Age, provenance and metamorphic conditions of the Mercara Shear Zone, Coorg Block, India

Abstract

High pressure granulite metamorphism has never been recorded during the Mesoarchaean. The Mercara shear zone in the Coorg Block, south west India plays host to kyanite-sillimanite-garnet-bearing felsic granulites that equilibrated at 11-13 kbar and 860 °C and 7.5-9 Kbar at 850-860 °C. LA-ICP-MS of U-Pb zircon and in-situ monazite geochronology reveal that the assemblages were first metamorphosed at ca. 3071 Ma, then again at ca. 676 ± _47 Ma. Pressure-temperature-time studies from the shear zone had not previously described conditions of this nature before, revealing how timing the suturing of the Coorg block will require extensive sampling methods. The conditions described however provide an insight into the thickened crust and low apparent thermal gradients existing during the stagnant-lid to first tectonism transitionary period, a period that usually only records high apparent thermal gradients and weakened crust. The ancient assemblage preserves regimes typical of contractional orogenic events, implying crustal thicknesses 35-40 km deep. Crustal thickening of this nature provides evidence for modern style tectonics operating during the Mesoarchaean.