et al., 2002). Tsunogae et

al. (2002) reported very high-density (up to 1.2 g/cm3) CO2-rich

inclusions within garnet, sapphirine, orthopyroxene, and quartz in

sapphirine-bearing UHT granulites from Tonagh Island. As they oc-

cur as primary inclusions, the fluids were inferred to have been

trapped during peak UHT metamorphism. Tsunogae et al. (2002)

also reported some late-stage secondary fluid inclusions, which

are also CO2-rich and which possess high density (1.1 g/cm3).

In this study, we obtained new fluid inclusion data for garnet–

orthopyroxene granulite (sample B98021001C) from the eastern

part of Tonagh Island. The island is composed of various UHT

granulites including felsic to ultramafic granulites, garnet gneiss,sapphirine granulite, and magnetite quartzite (Fig. 1c, Osanai et

al., 1999, 2001). The sample examined in this study contains gra-

noblastic garnet, orthopyroxene, quartz, ilmenite, and magnetite

(Fig. 2d). No hydrous mineral is present in the sample, which im-

plies high-grade metamorphism under dry conditions. Primary

fluid inclusions were found only rarely in garnet and quartz grains.

Those in garnet are rounded to ovoid in shape and fine-grained (2–

8 lm). They occur as aggregates of five to ten isolated inclusions

(Fig. 3f). The analyzed fluid inclusions show T m between 57.7

and 57.5 C and T h between 43.7 and 34.6 C (Fig. 4 and Table

1), which translate into very high CO2 densities in the range of

1.095–1.129 g/cm3. Primary inclusions in quartz, which are ovoid

in shape (Fig. 3g), display nearly consistent T m (57.2 to

57.0 C) and T h (32.4 to 25.2 C, d = 1.056–1.086 g/cm3).Garnet also contains secondary fluid inclusion clusters that com-

prise tens to hundreds of fine-grained rectangular and rounded

inclusions aligned along fractured secondary planes (Fig. 3h). Their

melting temperatures are consistent with those of primary inclu-

sions (57.6 to 56.9 C), although the homogenization tempera-

tures of the secondary inclusions show a wide range from

57.2 C to 27.2 C (d = 1.064–1.179 g/cm3). Such high-density

carbonic fluids are also present in quartz (Fig. 3i) (T m = 58.2 to

56.8 C, T h = 27.0 to 5.6 C, d = 0.960–1.064 g/cm3).

5. Isochores

The composition and density of the fluid phase trapped withininclusions can be represented through isochores (line of constant

Fig. 4. Histograms showing the distribution of melting (a) and homogenization and

(b) temperatures of carbonic inclusions in granulites from Southern India (sample

MD19-2D), Southern Africa (sample 3–3L), and East Antarctica (sample

B98021001C). See Table 1 for abbreviations.



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volume) in P–T space. The analyzed carbonic inclusions are corre-lated to the pure CO2 system, as supported by the T m data. The

isochores calculated using average and error (standard deviation)

of T h data for each category of inclusion are plotted in P–T diagrams

(Fig. 5) together with the available P–T conditions and paths from

petrological studies in the three areas. As the figure demonstrates,

the isochores plotted for the Palghat-Cauvery Suture Zone and the

Limpopo Complex are located far below the peak P–T conditions

inferred from the occurrence of sapphirine + quartz assemblages

and geothermobarometry. Although the peak pressure conditions

of the two localities are not known, judging from the presence of

Mg-rich staurolite inclusions in garnet, the peak pressures are in-

ferred to have exceeded 15 kbar. In contrast, the isochores esti-

mated from fluid inclusions in Tonagh Island in the Napier

Complex are consistent with available P–T conditions of the local-ity. The implications of these results are discussed in the next

section.

6. Discussion

6.1. Carbonic fluids in UHT metamorphism

Our new petrographic and microthermometric data of fluid

inclusions in high-grade metamorphic rocks formed at ultrahigh-

temperature conditions from the Palghat-Cauvery Suture Zone

(Southern India), Limpopo Complex (Southern Africa), and Napier

Complex (East Antarctica) confirm the presence of carbonic fluids

trapped as primary and secondary phases within the high-grade metamorphic minerals. Our data are consistent with the

observations in previous studies that identified CO2 as a dominantfluid component in UHT metamorphic rocks (e.g., Fonarev et al.,

