Insignificant Mantle Plume Contribution to the Generation of the Rajmahal Traps, India

Abstract Melting of mantle plumes has generated intra-plate volcanic chains and large igneous provinces (LIPs) throughout Earth’s history. Whereas the plume material itself is the major contributor, the ambient asthenosphere and overlying lithosphere may also contribute melts to the magma inventory. The ~118-Ma-old Rajmahal Traps in eastern India are thought to represent one of the earliest volcanic epicenters of the Kerguelen LIP, formed by melting the Kerguelen plume. Elemental abundances and Sr-Nd-Pb-Hf isotope ratios of basalts, basaltic andesites, and ultramafic lamprophyres from the Rajmahal Traps differ significantly from those of the recent Kerguelen hotspot. Basalts with very high Pb isotope ratios record high-μ (HIMU)-like compositions, but are unknown so far from the entire Kerguelen LIP, except for the felsic rocks from the Comei Province. Thermodynamics (energy and mass balance, using pMELTS) and geochemical modeling (Magma Chamber Simulator) show that melting a compositionally heterogeneous asthenospheric mantle source in the spinel stability field and assimilation of continental crust during transport of the melts through the lithosphere and crust can produce the rare earth and large ion lithophile elemental patterns and the Sr-Nd-Pb-Hf isotopic ratios of the Rajmahal rocks. These signatures do not necessitate direct melting of the Kerguelen mantle plume in the origin of the Rajmahal basalts. Rather, plume-assisted (heat) melting of a heterogeneous asthenosphere as the Kerguelen plume approached shallow mantle formed the Rajmahal Traps, which is consistent with the observation by others that the contribution of the Kerguelen plume was limited before 95 Ma.