Thermal architecture of cratonic India and implications for decratonization of the Western Dharwar Craton: Evidence from mantle xenoliths in the Deccan Traps

Abstract

The mantle beneath the Western Dharwar Craton of the Indian shield comprises a suite of refractory and fertile peridotites and mafic granulites. Detailed petrographic studies coupled with new mineral analysis and geothermobarometric estimations permit to decipher the thermal architecture and get an insight into the evolution of this ancient craton. The refractory rocks are coarse grained harzburgites/dunites, whereas the more fertile ones are at times, porphyroclastic lherzolites. Both show a similar range of equilibration temperatures and pressures indicating intermixing between the two at various levels. The peridotites contain undeformed interstitial REE-enriched clinopyroxene, phlogopite, apatite and carbonates recording post-kinematic modal and cryptic metasomatic events in the Precambrian cratonic lithosphere. Xenoliths of mafic granulite contain layers of clinopyroxenite which also vein the granulite. The P-T range of the granulites overlaps that of the ultramafic rocks. This study in combination with previous investigations reveals a distinct change in the thermal architecture of the craton from a warm/hot geotherm in the Proterozoic to a highly perturbed, still hotter geotherm of the Palaeocene. The Cenozoic thermotectonic rifting episodes heated, refertilized and thinned the bulk of the cratonic lithosphere beneath the Western Dharwar Craton, which has witnessed the most re-activation among cratons of the Indian shield. The waning of the Deccan Traps volcanism in Palaeocene time saw the reworking of ancient cratonic lithosphere and its replacement by non-cratonic, juvenile mantle and magmatic accretions, indicated by compound xenoliths. Differing petrological and geochemical characteristics of refractory xenoliths and fertile lherzolites serve to constrain the relative timing and composition of non-cratonic lithosphere. By the end of the Palaeocene the Western Dharwar Craton was characterised by a thermal high, an attenuated continental lithosphere (60-80 km), and a thin crust (less than 10- approx. 21 km), reflecting the decratonization of at least the western part of the Western Dharwar Craton.