Contrasting isotopic mantle sources for Proterozoic lamproites and kimberlites from the Cuddapah basin and eastern Dharwar craton: implication for Proterozoic mantle heterogeneity beneath southern India

Abstract

Kimberlites intruding the Precambrian basement towards the western margin of the Cuddapah basin near Anantapur (1090 Ma) and Mahbubnagar (1360 Ma) in Andhra Pradesh have initial Sr-87/Sr-86 between 0.70205 to 0.70734 and epsilon Nd between +0.5 to +4.68. Mesoproterozoic lamproites (1380 Ma) from the Cuddapah basin (Chelima and Zangamarajupalle) and its NE margin (Ramannapeta) have initial Sr-87/Sr-86 between 0.70520 and 0.7390 and epsilon Nd from -6.43 to -8.29. Combined Sr- and Nd-isotonic ratios suggest that lamproites were derived from "enriched" sources which have time-averaged higher Rb/Sr and lower Sm/Nd ratios than the Bulk Earth whereas kimberlites were derived from "depleted" source with lower Rb/Sr and higher Sm/Nd ratios. Calculated T-DM model ages suggest that the lamproite source enrichment (similar to 2 Ga) preceded that of kimberlites (similar to 1.37 Ga). Our work demonstrates the existence of isotopically contrasting upper mantle sources for Southern Indian kimberlites and lamproites and provides evidence for a lateral, isotopically heterogeneous mantle beneath the Cuddapah basin and eastern Dharwar craton. The significance of our results in the context of diamond exploration is also highlighted.