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 U-Pb 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 caic-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 Sr-87/Sr-86 values ranging from 0.7070 to 0.7109, but variable epsilon(Nd)(t) (-23.4 to -9.0) and zircon epsilon(Hf)(t) values (-46.9 to -2.7) as well as continental crust-like initial Pb isotopes with Pb-206/Pb-204(t) ratios of 17.1 to 17.9; Pb-207/Pb-20(4)(t) of 15.40 to 15.57 and Pb-208/Pb-204(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 epsilon(Nd)(t) values suggests that the juvenile material was silica-rich with slightly negative epsilon(Nd)(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. (C) 2020 Published by Elsevier B.V.