K-Rich Adakite-Like Rocks in Central Tibet: Fractional Crystallization of a Hydrous, Alkaline Primitive Melt

Abstract

K-rich adakite-like rocks (KARs) in post-collisional settings, such as in Tibet, have been widely linked with the melting of pre-existing thickened crust. Here, we investigate geochemical data of the Late Eocene (38–34 Ma) volcanic rocks including KARs from the southern Qiangtang terrane (SQT) of central Tibet. The data reveal that: (a) the volcanic rocks define a fractionation trend from high-K alkaline basalt to high-K calc-alkaline rhyolite, with a continuous compositional range and (b) they are characterized by a narrow range of depleted Sr–Nd isotopic compositions relative to the pre-Eocene SQT crust. We contend that the KARs in the SQT resulted from fractional crystallization of hydrous, alkaline melts derived from the lithospheric mantle where fractionation was dominated by amphibole and plagioclase. Partial melting of the lithospheric mantle beneath the SQT was possibly triggered by thermal perturbations owing to the north-directed subduction of the Indian continental lithosphere beneath southern Tibet.