U-Pb zircon geochronology of a pyroclastic rock from the Parsoi Formation, Mahakoshal Group: Implications towards age and tectonics of the Basin in Central Indian Tectonic Zone

The Mahakoshal Group of rocks in the Central Indian Tectonic Zone (CITZ) holds the key to documenting the early events in the CITZ evolution and initiation of collision/accretion between the North and South Indian cratons (NIC, SIC). Centimetre to decimetre-thick conformable pyroclastic beds, sandwiched within the siltstone-argillite metasedimentary succession of the Parsoi Formation allows us to generate (i) a robust depositional age, which otherwise is bracketed based on basement and intrusive age, and (ii) track the earliest record of accretion at the plate margin of NIC. A magmatic origin for the inferred pyroclastic unit is established from petrographic signatures and, in particular, the CL response of quartz grains. The dominance of the Dauphine twin in fine-grained (similar to 100 mu m and <20 mu m) quartz fraction implies sub-aquatic fast-cooling across the phase transition temperature (573 degrees C) of quartz within the pyroclastic rock. A volcanic arc to syn-collisional granite affinity is suggested for the pyroclastic unit based on (i) dacitic to rhyolitic composition, (ii) LILE-enrichment, (iii) HFSE depletion, and (iv) enrichment of Th, U, and Pb. The Pb-207/Pb-206 1894.3 +/- 9.4 Ma age of crystallization is estimated from the youngest and most prominent peak of near-concordant oscillatory zoned zircon grain with a high Th/U ratio. The age data is important in documenting the earliest accretion event at the plate margin in the collisional history of NIC and SIC.