Evidence for a Paleoproterozic event of metamorphism in the Bastar craton, Central India: P-T-t constraints from mineral chemistry and U-Pb geochronology of mafic dykes

Abstract

The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes. Many regions in the Bastar craton have been subjected to medium or high grade metamorphic conditions but the nature and timing of regional metamorphism is poorly understood. In this study, metamorphosed mafic igneous rocks collected from two different dyke swarms from the southern part of the Central Indian Bastar craton have been studied to evaluate the timing and conditions of metamorphism. In this region, two distinct metamorphosed dyke swarms are recognized based on differences in petrology and geochemistry: the Meso-Neoarchaean sub-alkaline BD1 swarm and the Neoarchaean boninite-norite (BN) swarm. The BD1 dykes are characterized by magnesio- and ferro-hornblende and sodic plagioclase (andesine to labradorite) whereas the BN dykes contain magnesio- hornblende and sodic-calcic amphiboles (barroisite and winchite types) with more calcic plagioclase. Temperatures calculated using the amphibole- plagioclase thermometer range from ∼575°C (BN dykes) to ∼700°C (BD1 dykes). Due to the absence of garnet in the studied rocks, metamorphic pressures were estimated based on the Al-content in amphibole barometer (∼4 kbar for the BD1 dykes). Together these data indicate the metamorphic grade in this region is medium amphibolite facies. U-Pb age results for metamorphic rutile from a BN dyke yielded a Palaeoproterozoic date of 21 18±2 Ma, interpreted to indicate the time of exsolution of retrograde rutile from Ti-rich actinolite. This represents a robust minimum age constraint for the timing of emplacement of the BN, and by inference the BD1 dyke swarm.