Petrology and geochemistry of greywackes from the Aravalli Supergroup, Rajasthan, India and the tectonic evolution of a Proterozoic sedimentary basin

Abstract

The Proterozoic Aravalli Supergroup in south-central Rajasthan, western India is predominantly composed of coarse, low-grade meta-arenites, carbonates, greywackes, interbedded fine-grained silty arenites and metapelites 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 concentrations. Tectonic setting for each facies type could only be discriminated using Q mFL t plots. Greywackes of facies A and D were derived from a recycled orogen and facies B was derived from arc-related andesitic source rocks. A continental block provenance is indicated for the silty arenites of facies C. This petrofacies-based interpretation shows good agreement with environmental reconstructions based on lithofacies and sedimentary structure analyses. The only exception is facies B, whose andesitic parentage as deduced from petrofacies analysis could not be demonstrated at its outcrop. These provenance discriminations were tested through tectonic setting interpretations based on major, trace and rare earth elements. The geochemical data broadly confirmed 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 discriminate provenance and tectonic setting fairly precisely. A Proterozoic plate model with subduction to the west has been proposed to explain the sedimentological variation in the Aravalli Supergroup.