Archaean High-Grade Gneiss Complex from Satnur-Halagur-Sivasamudram Areas, Karnataka, Southern India: Petrogenesis and Crustal Evolution

The high-grade gneiss complex of the Satnur-Halagur-Sivasamudram area in southern Karnataka forms a part of amphibolite-granulite facies transition zone of southern India. The major lithologies are metasediments (quartzites-pelites-BIF-Mn-horizons), amphibolite facies gneisses, foliated charnockites, mafic granulites, granite sheets (Closepet-type). All these lithologies show a prominent N-S fabric which appears to have been produced during late Archaean shear deformation. Rb-Srwhole rock isochron and U-Pb zircon and monazite ages suggest that much of the crust accreted during 2.96 Ga magmatic event followed by the 2.5 Ga granulite facies metamorphism; the occurrence of a previous high-grade metamorphism close to 2.9 Ga remains debatable.

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).