Easter Ghat Granulite Belt Metamorphism

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Geology

doi: 10.1130/0091-7613(2003)0312.0.CO;2

2003;31;51-54GeologySushmita Sarkar, M. Santosh, Somnath Dasgupta and M. Fukuokathe Eastern Ghats granulite belt, India

associated with ultrahigh-temperature metamorphism in2Very high density CO

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Geology; January 2003; v. 31; no. 1; p. 5154; 4 figures; Data Repository item 2003005. 51

Very high density CO2

associated with ultrahigh-temperature

metamorphism in the Eastern Ghats granulite belt, India

Sushmita Sarkar Department of Geological Sciences, Jadavpur University, Kolkata-700 032, IndiaM. Santosh* Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, JapanSomnath Dasgupta Department of Geological Sciences, Jadavpur University, Kolkata-700 032, IndiaM. Fukuoka Department of Earth and Planetary Sciences, Hiroshima University, Higashi-Hiroshima, Japan

Figure 1. Geological map of study area. Inset shows location of East-ern Ghats belt.

ABSTRACT

Spinel-bearing high Mg-Al granulites of the Vizianagram area

in the Eastern Ghats granulite belt show textural features clearly

establishing that the association spinelss quartz Fe-Ti oxide

solid solution sillimanite porphyroblastic orthopyroxene was

stable during peak metamorphic conditions. Pressure-temperature

(P-T) conditions estimated from both mineralogical thermobaro-

metry and phase-equilibrium limitations indicate that the peak

metamorphism occurred under ultrahigh-T conditions (1000 C)

at 89 kbar pressure. Retrograde P-T conditions of 750800 C, 6

7 kbar are deduced from the compositions of coronal garnet and

orthopyroxene, which have rims of spinel against quartz, indicat-

ing significant cooling with slight lowering of pressure. Quartz as-

sociated with the ultrahigh-T assemblage at Vizianagram contains

ubiquitous single-phase carbonic inclusions as isolated clusters thatbelong to two categories. Group I shows extremely high density

(homogenization temperature: 51 1.8 C; density 1.15 g/cm3)

and group II trapped relatively lower density fluids (homogeniza-

tion temperature: 18.4 2.4 C; density 1.05 g/cm3). The iso-

chores for group I inclusions pass through the peak metamorphic

P-T conditions, whereas those for group II coincide with the P-T

conditions of the formation of coronal garnet and orthopyroxene.

Our study is the first report of very high density CO 2 associated

with the Eastern Ghats granulite belt rocks and provides a strong

case for the presence of CO2-rich fluids during ultrahigh-T meta-

morphism at lower crustal levels.

Keywords: very high density CO2, ultrahigh-Tgranulites, fluid inclu-sions, Eastern Ghats Belt, India.

INTRODUCTION

Fluid-absent conditions are generally believed to be present during

crustal metamorphism at ultrahigh temperatures (900 C, e.g., Harley,

1998). Although fluids are important agents of heat transport, CO2influx as a cause of granulite metamorphism (e.g., Newton et al., 1980)

is no longer a popular mechanism. However, Tsunogae et al. (2002)

found ultrahigh-density CO2 inclusions in sapphirine granulites from

the Napier Complex, East Antarctica. As a part of our ongoing inves-

tigation of the ultrahigh-temperature (T) metamorphosed rocks of the

Eastern Ghats granulite belt, India, we have carried out detailed pet-

rological and fluid-inclusion studies on a suite of granulites from thisbelt. The polymetamorphic rocks in this belt achieved ultrahigh-T

metamorphic conditions during the first phase of metamorphism (e.g.,

Dasgupta and Sengupta, 2002). This study reports petrological and fluid-

inclusion data on spinel-bearing high-Mg-Al granulites related to this

ultrahigh-T metamorphism, and the results are important for evalu-

ating the role of fluids involved in metamorphism of deep continental

crust.

*Corresponding author. E-mail: [email protected].

