Late Eocene magmatism of the eastern Qiangtang block (eastern Tibetan Plateau) and its geodynamic implications

Abstract

Post-collisional magmatism offers significant clues to reveal the deep dynamic process and surface uplift of plateaus. New zircon U-Pb ages, biotite Ar-Ar ages, zircon trace elements, and whole-rock geochemistry, integrated with published data of igneous rocks from the eastern Qiangtang block (eastern Tibetan Plateau) were determined to unravel the geodynamics and constrain the uplift history. These igneous rocks consist of few mafic and many intermediate-felsic series. Geochronological studies reveal that the magmatism mainly occurred in ~43–33 Ma. The late Eocene igneous rocks are characterized by high potassium, enrichment in large ion lithophile elements (LILE, e.g., Rb, Ba, Th, U), depleted in high field strength elements (HFSE, e.g., Nb, Ta, Ti), negligible Eu anomalies, with a wide range of highly incompatible element ratios. The parental magmas of these rocks were most probably generated by different degrees of partial melting of the lithospheric mantle, which were heterogeneously metasomatized by subduction related multiphase fluids/melts. Whole-rock geochemistry, zircon trace elements, and thermodynamic modeling results suggest that the parental magmas experienced complicated and different differentiation processes. Mantle metasomatism, fractional crystallization, and magmatic recharge made contributions to the enrichment of incompatible elements and the wide range ratios between highly incompatible elements. In combination with the estimated Eocene crustal thickness, we suggest that the southward continental subduction of the Songpan-Ganzi block triggered the magmatism, thickened the crust, and induced the surface uplift to no more than ~3 km in the late Eocene.