Geochemistry and paleomagnetism of Late Cretaceous mafic dikesin Kerala, southwest coast of India in relation to large igneousprovinces and mantle plumes in the Indian Ocean region

摘要

New geochemical and paleomagnetic results are presented on two Late Cretaceous dikes of the 85-90 Ma leucogabbroic and doleritic dikes and the 65-70 Ma doler-ites in Kerala, India. The dikes are rich in incompatible elements, have fractionated patterns with light rare-earth element enrichment and are akin geochemically to Cretaceous basalts on the east coast of Madagascar. The magmas were formed at garnet lherzolite depths above the Marion plume, constituting part of a large igneous province in Madagascar. In contrast, the 65-70 Ma dolerites are moderately depleted in incompatible elements, with almost flat, rare-earth element patterns and resemble the upper formations of the Deccan Traps and the tholeiitic dikes of the Seychelles. These dolerites were formed by melting of spinel lherzolite over the Reunion plume. Paleomagnetic data from the dikes and the other coeval igneous units from south India provide the 90 Ma pole (latitude: 24°; longitude: 293°; A_(95) = 5.9; N = 18 sites) for India. The 65-70 Ma dolerites possess both normal and reverse polarities, and the mean pole (latitude: 36°; longitude: 283°; A_(95) = 5.7°; N = 10 sites) compares well with the Deccan superpole. Paleolatitude estimates indicate ～5° southward migration for the Marion plume and a northward migration for the Reunion plume, in conformity with global mantle-circulation models; however, distinguishing migration of the Reunion plume from the effects of true polar wander is difficult. Furthermore, the geodynamic reconstructions extending the shear zones of southern Madagascar into south India are not tenable.