2001; Tsunogae et al., 2002; Santosh et al., 2004, 2008; Sarkar

et al., 2003; Cuney et al., 2007; Santosh and Kusky, 2010). Although

CH4, N2, and H2O were previously detected by laser Raman spectro-

scopic analyses of some inclusions, these fluids occur only in trace

amounts (e.g., Tsunogae et al., 2008a, b). This is consistent with the

results obtained in the present study where the microthermomet-

ric measurements show melting temperature close to the triple

point of pure CO2. As some carbonic fluid inclusions are present

as primary phases in high-grade minerals (e.g., garnet, sapphirine,

corundum), the trapped fluid probably corresponds to prograde to

peak metamorphic fluid, although the timing of entrapment of car-

bonic inclusions in some granulite-facies rocks has been debated

(e.g., Lamb et al., 1987, 1991). Since Touret (1971) first reporteda transition from H2O-dominant to CO2-dominant fluid inclusions

in minerals while passing from amphibolite to granulite-facies

rocks in southern Norway, numerous studies in various regions

of the world documented CO2-rich fluids associated with granu-

lite-facies rock. The CO2-rich fluids are considered to play an

important role in lowering the water activity and thereby stabiliz-

ing dry mineral assemblages in granulites (e.g., Santosh, 1992;

Newton, 1992; Santosh and Omori, 2008), which is consistent with

the occurrence of anhydrous mineral assemblages such as sapphi-

rine + quartz, orthopyroxene + sillimanite + quartz, and gar-

net + orthopyroxene + quartz, which were stabilized at low H2O-

activity conditions. Although the ultimate source of such carbonic

fluids is not known, the carbon isotopic signature of CO2 in garnet

from the PCSZ suggests a mixture of mantle-derived and carbon-ate-derived CO2 (Tsunogae et al., unpublished data).

Table 1

Summary of petrography and microthermometric measurements of fluid inclusions in UHT granulites from Palghat-Cauvery Suture Zone (Southern India), Limpopo Complex

(Southern Africa), and Napier Complex (East Antarctica).

Sample no. Host minerala Melting temperature (C) Homogenization temperature

(C)

Density

(g/cm3)

Type of

inclusioncMorphologyd Size (lm) Referencee

Min. Max. Averageb Min. Max. Averageb

Palghat-Cauvery suture zone

MD19-2D Crn 57.0 56.9 56.9 ± 0.1 23.0 25.8 24.8 ± 0.9 0.699–0.739 PR RE,NC 15–20 1

Spr 57.0 56.9 57.0 ± 0.1 22.0 25.4 23.0 ± 1.2 0.705–0.752 PR IR 5–12 1

Crn 56.7 56.6 56.6 ± 0.1 16.7 18.3 17.4 ± 0.4 0.792–0.807 SC RE 3–25 1

Grt 56.9 56.8 56.9 ± 0.1 23.3 30.5 26.3 ± 3.1 0.569–0.735 SC IR, TU 5–20 1

MD16-1G PI 56.9 56.9 56.9 25.8 26.1 25.9 ± 0.1 1.058–1.060 PR RE 6–10 2

PI 56.9 56.8 56.9 ± 0.1 9.9 18.0 14.7 ± 2.8 0.795–0.863 PS OV, IR, RE 7–20 2

St 56.9 56.9 56.9 19.2 19.2 19.2 1.029 PS IR 8 2

St 57.3 56.6 56.8 ± 0.1 11.0 27.4 21.9 ± 5.0 0.670–0.854 PS IR, RE 3–30 2

Grt 56.9 56.6 56.7 ± 0.1 12.9 30.0 24.1 ± 3.6 0.596–0.839 SC IR, TU,OV 5–30 2

Limpopo complex

3–3L PI 61.1 58.7 59.3 ± 0.8 4.3 11.2 6.2 ± 4.6 0.853–0.953 SC OV 2–6 1

MPL1872 Qtz 57.2 56.8 57.0 ± 0.1 3.6 21.8 6.1 ± 3.1 0.754–0.906 PR NC. OV 2–10 3

Grt 58.6 56.4 57.6 ± 0.5 9.1 28.4 20.2 ± 4.6 0.647–0.868 PS IR, TU 5–25 3

PI 57.9 57.0 57.3 ± 0.3 7.9 16.1 12.2 ± 3.0 0.720–0.877 PS RE 3–5 3

Qtz 56.9 56.4 56.7 ± 0.2 27.0 27.7 27.3 ± 0.3 0.663–0.678 SC OV 2–4 3

Tonagh Island, Napier Complex

B98021001C Grt 57.7 57.5 57.6 ± 0.1 43.7 34.6 40.3 ± 3.4 1.095–1.129 PR OV 2–8 1

Qtz 57.2 57.0 57.1 ± 0.1 32.4 25.2 28.5 ± 2.6 1.056–1.086 PR OV 5–12 1Grt 57.6 56.9 57.1 ± 0.2 57.2 27.2 40.7 ± 9.5 1.064–1.179 SC NC 2–15 1