GEOLOGIC BACKGROUND AND PETROGRAPHY

The study area is 4 km north of the town of Vizianagram within

the Eastern Ghats, India (Fig. 1). Recent isotopic data identified several

distinct domains within the Eastern Ghats Belt (Fig. 1) (Rickers et al.,

2001). The study area is in domain II, where Nd model ages of ortho-

gneisses show extreme variation from 3100 to 1800 Ma, while those of

sediments range between 2500 and 2100 Ma (Rickers et al., 2001). The

age of the early ultrahigh-Tmetamorphism is somewhat conjectural, but

is definitely pre-Grenvillian (Dasgupta and Sengupta, 2002). This area

exposes a strongly deformed suite of rocks comprising khondalite

(quartz-perthite-sillimanite-garnet gneiss), leptynite (quartz-plagioclase-

garnet-perthite gneiss), orthopyroxene granulite (orthopyroxene-quartz-

plagioclase-garnet gneiss), calc-silicate granulite (calcite-quartz-

wollastonite-scapolite-garnet), quartzite, and spinel-bearing

high-Mg-Al granulite. Khondalite, leptynite, and Mg-Al granulites

show a gneissic foliation demarcated by quartzofeldspathic segrega-

tions (quartz mesoperthite plagioclase) alternating with layers of

ferromagnesian minerals.

The dark bands in the Mg-Al granulite contain mineral associa-

tion: Spl Fe-Ti oxide Opx Grt Sil minor Qtz Crd

Plag. Quartz, K-feldspar (mostly mesoperthite), and plagioclase con-

stitute the leucobands. (All mineral abbreviations used herein are after

Kretz, 1983.)

Most of the spinel grains are composite, containing intergrowths

of magnetite along crystallographic directions. In most places the total

volume of magnetite occurring both as exsolved lamellae within host

spinel or at its grain boundary roughly equals the volume of the spinel.

The composite spinel and Fe-Ti oxide aggregates are always shielded from

quartz by various retrograde coronas such as sillimanite-orthopyroxene

(Fig. 2A), sillimanite-garnet (Fig. 2B), and cordierite. Because the cor-

onal phases are continuous over the entire composite oxide aggregates,

including the phases occurring outside the host, we argue (following

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52 GEOLOGY, January 2003

Figure 2. A: Porphyroblastic spinel separated from quartz by doublecoronas of sillimanite and orthopyroxene. B: Porphyroblastic spinelseparated from quartz by double coronas of sillimanite and garnet.C: Distribution pattern of group I inclusions in host quartz grain. D:Enlarged view showing clusters of group I inclusions in host quartzgrain. E: Distribution pattern of group II inclusions in host quartzgrain. F: Enlarged view showing clusters of group II inclusions inhost quartz grain. Mineral abbreviations are after Kretz (1983).

Waters, 1988) that exsolution occurred after the formation of reaction

coronas. Coarse as well as coronal orthopyroxene and coronal garnet

are separated from sillimanite and quartz by a thin rim of cordierite.

MINERAL REACTIONS AND PRESSURE-TEMPERATURE

CONDITIONS

Mineral compositions were determined with a JEOL JXA 733

electron microprobe at the Hiroshima University, Japan, operated at 15

kV accelerating potential and 15 nA absorbed current. Chemical anal-

yses of different minerals are in the GSA Data Repository 1.

Garnet in all modes of occurrence is typically pyrope-almandine

solid solution with minor (6 mol%) grossular and spessartine content.

All varieties of orthopyroxene are magnesian and highly aluminous.

The highest alumina content is noted at the cores of coarse orthopy-

roxene (11 wt%), and the rims show depletion (8.2 wt%) at similar

XMg values (XMg Mg/Mg Fe2 0.680.70). Coronal and/or

symplectic orthopyroxene rimming spinel solid solution has slightly

lower XMg (0.660.68) and Al2O3 content of9 wt%. Spinel contains

very little zinc, and XMg (Mg/Mg Fe2) varies between 0.44 and

0.54. Cordierite is highly magnesian, XMg varying between 0.83 and

1GSA Data Repository item 2003005, Tables 1, 2, and 3, Chemical anal-yses, is available on request from Documents Secretary, GSA, P.O. Box 9140,Boulder, CO 80301-9140, [email protected] or at www.geosociety.org/pubs/ft2003.htm.

