A syn-collisional model for Early Cretaceous magmatism in the northern and central Lhasa subterranes

Abstract

Voluminous Early Cretaceous volcanic rocks from the northern and central Lhasa subterranes contain important information on the tectono-magmatic processes. In this contribution, we focus on the Lower Cretaceous volcanic rocks in the Nagqu area, northern Lhasa subterrane, and present their zircon LA-ICP-MS U–Pb ages, in situ Hf isotopic data, whole-rock major and trace element compositions, and Sr–Nd isotopic data. The Nagqu volcanic rocks are high-K calc-alkaline to shoshonitic (K2O: 1.41–8.52wt.%; K2O/Na2O: 0.36–6.65) and feature similar geochemical characteristics (e.g., subparallel distribution of incompatible elements and REEs). Assimilation–fractional crystallization of magmas from identical source (rather than diverse magma sources) was mainly responsible for the formation of the diverse volcanic rock types. High Hf/Sm (>0.7) and high U/Yb whole-rock values and low Y values in zircon grains suggest the involvement of terrigenous components rather than subducted oceanic crust in the magma source. Tectonic discrimination diagrams, sedimentary environment (marine–continental transition), magma compositions (K-rich and terrigenous components), crustal thickening, and spatio-temporal variations in subduction, syn-collisional and post-collisional processes, indicate that the geodynamic setting of the Early Cretaceous magmatism was associated with the collision between the Lhasa and Qiangtang terranes. The high-K characteristics were inherited from the melts derived from the partial melting of lower metasomatized lithospheric mantle (the K-rich layer), which was transported to great depths by the continuously thickening lithosphere, eventually triggering melting.