Plume Related (?) Acid Volcanic Activity in St. Mary's Islands, South Kanara District, Karnataka

Metaluminous (Alumina Saturation Index A/CNK < 1.04), acid volcanic rocks ranging in composition from rhyolite to dacite occur in St Mary's island. The rocks are rich in SiO, (68.58-71.27%), high field strength elemenls, moderately rich in AI₂O₃, (1 1.78-15.02%), and low in CaO, MgO. V, Cr, and Ni. They are characterized by fractionated light rare earth elements (La/Sm=6.8) and a flat heavy rare earth elements pattern with a large negative Eu anomaly (0.48). Variation in bulk- rock major and trace element composition together with mineral data on clinopyroxene composition, suggest a basaltic precursor and a cause related to rifting. The enrichments of incompatible elements point to an EM-I type mantle source. From the over all geochemical characteristics, it is inferred that the Marion plume caused the lower part of the basaltic crust to soften, stretch and rift in an extensional setting. The rise in the geotherm caused by the rising asthenosphere and the pressure change resulting from rifting lowered the solidus temperature and resulted in partial melting of the basaltic crust. Subsequent fractionation produced dacitic to rhyolitic magma, which was emplaced in a rift setting.