Petrology and geochronology of a suite of meta-supracrustal rocks from Madukkarai, Tamil Nadu: Implications for the Ediacaran-Cambrian orogenesis of the Granulite Terrane of South India

Abstract

The Granulite Terrane of South India (GTSI) has two major components. An orthogneiss dominated northern block of Archaean/early Palaeoproterozoic age and an Ediacaran-Cambrian, high to ultra-high temperature meta-supracrustal dominated southern block (the Madurai Block, MB). It is commonly perceived that the two blocks of GTSI came into juxtaposition along the Palghat-Cauvery Shear/Suture Zone (PCSZ) during a continent-continent collision. However, the timing and the nature of the contact relations between the two major components remains controversial. In this work, detail petrological and geochronological study of two metapelites along with a meta-psammite (only for geochronological study), from the Madukkarai Supracrustal Unit (MSU) within the PCSZ, are presented. The studied metapelites show two distinct assemblages:A.Garnet + biotite + sillimanite (both idioblasts and pseudomorphs after kyanite) + K-feldspar + plagioclase + quartz + ilmenite (rutile, zircon and monazite as accessory phases).B.Mineralogy is similar to A but contains cordierite and minor spinel, but no zircon.Both the metapelites develop a gneissic fabric that is defined by centimetre thick laterally discontinuous garnet-bearing leucosomes. ‘Frozen in’ reaction textures, phase equilibria modelling and mineralogical thermobarometry yielded “peak” temperature and pressure of metamorphism in the range of ~730–790 °C, ~5.5–7 kbar (considering sillimanite and ilmenite to be stable). Subsequently, the rocks followed a decompressive retrograde P-T path recorded till ~630 °C and ~4 kbar. The geometry of the construed retrograde P-T path is similar to that commonly found in rocks of a continent-continent collision zone. LA-ICP-MS UPb zircon dating and U-Th-total Pb monazite dating constrain the major high-grade metamorphism and deformation during ~550–520 Ma with vestiges of ~600 Ma old tectonothermal/ thermal event. The geological history of the MSU shares many characteristics with the Ediacaran-Cambrian meta-supracrustal rocks lying on both sides of the PCSZ, in terms of lithological association, timing of protolith formation, deformation and metamorphism. The variation in intensity of metamorphism in the meta-supracrustal rocks of the GTSI could be linked to differential upliftment of the GTSI during the neotectonic activities, followed by erosion. Integrating all the geological, geochronological and geophysical information, it has been argued that the GTSI behaved as a coherent continental block that was involved in continent-continent collision during the formation of the Ediacaran-Cambrian supercontinent Gondwanaland. The findings of this study do not corroborate the view that PCSZ is an Ediacaran-Cambrian terrane boundary.