Petrogenesis of Proterozoic Lamprophyres from the Western Dharwar Craton, southern India

Abstract

The Proterozoic Harohalli dyke swarm lies in the Dharwar craton. Some of these dykes were sampled from Mysore to investigate the sub-surface geology of the region. Lamprophyre samples were collected since they act as windows to the SCLM. Microscopy gave characteristic lamprophyric features – panidiomorphic-porphyritic texture with amphibole and clinopyroxene as phenocrysts and feldspar as groundmass. EPMA analysis revealed the presence of two types of amphiboles – magnesiohastingsite (primary) and actinolite (secondary). Diopsidic augite was also present. The lamprophyre was identified to be Vogesite – a calc-alkaline lamprophyre. The lamprophyres are shoshonitic and high-K in nature. Whole-rock geochemistry showed Nb-Ta-Ti negative anomaly along with Pb, K and Sr enrichment. A negative Hf-Zr anomaly and depletion of HREE was observed. However, due to U depletion, low Mg# and Ni concentration, crustal contamination appeared to be practically absent, and with the help of tectonic discrimination diagrams, a subduction zone tectonic setting has been suggested. The petrogenesis was pinned down to low degrees of partial melting of subduction fluid-metasomatised depleted mantle followed by high degrees of fractional crystallisation, through discrimination diagrams and petrogenetic modelling. A phlogopite garnet lherzolite source was estimated, and the metasomatized mantle was simulated by mixing average global subducting sediments and depleted mantle.