Contrasting high and intermediate pressures of metamorphism in the Archaean Sargur Schists of Southern India

Abstract

The Archaean Karnataka craton of southern India contains Eastern and Western crustal blocks (separated by a major thurst) in which Sargur Schists occur as lenses within tonalitic Peninsular Gneisses. The Schist complex comprises pelites, quartzitic psammites, carbonates and calc-silicates, iron formations, and basic rocks, and thus provides many mineral assemblages ideal for the calculation of PT conditions. With their gneisses the Sargur rocks are unconformably overlain by the Dharwar greenstone belts, and are generally thought to be older than 3,000 my. In the Western block maximum metamorphic conditions are given by meta-basic rocks as 790±50°C and 13±2 kb, but adjacent meta-sediments give a pressure of 9 kb, suggesting that the differences in P and T recorded in this block mark a polychronic metamorphic geotherm related to the exhumation of the terrain by uplift and erosion. In the eastern block maximum temperatures were in the range 750°-850°C and maximum pressures were 7 kb. The rocks of the two blocks were sampled 100 km apart, and thus there was probably a regional pressure difference between the two blocks caused by differentiated crustal thickening prior to or during metamorphism. The shape of the geotherm from the Western block shows near-isothermal decompression over 20 km. Our data suggest that during Sargur metamorphism maximum crustal thicknesses were in excess of 45 km and that there was a minimum difference of 20 km in crustal thickness between the Eastern and Western blocks.