Qtz 58.2 56.8 57.2 ± 0.3 27.0 5.6 20.4 ± 5.7 0.960–1.064 SC NC 2–10 1

B98021104A Qtz 56.8 56.4 56.6 ± 0.1 31.5 13.2 23.6 ± 3.9 1.000–1.083 PR, PS IR,NC 2–40 4

Spr 56.7 56.4 56.6 ± 0.1 14.3 14.3 14.3 1.005 PR IR 3–4 4

B98022208C Qtz 57.2 56.6 56.9 ± 0.1 29.1 2.5 26.3 ± 1.7 0.913–1.073 PS NC 2–5 4

Grt 56.7 56.4 56.6 ± 0.1 24.8 4.2 12.9 ± 7.4 0.902–1.054 PS NC 2–8 4

Opx 56.6 56.6 56.6 3.7 3.7 3.7 0.950 PS NC 2–10 4

a Crn: corundum, Grt: garnet, Opx: orthopyroxene, PI: plagioclase, Spr: sapphirine, St: staurolite, Qtz: quartz. Mineral abbreviation with asterisk indicates higher-density

inclusions.b Error indicates standard deviation.c PR: primary, PS: pseudosecondary, SC: secondary.d IR: irregular shape, TU: tubular shape, OV: ovoidal shape, RE: rectangular shape, NC: negative crystal.e 1: This study, 2: Ohyama et al. (2008), 3: Tsunogae and van Reenen (2007), 4: Tsunogae et al. (2002).



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6.2. Implications for P–T path

Although our fluid inclusion data from ultrahigh-temperature

granulites in three major continental collision belts demonstrate

that CO2 was the dominant metamorphic fluid present during the

development of the hot orogens, there is a significance difference

in fluid density among the granulites which were exhumed along

a clockwise P–T path (PCSZ and Limpopo Complex) and those

which were brought up along an counterclockwise path (Napier

Complex). The CO2 isochores derived from fluid inclusion datafrom the UHT rocks in the PCSZ and the Limpopo Complex yield

a significantly lower-pressure estimate than those predicted from

peak metamorphic conditions by geothermobarometry (see Fig.

5a and b). Since the near-pure CO2 fluid composition corresponds

well with the anhydrous conditions required for the equilibration

of the peak mineral assemblages in these rocks, we interpret the

low density as a reflection of significant density reversal due to

inclusion cavity volume changes following rapid decompression

of the rocks along a clockwise P–T trajectory. A wide distribution

of T h values continuously from +16.7 to +30.5 C for the inclusions

in the granulite from PCSZ (Fig. 4) further attests to this inference.

Similar primary fluid inclusions with markedly low-density CO2have also been reported from some UHT terranes elsewhere such

as from the Trivandrum Block (Fonarev et al., 2001) and MaduraiBlock (Tsunogae et al., 2008a) in southern India, and the Bunt Is-

land in East Antarctica (Tsunogae et al., 2008b). All these rocks

show petrological evidence for rapid decompression after the peak

metamorphism along a clockwise P–T history. Such low-density

CO2 inclusions are also found in some eclogites (e.g., Fu et al.,

2003), with totally offset isochors that do not correspond with

the peak metamorphic P–T conditions of the rocks, but intersect

the P–T trajectory at the late retrograde stage. The isochores which

define very low-pressure conditions from the PCSZ samples (Fig.

5a) suggest that the fluid density has been modified even after

the retrograde metamorphism probably during the late stages of

exhumation.

In contrast, the estimated isochores for primary inclusions in

garnet–orthopyroxene granulite from the Napier Complex are con-sistent with the peak P–T conditions of UHT rocks in Tonagh Island

(1000–1100 C at 9 kbar), suggesting that most of the fluid

inclusions in the Napier rocks preserve the original density. This

is probably due to the style of the exhumation path characterized

by near-isochoric exhumation, that is, P–T path running parallel to

the slope of the isochores of the high-density inclusions (Tsunogae

et al., 2002). Our results therefore suggest that the preservation or

absence of primary high-density fluid inclusions might be domi-

nantly controlled by the P–T path and the post-peak exhumation

history of the orogens.

Acknowledgements

We express our sincere thanks to Mr. Syed Meha Fooz (Babu),

Ms. Preetha Warrier, late Prof. Takashi Miyano, and members of

JARE-39 geology team for valuable field support and discussion.

Dr. Shoujie Liu and Prof. Zeming Zhang provided helpful comments

that aided in improving our presentation. We thank these referees

as well as Dr. T.R.K. Chetty for his editorial comments. Partial fund-

ing for this project was produced by a Grant-in-Aid for Scientific

Research (B) from the Japanese Ministry of Education, Culture,

Sports, Science, and Technology (MEXT) to Tsunogae (Nos.

20340148 and 22403017) and JSPS-INSA Joint Research Program.

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