ALKALINE NATURE AND TAPHROGENETIC AFFINITY OF FELSIC VOLCANIC-ROCKS OF ST-MARY-ISLANDS, OFF MANGALORE COAST

The St. Mary Islands located off the Mangalore coast is an isolated pa tch of volcanic rocks presumed by some of the earlier workers to be re lated to the Deccan Trap volcanism. Geochemical analysis of the rocks of the islands reveals their essentially felsic but alkaline nature wh ich indicates a possible relation with continental rift. In the light of the available age-data (of 93 Ma) and their location in the reconst ructed Gondwanaland, the genesis of the St. Mary Islands volcanics can be related to the crustal thinning preceeding the Indo-Madagascar rif t which predates Deccan volcanism significantly. This study necessitat es identifying such expressions of rift-related magmatism of pre-Decca n age from other areas of the western margin of the Indian subcontinen t.