0.88. Plagioclase has the composition Ab5557An4341Or2, but is more

sodic in the lamellae within perthite (Ab61An38Or1). Perthitic K-feld-

spar has the composition Ab1019An12Or8979. Sillimanite contains

significant Fe2O3 (total Fe), to 1.45 wt%. The XMg of coexisting phases

decreases in the order: XMgCrd XMgOpx XMgGrt XMgSpl.

The textural features described here indicate clearly that spinelss quartz sillimanite Fe-Ti oxidess orthopyroxene were stable

during peak metamorphic conditions in the dark bands. Mineral re-

actions deduced from textural and compositional criteria include the

following:

Spl Qtz Grt (coronal) Sil (coronal). (1)

Spl Qtz Opx (coronal) Sil (coronal). (2)

Exsolution and/or oxidation exsolution of the oxide phases subsequent-

ly produced different mineral aggregates.

Spinel hercynite magnetite. (3)ss

Fe TiO O (FeTiO Fe O ) Fe O in spinel. (4)2 4 2 3 2 3 ss 3 4

Cordierite appeared subsequently.

Opx Sil Qtz Crd (coronal). (5)

Grt Sil Qtz Crd (coronal). (6)

Spl Qtz Crd. (7)

Reactions 1 and 2, which produced coronal garnet orthopyroxene

sillimanite, ensued during cooling from ultrahigh peak metamorphic

temperatures (e.g., Harley, 1989; Dasgupta and Sengupta, 1995). This

also establishes the notion that quartz (from which fluid inclusions

were obtained) was a part of the peak metamorphic assemblage in the

studied rocks.

The peak mineral assemblage of spinelss, quartz, orthopyroxene,

and sillimanite, when considered in the high fO2 petrogenetic grid in

the systems KFMASH and FMAS (Dasgupta et al., 1995; Dasgupta

and Sengupta, 2002), defines pressure (P)-T conditions of 8 kbar,

950 C. Reintegrated ternary feldspar compositions give temperaturesas high as 1100 C (e.g., Kroll et al., 1993; Hokada, 2001). Reintegra-

tion of Fe-Ti oxide aggregates shows more than 10 mol% ulvospinel

in the preexsolution stage, which suggests cooling from temperatures

1000 C (e.g., Sack and Ghiorso, 1991). Therefore, the studied spinel

granulites record peak metamorphism under ultrahigh-T conditions

(1000 C) at 89 kbar. Grew et al. (2001) estimated similar P-T

conditions in this area.

Retrograde P-T conditions are deduced from the compositions of

the coronal garnet and orthopyroxene, which rim spinel against quartz.

Simultaneous solution of the garnet-orthopyroxene thermometric equa-

tion (Lee and Ganguly, 1988) and GOPQ barometric equation (Bhat-

tacharya et al., 1991) gives 750800 C, 67 kbar. This indicates sig-

nificant cooling with slight lowering of pressure during corona

formation. The alumina content of coronal orthopyroxene coexistingwith garnet gives 800 C (following Harley, 1998), and 1025 C

(following Harley and Motoyoshi, 2000). Cordierite appeared even lat-

er than the coronal garnet and orthopyroxene, and is therefore unrelated

to the ultrahigh-Tmetamorphism. Cordierite thermobarometry records

much lower P-T conditions of 56 kbar, 650700 C.

FLUID-INCLUSION STUDY

Fluid-inclusion studies were carried out on rock wafers prepared

from three representative ultrahigh-T samples of the study area (V98,

V21, and NLM1), from the same rock chips used for petrologic studies.

