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Authors
Affiliations
1 Civil Engineering Department, S.D.M. College of Engineering and Technology, Dharwar -580 002, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 70, No 1 (2007), Pagination: 43-52
Abstract
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.
Keywords
Plume related, Acid volcanic activity, Columnar structure, Rhyolite-Dacite, Geochemistry, Petrogenesis, St. Mary's Island, South Kanara District, Karnataka.