Basal Aravalli Volcanism : Evidence for an Abortive Attempt to form Proterozoic Ensialic Greenstone Belt in Northwestern Part of Indian Shield

Early Proterozoic mafic volcanic rocks, occurring at the base of Aravalli Supergroup of northwestern Indian shield, have komatiitic, tholeiitic compositions. In discrimination diagrams based on immobile minor and trace elements they show a close affinity with MORB, However, the immobile incompatible element patterns and element ratios resemble those of enriched continental tholeiites which erupt at the time of initial rifting. Geochemical and geological evidences suggest a transitional nature of basal Aravalli sequence between Archaean greenstone belts and younger continental suites. It appears that the basal Aravalli volcanism was an attempt to form a Proterozoic greenstone belt on the stable continental crust through lithospheric rifting and lava eruption. The attempt failed with cessation of volcanism, probably due to falling geothermal gradient and basin subsidence by sediment loading. The occurrence of similar rock associations in other shields of the world indicates the existence of particular tectonic-thermal regimes during the early Proterozoic.