Drillhole lavas from the northwestern Deccan Traps, and the evolution of Réunion hotspot mantle

Picritic and basaltic lavas from three drillholes in the northwestern Deccan Traps provide new evidence on the composition of the Deccan mantle source. Age-corrected NdPbSr isotopic ratios of these lavas define two distinct trends. One, Trend 1, reflects mixing between a mantle end-member very similar to that of modern Réunion Island and high-208Pb/204Pb, high-207Pb/204Pb continental lithospheric (sensu lato) material unlike any seen in the well-studied southern Deccan formations (but broadly like a lithospheric end-member in the Madagascar province). The other trend, Trend 2, records mixing between the same or similar continental lithospheric material and a mantle end-member resembling that of the thick Ambenali Formation of the southwestern Deccan. Trend 2 flows are interbedded with Trend 1 flows in one drillhole, indicating that the mantle source region was compositionally zoned and, in the context of a Réunion plume model for the province, consistent with a zoned plume head (e.g., [1]). In the same context, the Trend 1 data imply that (1) the isotopic composition of the Réunion end-member has changed relatively little in the last ∼ 66 m.y., and that (2) the source feeding the recent shield lavas of Réunion and Mauritius Islands may contain a small fraction of MORB-type mantle entrained with the presumed plume-source material. critic flood-basalts have previously been suggested to be high-degree melts from the hottest part of the Réunion plume head, dominated by plume-source mantle, and basalts lower in MgO have been suggested as corresponding to cooler regions of the head characterized by a significant amount of entrained non-plume-source mantle [1]. However, both Trend 1 and Trend 2 include picritic members, and the most Réunion-like lavas are non-picritic basalts. Moreover, elemental data indicate that the somewhat alkalic drillhole lavas probably represent lesser degrees of partial melting than the strongly tholeiitic Ambenali basalts to the south. Indeed, regional north-to-south Deccan-wide gradients are present in incompatible element ratios and suggest that the average degree of melting increased gradually southward in the Deccan, perhaps as a result of progressive lithospheric thinning.