Zircon U-Pb-Hf, geochemical and Sr-Nd-Pb isotope systematics of Late Mesozoic granitoids in the Lantian-Xiaoqinling region: Implications for tectonic setting and petrogenesis

The Lantian-Xiaoqinling region, located at the southern margin of the North China Craton (S-NCC), exposes large-scale Late Mesozoic granitoids associated with molybdenum and hydrothermal gold deposits. This study presents zircon U-Pb-Hf isotope, whole-rock element and Sr-Nd-Pb isotope data of granitoids from two representative plutons (Laoniushan and Huashan) of the Lantian-Xiaoqinling region including previous data from the Lantian pluton. Zircon UPb dating supports a prolonged magmatic history from c. 150 to 130 Ma. Different to the thick continental crust-derived high Sr/Y granitoids from the Dabie Mts., the granitoids of the Lantian-Xiaoqinling region are ordinary calc-alkaline granitoids which show evidence for fractional crystallization, implying no need for the existence of a thick continental crust during magmatism. The plutons share similar high whole-rock initial 87Sr/86Sr values ranging from 0.7070 to 0.7109, but variable εNd(t) (−23.4 to −9.0) and zircon εHf(t) values (−46.9 to −2.7) as well as continental crust-like initial Pb isotopes with 206Pb/204Pb(t) ratios of 17.1 to 17.9; 207Pb/204Pb(t) of 15.40 to 15.57 and 208Pb/204Pb(t) of 37.6 to 38.35. Nd-Hf-Pb isotopic compositions are more depleted compared to ancient lower NCC crust as represented by the Taihua and Xiong'er Groups and suggest involvement of some juvenile materials into the magma source. Positive correlation between SiO2 and εNd(t) values suggests that the juvenile material was silica-rich with slightly negative εNd(t) values. Such material cannot be produced from a mantle source as previously stated. These compositional features make Neoproterozoic continental crust of Yangtze Block a likely source for the juvenile material. It is argued that the juvenile material originated from partial melting of subducted Neoproterozoic continental crust with abundant black shale. We propose that the Late Mesozoic granitoid plutons in the Lantian-Xiaoqinling region stem from a composite source, including rocks similar to the ancient lower NCC crust and a juvenile felsic melt derived from the subducted Yangtze plate. Heat input and melting was triggered by asthenosphere upwelling caused by subduction of the Paleo-Pacific oceanic plate. [Display omitted] •Late Mesozoic granitoids of the Lantian-Xiaoqinling region are pseudo-adakites.•Subducted Yangtze crust was involved in the granitoid magmas.•Subduction of the Paleo-Pacific plate triggered the heat input and melting.