Geochemical and Nd, Pb, and Sr Isotope Data from Deccan Alkaline Complexes-Inferences for Mantle Sources and Plume-Lithosphere Interaction

Abstract

Previous chemical and isotopic studies based on alkaline rocks and carbonatites associated with large, continental flood basaltic provinces indicate their important role in monitoring plume–lithosphere interaction. We report new major and trace element data, and Nd, Pb, and Sr isotope ratios for various alkaline silica-undersaturated rocks and carbonatites from several Decca alkaline complexes in an attempt to evaluate the relative contributions of Réunion plume and Indian sub-continental mantles in their source regions. Major and trace element abundances for the most primitive silicate samples are consistent with an origin via small-degree partial melting of metasomatized mantle. Initial 87Sr/86Sr, 143Nd/144Nd and Pb isotope ratios for the most primitive alkaline silicate samples and associated carbonatites exhibit a large variation, and are attributed to mixing of three distinct mantle components—Réunion plume, continental lithosphere and asthenosphere (Indian MORB-like). For the silicate rocks, isotope ratios correlate with major and trace element composition and support derivation from distinct mantle sources. The data obtained here are consistent with previous models invoking Réunion plume–continental lithosphere interaction to explain the origin of Deccan alkaline complexes, which suggest a more prominent role of Réunion mantle during the early stages of Deccan volcanism involving small-degree melting of plume-modified lithosphere. With time, the isotope systematics of both alkaline and tholeiitic magmatism record a larger lithospheric imprint.