Microthermometric measurements were performed with a LINKAM



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GEOLOGY, January 2003 53

Figure 3. Histograms of melting (Tm

, A, B) and homogenization tem-peratures (T

h, C, D) of group I and group II inclusions. A and C:

Group I CO2

inclusions in quartz from samples V21 and V98. B andD: Group II CO

2inclusions in quartz from sample NLM 1.

Figure 4. Combined pressure-temperature (P-T) diagram showingmineral reaction grid, fluid-inclusion isochores, two P-Tboxes, andpossible uplift path (coordinates of invariant points are after Das-gupta and Sengupta, 2002). Mineral abbreviations are after Kretz

(1983).

freezing stage at the University of Kochi, Japan, calibrated with a pre-

cision of0.1 C for melting and 0.2 C for homogenization tem-

perature. Representative inclusions were analyzed by laser Raman

spectroscopy using a Jobin-Vyon laser Raman microsocope housed at

the Fukuoka University, Japan.

Fluid inclusions occur in various minerals in the studied rocks,

and are more abundant in quartz and cordierite; the inclusions range

from 5 to 40 m. Those in orthopyroxene and plagioclase are small(5 m). Here we report the results on fluid inclusions in early quartz

grains in textural association with the ultrahigh-T assemblage. From

their mode of occurrence and microthermometric characteristics, the

fluid inclusions in quartz in the studied samples can be grouped as

follows.

Group I inclusions were observed in samples V98 and V21 and

occur as clusters of monophase inclusions filled with a single liquid

phase. The inclusions are concentrated toward the core of the host

quartz grains (Figs. 2C, 2D) occurring adjacent to spinel, but separated

by various coronas, e.g., sillimanite-orthopyroxene and sillimanite-garnet

(cf. Figs. 2A, 2B). These quartz grains do not have any visible frac-

tures, and the concentration of the inclusions in the form of clusters at

the grain core suggests that the inclusions were trapped during the

formation of the host mineral. The inclusions range in size from 5 to30 m, although most are between 10 and 20 m and have shapes

varying from euhedral negative crystal to oval, elongated, and irregular.

Group II inclusions are seen in sample NLM1 within early quartz

that constitutes the leucoband (Figs. 2E, 2F) and are distinguished from

group I inclusions by their slightly lower density (discussed later).

These inclusions also occur as clusters or as scattered inclusions away

from the microfractures within host quartz. They vary in size from 10

to 30 m, the dominant size being between 15 and 25 m, and are

single phase at room temperature. Their shape varies from euhedral

negative crystal to elongate, ovoid and tubular, although the majority

has ovoid or elongate cavities.

Following the recent nomenclature proposed by Touret (2001),

both group I and group II inclusions of our study are inclusion clusters

or a group of synchronous inclusions.

The data from microthermometric studies of inclusions are pre-

sented in Figure 3. Types I and II single-phase inclusions show abrupt

melting at temperature (Tm) close to 56.6 C, which is the triple point

of pure CO2 (Figs. 3A, 3B). Laser Raman spectroscopy of represen-

tative inclusions yielded sharp peaks at 13821386 cm1, indicating a

pure CO2 composition for the fluid. Although depression in melting

temperatures of pure CO2 is known to result from the presence of

additional volatiles, no peaks for other volatiles such as CH4 or N2were detected in these inclusions during Raman analyses. The variation

and slight depression in melting temperatures are therefore not due toany significant compositional control.

On continued heating, homogenization always occurred into the

liquid phase. The homogenization temperature for group I inclusions

ranges between 52 and 24 C (Fig. 3C). The peak homogenization

at 51 1.8 C for sample V21 corresponds to a density of 1.15 g/cm3.

Inclusions in sample V98 show peak homogenization at 42 3.0

C, corresponding to a density of 1.13 g/cm 3 (39 cm3 /mol). Group II

carbonic inclusions show a peak homogenization temperature of18.4

2.4 C, which translates into a density of 1.05 g/cm3. The homog-

enization temperatures in this sample range from 26.1 to 10.1 C

(Fig. 3D).

