Mantle Input to the Crust in Southern Gangdese, Tibet, during the Cenozoic: Zircon Hf Isotopic Evidence

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 ɛ_Hf(t) values of 6.3–14.7. Their Hf depleted-mantle modal ages (T_DM) are in the range of 137–555 Ma, and the zircon Hf isotope crustal model ages (T_DM ^C) range from 178 to 718 Ma. The mantle-like high and positive ɛ_Hf(t) values in these samples suggest a mantle-dominated input of the juvenile source regions from which the batholith originated. The large variations in εHf(t) values, up to 5-ɛ unit among zircons within a single rock and up to 15-ɛ 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.