首页|CO2-rich fluid inclusions in staurolite and associated minerals in a high-pressure ultrahigh-temperature granulite from the Gondwana suture in southern India

CO2-rich fluid inclusions in staurolite and associated minerals in a high-pressure ultrahigh-temperature granulite from the Gondwana suture in southern India

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We report the first detailed fluid inclusion study on Mg-Al rocks from the Gondwana suture zone along the Palghat-Cauvery Shear Zone System in southern India.These rocks underwent high-pressure(P>12 kbar)and ultrahigh-temperature(T~990°C)metamorphism during the late Neoproterozoic associated with the collisional assembly of the Gondwana supercontinent.Fluid inclusions of texturally primary and pseudosecondary categories occur in plagioclase,staurolite,and kyanite in these rocks.Inclusions of secondary nature are found in garnet and apatite.All the trapped fluids show melting temperatures in the range of-58.6 to-56.6°C,suggesting a dominantly pure CO2 composition.The secondary inclusions show high homogenization temperatures(T_h)of+12.9 to+30.0°C,indicating low density of 0.59-0.83 g/cm3.Although most of the primary and pseudosecondary inclusions also contain moderate to low-density fluids(0.67-0.95 g/cm3;T_h=-3.5 to+27.4°C),rare high-density inclusions(1.00-1.06 g/cm3;T_h=-26.1 to-14.1°C)were discovered in matrix plagioclase and porphyroblastic staurolite.The estimated CO2 isochores for the high-density primary and pseudosecondary inclusions intersect the clockwise P-T trajectory of the rocks at around 8.3-9.0 kbar at 1000°C,which broadly coincide with the peak metamorphic conditions of the area.Since these fluid inclusions show textural evidence for entrapment during the growth of the host high-grade minerals,we interpret the high-density CO2 fluids to be traces of a CO2-rich synmetamorphic fluid that was present during the peak ultrahigh-temperature metamorphism.Despite the preservation of high-density CO2 inclusions in staurolite and plagioclase,the primary and pseudosecondary inclusions in most of the other minerals show wide density variation which suggests the fluids in these inclusions underwent significant density reversal(density decrease)during decompression of the rocks from high-pressure through the peak ultrahigh-temperature stage along a clockwise P-T trajectory.The finding of pure CO2 inclusions in these Mg-Al granulites conforms with similar observations from many other granulite-facies terrains with the carbonic fluids probably derived from mantle reservoirs and transferred through deep-seated shear zones that characterize this Gondwana suture.

Carbonic fluid inclusionUltrahigh-temperature granulitePalghat-Cauvery Shear Zone SystemGondwanaSouthern India

Hiroyuki Ohyama、Toshiaki Tsunogae、M.Santosh

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Graduate School of Life and Environmental Sciences(Earth Evolution Sciences),University ofTsukuba,Ibaraki 305-8572,Japan

Department of Geology,University of Johannesburg,P.O.Box 524,Auckland Park 2006,Republic of South Africa

Department of Natural Environmental Science,Faculty of Science,Kochi University,Akebono-cho 2-5-1,Kochi 780-8520,Japan

2008

Lithos

Lithos

ISTP
ISSN：0024-4937
年,卷(期)：2008.101(3/4)