Multistage evolution of subcontinental lithospheric mantle of northwestern Deccan volcanic province, India: Constraints from the ultramafic xenoliths in alkali magma

We report multistage geochemical evolution of off-cratonic subcontinental lithospheric mantle by studying mantle xenoliths hosted in alkali basalts from the northwestern part of the Deccan volcanic province in Kutch. The xenolithic fragments are peridotites, composed of Cr-rich diopside, Mg-rich olivine, orthopyroxene and aluminous spinel. Compositionally, these peridotites belong to type I: Cr-diopside lherzolite group. Primarily they possess protogranular texture, but, secondary textures such as reaction corona are also present. High forsterite content of primary olivine (Fo(89-92)), high Mg# (>92) in orthopyroxene along with low Cr# (0.09-0.35) and TiO2 content (up to 1.12 wt.%) in spinel indicate their upper mantle origin. Higher values of Al-VI/Al-IV (0.99-2.4) of primary clinopyroxenes provide evidence for their high-pressure upper mantle origin, consistent with their high Mg#. Clinopyroxene displays light rare-earth element- and mid-rare-earth element-enriched pattern along with pronounced negative spike for Nb, Ta, Ti, Zr and Hf. They record multiple metasomatic imprints; high Ti/Eu (>1500) ratio attests silicate metasomatism whereas negative high-field-strength element anomalies indicate a carbonate metasomatism. Olivine-spinel mantle array trend suggests low degrees of partial melting and melt extraction (1-15 wt.%). The primary mineralogy of peridotite xenoliths affirms that they have been equilibrated at a temperature and pressure range of 786-1159 degrees C and 8-23 kb, respectively. They record high mantle heat flow between 62 and 66 mW m(-2), may be interpreted as a consequence of thermal perturbation by additional heat input into a relatively thin mantle lithosphere via upcoming magma(s) due to upwelling of Reunion plume beneath western part of the Indian subcontinent. Summarising we conclude that the lithospheric mantle beneath western India suffered a complex multistage evolutionary history followed by rifting at the end of the Mesozoic era.