Timescale of evolution of a late archean collision zone from Coorg, S. India: constraints from zircon and garnet geochronology

Abstract

Petrology, geochemistry and geochronology of a metapelite (sillimanite-garnet-biotite-plagioclase-quartz) from the vicinity of the Archean Mercara Shear Zone in Coorg, S. India show that metamorphism at temperatures>850 °C occurred between 2700–3300 Ma (Phase equilibria, thermobarometry, U-Pb dating of zircons and Lu-Hf dating of garnets). Subsequently, the rocks experienced thermal events at lower temperatures at 2400-2600 Ma as well as at 600-640 Ma (U-Pb dates from rutile). There are indications of multiple episodes of metasomatic/ (high temperature) hydrothermal activity during the Archean events. Residence of the rocks at lower temperatures between the high temperature events is indicated by the kinetics of dissolution of zircon in melt. Taken together, this history shows that (a) P-T-t evolution in this Archean collisional setting happened along an overall clockwise path but not in a single continuous loop - episodes at high temperatures were interspersed with residence at cooler temperatures in between, (b) subtle effects of metamorphism that occurred at temperatures below the peak temperature could help to resolve some controversies related to tectonothermal reconstructions in the region (e.g. whether signatures of both - amalgamation of Dharwar and Coorg cratons and activity along an equivalent of the Betsimisaraka suture zone in east-central Madagascar may be present in the region), and (c) the duration of high-temperature events (several 100 million years at ~800 °C) are consistent with an early Earth peel-back style of plate tectonics, rather than modern day plate tectonics, operating in the region at the time.