Ultramafic Xenoliths from Kutch, Northwest India: Plume-Related Mantle Samples?

Abstract

The Deccan flood basalt province of west-central India has been linked to the Reunion plume, and reconstructions suggest that the Kutch region was over the plume at the time of Deccan volcanism at 65-68 Ma. Field relations and isotopic data indicate that the alkaline basalts of Kutch, which occur to the NNW of the main Deccan tholeiitic province, preceded the main flood-basalt volcanism and are related to the limited plume incubation period. Several plugs of these alkali basalts contain small spinel peridotite xenoliths of mantle origin. The minerals of the spinel peridotites have been analyzed for their major, trace, and rare-earth element (REE) concentrations using electron micro-probe and LAM-ICPMS techniques. The modes and mineral chemistry, especially of the clinopyroxenes, indicate a fertile mantle; modeling of the clinopyroxene REE data is consistent with <5 to 15% of partial melting of a primitive mantle source material in the spinel peridotite field. Subsequent cryptic metasomatism introduced LREE, U, Th, and Zr. The xenoliths may represent: (1) young lithosphere generated during the lithospheric extension that preceded the main Deccan volcanism; or (2) material from the uppermost parts of the rising plume, brought to the surface by the first stages of the volcanism. Their low equilibration temperatures (≤900°C) and their textural and chemical similarity to xenolith suites from other Phanerozoic intraplate settings favor the first alternative. However, the extensive cryptic metasomatism may reflect the influence of the rising Deccan plume.