Zircon U–Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet

During the Mesozoic–Cenozoic, northward Neotethyan subduction and subsequent India–Asia collision gave rise to the extensive Transhimalayan magmatism that stretches from Burma and western Yunnan through southern Tibet to the Ladakh and Kohistan complexes. To understand the age distribution and petrogenesis of the Gangdese batholith, the largest intrusive exposure along the Transhimalayan magmatic belt, fifty granitic samples were selected for in situ zircon U–Pb and Hf isotopic analyses. The U–Pb data suggest four discrete stages of magmatic activity, i.e., ~ 205–152, ~ 109–80, ~ 65–41 and ~ 33–13 Ma, respectively, with the 65–41 Ma stage being the most prominent. The Hf isotopic data indicate that the Gangdese batholith is overwhelmed by positive εHf(t) values, which are comparable to those of the Kohistan–Ladakh batholiths in the west but differ markedly from those of the Chayu–Burma batholiths in the east. Most of the Gangdese granites show similar and young Hf model ages (1000–300 Ma), indicating their derivation from juvenile crust. However, those formed in the 65–41 Ma stage exhibit more heterogeneous Hf isotopic ratios, with negative εHf(t) values being observed in some granites younger than 50 Ma, suggesting the involvement of old Indian continental crust in their petrogenesis. This age may thus mark the onset of the India–Asia collision. The newly established zircon U–Pb age and Hf isotope database of the Gangdese batholith can be used as a powerful tracer or “fingerprint” when studying the source-to-sink relation of the sediments eroded from the southern Tibetan Plateau.