Rajmahal Basalts, Eastern India: Mantle Sources and Melt Distribution at a Volcanic Rifted Margin

Abstract

Basalts in the Rajmahal Hills represent the edge of a >200-m-thick sequence of predominantly tholeiitic lavas erupted in the Bengal Basin, eastern India. The 40 Ar/ 39 Ar ages of Rajmahal lavas and dikes cluster at 116-113 Ma. These ages indicate that volcanism in eastern India occurred within a longer period of igneous activity in west Australia (130-100 Ma) and was contemporaneous with the final stages of basaltic volcanism at Ocean Drilling Program Sites 749 and 750 on the central Kerguelen Plateau. Pb-Nd-Sr isotope ratios of the least crustally contaminated Rajmahal basalts differ from those of most Kerguelen Plateau lavas and appear to reflect an Indian mid-ocean ridge basalt (MORB)-like source with 206 Pb/ 204 Pb ∼17.9, 207 Pb/ 204 Pb ∼15.5, e Nd (t) >+5, and 87 Sr/ 86 Sr (t) <=0.7037. The isotopic similarities with Indian MORB are best explained if Rajmahal basalts were generated by decompressional melting of asthenosphere welling up passively beneath the rifted margin of eastern India. Upwelling could have been initiated and driven by viscous coupling of MORB-source mantle to flow in the conduit of the Kerguelen plume, located about 1000 km to the south of eastern India at 116 Ma. The MORB-source mantle lay at the edge of the thermal halo defining the plume boundary and may have been heated by the plume. We infer that the Rajmahal basalts are examples of lavas which, though associated spatially and temporally with the magmatic products of hot spot activity, were derived from compositionally 'normal' asthenosphere.