Retrograde, charnockite- gneiss relations in Southern India

Srikantappa, C.; Ashamanjari, K. G.; Narasimha, K. N. Prakash; Raith, M.

The high-pressure charnockites (P = 8 to 9 Kb., T = 700 to 800 C CO2-rich fluid regime) of the Nilgiri hills show evidence of retrogression related to shear deformation within the Moyar and Bhavani shear belts. Two types of retrogression have been noticed: (1) retrogression along shear planes, and (2) retrogression along pegmatitic veins. Initial stages of retrogression results in the formation of irregular, 2 to 3 cm to one meter wide bleached zones with the removal of greasy grey color of charnockites. Minor structures which were earlier obscured in charnockites are clearly seen in bleached areas. In intensely shear areas, formation of highly fissile grey gneiss results often with the development of flaser and mylonitic structures. Fluid inclusion studies and geochemical investigations carried out for serial samples collected from charnockite to gneiss indicate following features: (1) there is a gradual decrease in density of CO2-rich fluids from 1.073 to 0.821 g/cu cm; (2) interestingly, in many sections of the gneisses studied, there is almost complete absence of fluid inclusions suggesting that they would have decrepitated (this may be due to large pressure difference (2 to 3 Kb.) created between the interior and exterior of the fluid inclusions); and (3) presence of mixed CO2-H2O inclusions were noticed.

Document ID

19890012881

Acquisition Source

Legacy CDMS

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Conference Paper

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Date Acquired

September 5, 2013

Publication Date

January 1, 1988

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Publication: Lunar and Planetary Inst., Workshop on the Deep Continental Crust of South India

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Accession Number

89N22252

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Public

Work of the US Gov. Public Use Permitted.

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