The Geochronology and Geochemistry of Highly Fractionated I-type Granite within Gangdese batholith in Sangri Area, South Tibet

Long-term subduction of Neo-Tethyan oceanic lithosphere beneath an active continental margin arise prolonged plutonic activity, which is illustrated by the vast chain of Gangdese batholith in the south margin of Lhasa Terrane, Southern Tibet. Although extensive studies have performed on the source regions, spatial-temporal associations and geotectonic implications of the granitic rocks within the batholith, the magmatic evolution and petrogenesis of plutons in Sangri area have remained poorly studied. Here we present zircon U-Pb geochronology, Hf isotope and bulk-rock geochemistry of the late Cretaceous Sangri biotite granites (SBG) in order to shed light on this issue. Zircon U-Pb geochronology demonstrates the plutonic activity emplaced at 67~65Ma. The SBG belong to high-K calc-alkaline series, displaying highly fractionated I-type signature with high content of SiO2 (74.26~76.93%), K2O+Na2O (7.87~8.56%), but low content of CaO (0.28~1%) and P2O5 (0.02~0.04%) and pronounced depletion in Sr, Ba, Eu, Nb and Zr. The affinity of mantle component in the SBG that was elucidated by positive zircon Epsilon Hf(t) of 4.6~10.9, which makes it indistinguishable from other granitic rocks within Gangdese batholith. Tectonic models of evolution of the Neo-Tethyan Ocean suggest that the break-off of the oceanic slab is required in order to generate enormous magmatism in Gangdese batholith during 66~50Ma. Associated with the coeval gabbroic and dioritic intrusions occurred in Sangri area, we suggest the magma of SBG had been derived from juvenile lower crust mingled with mantle component upwelling through the window of Neo-Tethyan oceanic slab, and were subjected to high degree of fractionation in the magma chamber.