DISCUSSION

The fluid-inclusion and mineral-phase equilibria data from Vi-zianagram are combined into a single P-T grid in Figure 4, which

shows the available phase relations for pelitic rocks in the FMAS sys-

tem (e.g., Hensen, 1986). The isochores for type I (early) inclusions

pass through the peak metamorphic P-T conditions (1000 C and 89

kbar) estimated for the rock, suggesting that the fluids in type I inclu-

sions were trapped at the time of peak ultrahigh-Tmetamorphism. Type

II inclusions are of lower density and their isochores broadly match

the P-T conditions of formation of coronal garnet and orthopyroxene

at 67 kbar and 750800 C, passing through the stability field of

garnet-sillimanite-orthopyroxene. The isochores for early inclusions in

quartz associated with spinel in this rock intersect the P-Tpath in the



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54 GEOLOGY, January 2003

stability field of spinel-quartz, which corresponds to the peak P-Tcon-

ditions of this area. The close correspondence of CO2 isochores with

the P-T estimates from mineral phase equilibria suggests fluid entrap-

ment at peak ultrahigh-T stage and subsequent cooling of the rocks.

The ultrahigh-T metamorphism at Vizianagram in the Eastern

Ghats Belt is characterized by the presence of dry mineral assemblages,

the stability of which requires anhydrous conditions. Low water activ-

ities leading to the generation of anhydrous assemblages in granulites

can result from a variety of processes (e.g., Waters, 1988; Clemens and

Vielzeuf, 1987; Valley, 1985; Santosh, 1992). The results presented in

this study are consistent with CO2-rich fluids being instrumental inbuffering water activity to low values in the Vizianagram granulites.

The occurrence of inclusions as isolated clusters within early formed

quartz in association with spinel within the ultrahigh-Tassemblage and

extremely high density values that correspond closely to metamorphic

P-T conditions provide a strong case in favor of the synmetamorphic

nature of the trapped fluid.

Our study is the first report of very high density CO 2 associated

with granulites from the Eastern Ghats. Tsunogae et al. (2002) found

very high density fluid inclusions, 0.91.1 g/cm3, from the ultrahigh-T

granulites of Tonagh Island in the Archean Napier Complex, East Ant-

arctica. The estimated CO2 isochores for sapphirine granulite intersect

the counterclockwise P-T trajectory of Tonagh Island rocks at 69

kbar at 1100 C, corresponding to the peak metamorphic conditions of

the terrain derived from mineral assemblages. These values are closeto the fluid densities reported from the granulites of the Eastern Ghats

in our study.

An alternate model for the occurrence of CO2 inclusions is their

entrapment as residual fluids from a mixed CO2 H2O fluid expelled

from deep-seated magmas. If dehydration melting played a critical role

in generating ultrahigh-T assemblages in this locality, such a process

would result in the selective extraction of water into melt segregates.

This would leave a residue rich in anhydrous fluids, notably CO2, with-

in the bands containing ultrahigh-T assemblages.

Our study shows that fluid inclusions trapped from extreme crustal

metamorphism could preserve near-peak fluid densities. This might be

a result of their postpeak cooling path along the isochore (e.g., Touret,

2001). Our results show that a combination of mineral phase equilibria

and fluid-inclusion studies can provide important information relatingto the P-Tevolutionary history and thermal regime of deep continental

crust.

ACKNOWLEDGMENTSWe thank S. Yoshikura and Pulak Sengupta for their helpful suggestions,

S. Taguchi for the Raman laser analytical facility, and M. Tagawa and R. Katorifor support during preparation of fluid-inclusion samples. Sarkar and Dasguptaacknowledge the Council of Scientific and Industrial Research and the Depart-ment of Science and Technology, Government of India, respectively, for finan-cial support. Santosh thanks Kochi University for facilities, and for projectsupport for the fluid-inclusion laboratory facility from the President of KochiUniversity. We acknowledge with thanks the constructive comments from two

journal referees.

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Manuscript received May 30, 2002Revised manuscript received September 19, 2002Manuscript accepted September 22, 2002

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Easter Ghat Granulite Belt Metamorphism

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