Intra-Oceanic Subduction and Tectonic Framework of theEasternJiangnanOrogen in Neoproterozoic:Evidence from the Basic and Ultrabasic Rocks in Qingdingshan Area of Zhuji

Neoproterozoic magmatic rocks, which extensively developed in the Northwest Zhejiang Province of the eastern Jiangnan orogen, are the key for discussing the pattern and evolution of the Jiangnan orogen. Although they have a great deal of research, their petrogenesis and tectonic setting remain ambiguous. In this paper, a set of basic-ultrabasic rocks from Qingdingshan area, Zhuji, Zhejiang Province are selected as objects, and detailed zircon chronology,Hf isotopic composition and geochemistry of rocks are studied. Zircon LA-ICP-MS U-Pb dating of Qingdingshan basic-ultrabasic rocks shows that these rocks were formed at 860 Maapproximately. The zircon ε_Hf(t) values are 9.7 and 9.5, respectively. According to the geochemical characteristics of rocks, Qingdingshan basic-ultrabasic rocks are divided into two types: “low Al and high Mg” and “high Al and low Mg”. The former rocks are characterized by high TiO₂, MgO, Fe₂O₃^T, low Al₂O₃, rich in Cr-Ni and low Sr/Y value, while the latter is characterized by low TiO₂, MgO, Fe₂O₃^T, high Al₂O₃, poor Cr-Ni and high Sr/Y value. The geochemical characteristics and zircon Hf isotopic composition indicate that the Qingdingshan basic-ultrabasic rocks were formed in the oceanic island arc environment, and their genesis may be related to the subduction of the oceanic plate and the mantle wedge metasomatism at different depths. Based on the previous research results, the tectonic framework of the Neoproterozoic multi-arc-basin system in the eastern Jiangnan orogen has been preliminarily constructed for the “Longmenshan intraoceanic arc (1 172－840 Ma)-Anhui-Jiangxiinter-arcbasin (1 061－827 Ma)-Wannian continental margin arc (～840 Ma)-Jiangxi-Hunan back-arc basin (820－830 Ma)”. It provides a new perspective for the study of Jiangnan orogen.