Synmetamorphic High-Density Carbonic Fluids in the Lower Crust: Evidence from the Nilgiri Granulites, Southern India

Srikantappa, C. and Raith, M. and Touret, J.L.R. (1992) Synmetamorphic High-Density Carbonic Fluids in the Lower Crust: Evidence from the Nilgiri Granulites, Southern India. Journal of Petrology, 33 (4). pp. 733-760.

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Abstract

High-density CO2 inclusions occur abundantly in granulite facies rocks (age of metamorphism approximately 2.5 b.y.) of the Nilgiri massif, southern India. The chronology of carbonic inclusions in the widespread enderbitic granulites studied in relation to the development of micro-textures and mineral assemblages indicates that randomly oriented, negative-crystal-shaped CO2 inclusions (4-20-mu-m) in garnet and quartz grains (qtz I) armoured by garnet entrap syn-peak-metamorphic pore fluids. The more abundant trail-bound CO2 inclusions in the deformed, polygonized, and partially recrystallized matrix quartz grains (qtz II and III) and plagioclase grains were formed in connection with a stage of compressional deformation and subsequent annealing related to the development of the late-Proterozoic Bhavani shear zone. These inclusions resulted from local re-equilibration of the former peak-granulitic carbonic inclusions and re-entrapment of released fluids. The presence of pure CO2 in all the inclusions is confirmed by microthermometric data and laser-excited Raman microspectrometry. Temperatures of homogenization (liquid phase) are in the range of -50 to +20-degrees-C, and the corresponding CO2 densities are between 1.154 and 0.807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documents a continuous gradient of near-peak metamorphic conditions from 750-degrees-C, 9-10 kb in the northern part to 730-degrees-C, 7 kb in the southwestern part of the Nilgiri massif. Uniform P, T estimates (600-650-degrees-C, 6-7 kb) for late coronitic garnet + quartz assemblages in enderbites and metadolerites indicate that differential uplift of the massif to mid-crustal levels was accomplished before late compressional deformation. In conformity, carbonic inclusions in quartz II and III are characterized by uniformly high density (1.154-1.08 g/cm3). In contrast, early carbonic inclusions in garnet and quartz I preserve the density contrast reflecting the regional P, T gradient during near-peak metamorphic fluid entrapment. The fluid inclusion systematics indicate `near-isochoric; uplift of the northern high-P domain, but near-isobaric cooling of the southwestern low-P domain. The carbonic fluids are thought to have been derived either from internal sources during dehydration-melting processes or from freezing synmetamorphic intrusives into the lower crust.

Item Type:	Article
Subjects:	F Earth Science > Geology
Divisions:	Department of > Earth Science
Depositing User:	Users 23 not found.
Date Deposited:	15 May 2021 07:36
Last Modified:	15 May 2021 07:36
URI:	http://eprints.uni-mysore.ac.in/id/eprint/16431

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