CRUSTAL TRAJECTORY OF SAPPHIRINE-BEARING GRANULITES FROM GANGUVARPATTI, SOUTH-INDIA - EVIDENCE FOR AN ISOTHERMAL DECOMPRESSION PATH

Ganguvarpatti is part of a Precambrian terrane characterized by granul ite facies rocks, including charnockites, mafic granulites, sapphirine -bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on t he metamorphic history of this area, as they formed in response to dec ompression during uplift. Geothermobarometry and constraints from reac tion textures define a segment of a P-T path traversed by the granulit es of Ganguvarpatti. Near-peak metamorphic conditions of c.800 degrees C and 8 kbar were succeeded by a symplectitic stage at a significantl y lower pressure (c.700 degrees C and 4.5 kbar), documenting a nearly isothermal decompression P-T path and rapid uplift (c.12km) followed b y cooling. The presence of many fluid inclusions of extremely low dens ity in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure of c.8 kbar indicates c.27km de pth of burial during metamorphism. This would imply a total crustal th ickness of c.65-70km at 2.6-2.5 Ga. Such a profound crustal thickness and a clockwise decompressive P-T path is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat-Cauvery shear zone via northward subduction.