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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
Printed in USA
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