Geochemistry and petrogenesis of metabasic rocks from the Lesser Himalayan Crystallines, Western Arunachal Himalaya, Northeast India

Abstract

In the Lesser Himalayan Crystallines of the western Arunachal Himalaya, numerous metabasic rocks occur as disrupted dykes / sills and boudins mostly within gneiss and rarely interbedded with quartzite and phyllite. These metabasic rocks are tholeiitic in composition, resemble ortho-amphibolites, enriched in light rare earth elements and large ion lithophile elements, and depleted in high field strength elements (Nb, P, Ti). Amphiboles present in these rocks are typically calcic in nature and range from tschermakite to magnesiohornblende-actinolite, whereas plagioclase varies from oligoclase to labradorite (An29 to An52). Their mineral chemistry suggests that these rocks were metamorphosed at temperatures between 566 °C and 633 °C at a pressure >5 kb. Geochemical characteristics and petrogenetic modelling imply that the protolith of these metabasic rocks was probably derived from an enriched lithospheric mantle source by partial melting (5% to 20%) leaving a residue with 35% olivine, 50% orthopyroxene, 15% clinopyroxene; subsequently, the melt was modified by fractional crystallization of clinopyroxene and plagioclase. It is also suggested that the protoliths of these rocks were emplaced in a continental rift environment and were metamorphosed into amphibolites during the Himalayan orogeny.