Abstract
Early Cretaceous NNW- to WNW-trending dolerite dykes of the eastern Indian Shield, collectively termed the Raniganj-Koderma swarm, are studied for their emplacement ages and petrogenetic history, and assessed for a possible linkage with the Greater Kerguelen plume. 40Ar/39Ar dates of four dolerite dykes from different locations in the Chhota Nagpur Gneissic Terrane, indicate three pulses of emplacement ca. 118–116 Ma, ca. 112–111 Ma, and ca. 109 Ma. Geochemistry of 19 samples analysed herein (and an additional 12 samples from literature) shows sub-alkaline high-Mg tholeiitic basaltic andesite compositions. All the dyke sets belonging to the Raniganj-Koderma swarm with sub-sets WNW-trending ca. 118 Ma, NNW-trending ca. 116 Ma Salma, and a N-S trending ca. 109 Ma dyke showing similar chemistry. However, NW-trending dykes emplaced ca. 112–111 Ma have slightly different geochemical characteristics. Key trace element geochemistry data, particularly Nb/Y, Zr/Y, Nb/Yb, Ti/Yb, Th/Nb and Th/Yb ratios, indicate derivation from mantle melts generated from interaction of a plume with a spreading ridge (producing OIB – E-MORB melts) with a minor role for interaction with an enriched lithospheric mantle that was metasomatized during an earlier (pre-Mesozoic time) subduction event. Spatiotemporal distribution of the studied dolerite dykes connects them with the second plume head phase of the Greater Kerguelen mantle plume.
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