Archaean High-Grade Gneiss Complex from Satnur-Halagur-Sivasamudram Areas, Karnataka, Southern India: Petrogenesis and Crustal Evolution
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Both gneisses and foliated charnockites show similar chemical characteristics except in LIL elements, which appear to have been disturbed during granulite metamorphism. Low Mg# (0.18-0.27), low content of HFS elements (Ti, Nb, Ta, Hf and Y) and strongly fractionated REE patterns with HREE depletion suggest that magmatic history of the gneisses and charnockites are similar to the classical Archaean trondhjemite, tonalite, granodiorite suites (TTG).
The geodynamic evolution of the Satnur-Halagur-Sivasamudram area implies a change in the thermal structure and melting conditions of lithosphere from 3.0 to 2.5 Ga. Following the model of Martin (1987), the magmatic protoliths of gneisses and charnockites could have been generated in an accretion zone (subduction-mantle plume ?)2.9 Ga ago, where young oceanic crust melted before dehydration. Granulite facies metamorphism occurred during latest Archaean: synchronous mantle derived melts tapped into mid-crustal levels along a major shear-zone where they induced migmatization (Closepet-type batholith; Jayananda et al. 1994) immediately followed by granulite metamorphism, related to a possible 2.5 Ga old mega-plume (Peucat et al. 1993b).
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