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.
