LAUSR

Abstract The properties of accreted terranes in a large collisional orogen determine, in large part, the nature and subsequent development of the orogen. In this paper we present a detailed study of Late Mesozoic sodic mafic magmatic rocks from Central Tibet. These sodic mafic rocks from Bandatso provide an excellent opportunity to assess the nature and evolution of pre-collisional lithospheric mantle in the Himalayan–Tibetan region, and will thus aid our understanding of the evolution of the orogeny. The Bandatso sodic mafic rocks are characterised by marked oceanic island basalt (OIB)-like enrichment in incompatible elements, such as Nb (up to 139 ppm) and light rare-earth elements. This Late Mesozoic (ca. 148 Ma and 90 Ma) mafic magmatism is likely to be the result of two-stage mantle evolution in the northern Qiangtang. The late Jurassic rocks were derived from carbonated lithosphere metasomatized by subducted oceanic sediments during flat subduction. Whereas the late Cretaceous basanites were likely derived from interaction of lithosphere with upwelling asthenosphere induced by removal of lithosphere that had previously been thickened by relamination. Our study suggests the presence of an inherited carbonated lithospheric mantle beneath the northern Qiangtang prior to continental collision. The conclusions from this work suggest that the enrichment of the mantle source of Cenozoic potassic rock closely related to plateau uplift, more likely result from continental subduction after the late Cretaceous (ca. 90 Ma). These conclusions will help our understanding of the Cenozoic geology of this multi-terrane collision system and uplift mechanism of the Tibetan Plateau.