Plume-lithosphere interaction: Petrology of Rajmahal continental flood basalts and associated lamproites, northeast India.

The Rajmahal basalts of Bihar State, Northeast India, are a suite of Early Cretaceous tholeiites and ferrotholeiites emplaced along the Indian continental margin during the separation of India from Australia-Antarctica. The basalts show a wide range in Pb-Sr-Nd isotopic composition, ranging from ratios typical of Indian Ocean P-type MORB, to those of ancient (>1 b.y. old) continental lithosphere. Relationships between mg#, Rb/Sr, Sm/Nd and eNd suggest that the range in Sr-Nd isotopes is not a mantle source feature, but was produced by interaction between tholeiitic liquids and small-volume partial melts of amphibolite-facies upper crust. However, the range in incompatible trace element concentrations shown by the Rajmahal basalts cannot be explained solely by crustal processes, and must also involve differences in the depth and degree of partial melting. A dynamic melting model is proposed in which polybaric decompression melting of clinopyroxene-garnet lherzolite results in the formation of picritic parent magmas. The generation of picrites requires high asthenospheric potential temperatures (>1400 °C), consistent with melting in close proximity to the axis of a mantle plume. Trace element and isotopic similarities between the Rajmahal basalts and tholeiites from the central and southern Kerguelen Plateau, and palaeomagnetic reconstructions of the eastern Indian Ocean at 118 Ma encourage a link with the Kerguelen plume. Geochemical data from lamproites emplaced shortly before the basalts suggest that the chemical and isotopic composition of the continental mantle beneath eastern India had been extensively modified by heat and material input from the Kerguelen plume. The data imply that the DUPAL isotopic signature of the Indian lithosphere results from 'recent' (~190-140 Ma) metasomatic enrichment of a harzburgitic protolith by asthenospheric small-volume melts. The enrichment event produced a laterally-extensive (>150 km) light REE- and LILE-enriched lithospheric mantle domain, distinct from P-type MORB mantle underlying the Bengal basin.