The Quxu （曲水） complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang （棉将） and Niedang （聂当） villages in Quxu County, including gabbro, mafic micro-enclaves （MME）, and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis （47-51 Ma）. In this article, the same zircons of the 6 samples were applied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176Hf/177Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to εHf（t） values of 6.3-14.7. Their Hf depleted-mantle modal ages （TDM） are in the range of 137-555 Ma, and the zircon Hf isotope crustal model ages （TDMC） range from 178 to 718 Ma. The mantle-like high and positive Era（t） values in these samples suggest a mantledominated input of the juvenile source regions from which the batholith originated. The large variations in εHf（t） values, up to 5-ε unit among zircons within a single rock and up to 15-ε unit among zircons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.
The Cretaceous granitoids in the middle and northern Gangdese, Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision. This paper reports bulk-rock major element, trace element and Sr-Nd isotopic data, zircon U-Pb age data, and zircon Hf isotopic data on the Zayu pluton in eastern Gangdese, Tibet. These data shed new light on the petrogenesis of the pluton. Our SHRIMP zircon U-Pb age dates, along with LA-ICPMS zircon U-Pb age dates recently reported in the literature, indicate that the Zayu pluton was emplaced at about 130 Ma, coeval with Early Cretaceous magmatic rocks in other areas of eastern Gangdese (e.g., Rawu, Baxoi areas) and the Middle Gangdese. The Zayu pluton samples lack amphibole and muscovite, and are compositionally characterized by high SiO2 (69.9%―76.8%), K2O (4.4%―5.7%), and low P2O5 (0.05%―0.12%). These samples also have A/CNK values of 1.00-1.05, and are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, P, Ti, and Eu. These geochemical features suggest that the Zayu pluton samples are metaluminous to slightly peraluminous and are of highly fractionated I-type granite. The Zayu pluton samples have high εNd(t) values (-10.9--7.6) and low initial 87Sr/86Sr ratios (0.7120- 0.7179) relative to melts derived from mature continental crust in the Gangdese (e.g., Ningzhong Early Jurassic strongly peraluminous granite). The Zayu pluton samples are heterogeneous in zircon εHf(t) values (-12.8--2.9), yielding ancient zircon Hf crustal model ages of 1.4―2.0 Ga. The data obtained in this study together with the data in the recent literature suggest that the Early Cretaceous granitoids in eastern Gangdese represent the eastward extension of the Early Cretaceous magmatism in the middle Gangdese, and that the Lhasa micro-continent block with ancient basement may extend for ~2000 km from east to west. Zircon Hf isotopic data and bulk-rock zircon saturation temperature (789-821℃) indicate that mantle-d
