Geochemical characteristics of the Abor volcanic rocks, NE Himalaya, India: nature and early Eocene magmatism

Abstract

The Abor volcanic rocks. exposed in the Siang window, NE Himalaya. occur as dismembered thrust-hound packages interbanded with fossiliferous shelf sediments of Upper Palaeocenc to Middle Eocene age. Their age is broadly contemporaneous with the age of collision between the Indian and Tibetan continents. They were. therefore, erupted in a convergent setting through channel ways in the leading edge of the Indian continent. The volcanic rocks occur beneath the Lesser Himalayan thrust sheets and were transported with them during their southward propagation. The Abor volcanic rocks were erupted in an early Palaeogene shallow basin located within marginal parts of the Himalayan fold-thrust belt south of the collision zone. They may thus represent foreland basin magmatism. The abundances of some major and large-ion-lithophile elements have been affected by alteration of the samples. Ti and some other trace elements remain unaffected however, and their abundances indicate that the volcanic rocks form a chemically coherent group of tholciites and alkalinc basalts. The tholeiitic and alkaline basalts reflect different degrees of melting, and low-pressure fractional crystallization involving olivine and plagioclase has played a significant role in the evolution of the basalts. On the basis of ratios of strongly incompatible elements, the two basalt types appear to have been generated from sources having similar character. This source is inferred to he enriched sub-continental mantle.