A long-lived enriched mantle source for two Proterozoic carbonatite complexes from Tamil Nadu, southern India

Abstract

We report new neodymium and strontium isotopic data for two Proterozoic carbonatites and related alkalic rocks, at Hogenakal and Sevathur in southern India. These complexes were emplaced into the crust at 2.4 Ga (Hogenakal) and 0.77 Ga (Sevathur). Their initial strontium and neodymium isotopic compositions, together with oxygen isotope data, suggest the involvement of a single long-lived enriched mantle source in their origin. The isotopic evolution of this source indicates that it formed approximately contemporaneously with the accretion and metamorphism of the overlying crust at the southern margin of the Dharwar craton and survived convective disruption in the mantle from early Proterozoic until at least 770 Ma ago. The older of the two carbonatites was intruded into young crust that was not older than about 150 Ma at the time of emplacement. The isotopic data contrast with those from carbonatites of the Canadian Shield, for which isotopic evidence also suggests origin from a long-lived lithospheric source, but one with a depleted chemical signature. They, therefore, indicate that there is no geochemically unique lithospheric source for carbonatites.