PETROLOGY AND GEOCHEMISTRY OF GREYWACKES FROM THE ARAVALLI SUPERGROUP, RAJASTHAN, INDIA AND THE TECTONIC EVOLUTION OF A PROTEROZOIC SEDIMENTARY BASIN

The Proterozoic Aravalli Supergroup in south-central Rajasthan, wester n India is predominantly composed of coarse, low-grade meta-arenites, carbonates, greywackes, interbedded fine-grained silty arenites and me tapelites resting on an Archaean basement complex of gneisses, schists and high-grade metasediments. We have differentiated four lithofacies (A, B, C and D) within the greywacke suite, each with characteristic QFL composition, grain size and major, trace and rare earth element co ncentrations. Tectonic setting for each facies type could only be disc riminated using Q(m)FL(t) plots. Greywackes of facies A and D were der ived from a recycled orogen and facies B was derived from arc-related andesitic source rocks. A continental block provenance is indicated fo r the silty arenites of facies C. This petrofacies-based interpretatio n shows good agreement with environmental reconstructions based on lit hofacies and sedimentary structure analyses. The only exception is fac ies B, whose andesitic parentage as deduced from petrofacies analysis could not be demonstrated at its outcrop. These provenance discriminat ions were tested through tectonic setting interpretations based on maj or, trace and rare earth elements. The geochemical data broadly confir med the tectonic interpretations made on the basis of the petrological attributes. Use of these laboratory-based methods in conjunction with field measurements of primary sedimentary structures allow us to disc riminate provenance and tectonic setting fairly precisely. A Proterozo ic plate model with subduction to the west has been proposed to explai n the sedimentological variation in the Aravalli Supergroup.