Neoproterozoic tectonic evolution of the northwestern Yangtze Block: Constraints from the Bijigou intermediate-acid magmatism in the Hannan massif

The Bijigou intrusion is one of the largest and most well-differentiated Fe-Ti oxide-bearing layered intrusions in the Hannan massif located in the northwestern margin of the Yangtze Block, South China. Besides the mineralization-related mafic-ultramafic rocks, the intermediate-acid intrusive rocks are also exposed in the mining area, which is of great significance for the understanding the Neoproterozoic tectonic evolution of the Yangtze Block, but studies on these intermediate-acid rocks are scarce. The Bijigou mafic-ultramafic layered intrusion is surrounded by granite and cut by syenite veins. Here, we report new zircon U-Pb ages, Lu-Hf isotope composition and bulk rock geochemical data of the Bijigou syenite vein and wall-rock granite in the northwestern margin of the Yangtze Block. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating results show that the Bijigou syenite vein and wall-rock granite formed at 770 +/- 3.5 Ma (MSWD = 0.17, n = 28) and 810 +/- 4 Ma (MSWD = 0.84, n = 26), respectively. The zircon epsilon Hf(t) values of the syenite veins range from + 1.52 to + 5.33 (average of + 3.05), combined with its high potassium contents, negative Nb-Ta anomalies and positive Pb anomalies, suggesting that they may have originated from mantle-derived basaltic magma, which was modified by materials from subducting oceanic slab. The zircon epsilon Hf(t) and TDM2 ages of the wall-rock granite range from + 0.71 to + 5.71 (average + 3.06) and 1344 to 1659 Ma (average 1519 Ma), respectively, indicating that the granite was produced by partial melting of juvenile crust. The geochemical characteristics of the Bijigou syenite and granite indicate that they were formed in a continental margin arc setting. Thus, combined with previous studies, it suggests that there was a major subduction system along the northwestern margin of the Yangtze Block during 824-720 Ma, and the magmatism in the Hannan massif was divided into two episodes: (1) early magmatism (824-790 Ma), such as the Bijigou, Hongmiaozhen and Huangguanzhen granitoids, was derived from partial melting of the juvenile or pre-existing crust in a continental arc setting; (2) later magmatism (789-718 Ma), including Bijigou syenite, Wudumen, Erliba and Zushidian granitoids, formed in a subduction-related back-arc extensional environment. The long-term subduction system along the northwestern margin of the Yangtze Block during 824-720 Ma suggests that the Yangtze Block was previously located at the periphery of the Rodinia supercontinent.