GEOCHEMICAL CHARACTERISTICS OF THE ABOR VOLCANIC-ROCKS, NE HIMALAYA, INDIA - NATURE AND EARLY EOCENE MAGMATISM

The Abor volcanic rocks, exposed in the Slang window, NE Himalaya, occ ur as dismembered thrust-bound packages interbanded with fossiliferous shelf sediments of Upper Palaeocene to Middle Eocene age. Their age i s 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 contin ent. The volcanic rocks occur beneath the Lesser Himalayan thrust shee ts and were transported with them during their southward propagation. The Abor volcanic rocks were erupted in an early Palaeogene shallow ba sin located within marginal parts of the Himalayan fold-thrust brit so uth of the collision zone. They may thus represent foreland basin magm atism. The abundances of some major and large ion-lithophile elements have been affected by alteration of the samples. Ti and some other tra ce elements remain unaffected however, and their abundances indicate t hat the volcanic rocks Form a chemically coherent group of tholeiites and alkaline basalts. The tholeiitic and alkaline basalts reflect diff erent 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 incompat ible elements, the two basalt types appear to have been generated From sources having similar character. This source is inferred to be enric hed sub-continental mantle.