Sr-Nd isotopic study of dolerite dykes in the Western Dharwar craton, southern India: Implications for the evolution of the subcontinental lithospheric mantle in late Archean

Abstract

Mafic dyke swarms are one of the major geologic features that represent crustal extension and rifting episodes which result in the transfer of large volumes of mantle materials to the crust. The rapid emplacement of dykes within a short span of time helps to preserve the composition of subcontinental lithospheric mantle (SCLM) and understand its evolution over time. Here we report major and trace element geochemistry and Sr-Nd isotopic characteristics of dykes from the Tiptur area in the late Archean Western Dharwar craton of southern India. The studied mafic dykes are un-metamorphosed and preserve pristine igneous textures and mineralogy. Three different groups of dykes were identified and categorized as olivine dolerites, dolerites and REE enriched dolerites, based on the distinct petrographical and geochemical characteristics. Olivine dolerites are having a high Mg# suggesting a lower degree of fractionation and is indicative of a less evolved magma. Dolerites have comparatively lower Mg# than the olivine dolerites indicating that the former have undergone significant fractional crystallization as confirmed by the higher Ni contents. The low initial Sr isotopic ratio for olivine dolerites (87Sr/86Sr = 0.70088) with negative epsilon Nd values are suggesting a late Archean enriched subcontinental lithospheric mantle (SCLM) in the source region. The dolerites (87Sr/86Sr = 0.70132 to 0.70327) and REE enriched dolerites (87Sr/86Sr = 0.70405 to 0.70653) show signatures of a more enriched source. The evolution of SCLM from a DMM source enriched by the subduction of the mafic oceanic crust in the late Archean to a mantle enriched by continent-derived sediments towards the early Proterozoic are reflected in the dolerite and REE enriched dykes.