Zircon U-Pb ages of the Neoproterozoic Jiuling complex granitoid in the eastern segment of the Jiangnan orogen and its tectonic significance

Located in eastern Jiangnan orogen, the Jiuling pluton is the largest Neoproterozoic granitoid intrusion in South China. According to mineral assemblage, structure and intrusive contact relationships, Jiuling pluton can be divided into complex massif with 3 intrusion orders, followed by biotite-granodiorite, tonalite and biotite-monzogranite from early to late respectively. Among them, granodiorite are distributed most widely, followed by biotite-monzogranite, while tonalite is only distributed in a minimum area. They intruded into the surrounding epimetamorphic rocks of Neoproterozoic period. The LA-ICP-MS zircon U-Pb dating results show that the biotite-granodiorite, tonalite and biotite-monzogranite were formed in 821.6-821.6 Ma, 819.5-823.6 Ma and 820.4-824.5 Ma, respectively, which indicates that they belong to the product of contemporary magmatic activity. On the whole, however, the ganitoids tend to be younger from the southeast side to the northwest side. Besides, the dip angle of contact interface between Jiuling pluton and surrounding rocks (Shuangqiaoshan Group) is steep in the south and smooth in the north, hornfelsic belt is narrow in the south and wide in the north, and the xenoliths from the surrounding rock (Shuangqiaoshan Group) and the captured zircons also tend to decrease in size from south to north. These characteristics show that the rocks on the southeast side was eroded deeper than those on the northwest side, probably implying that, during the collision between Cathaysia plate and Yangtze plate in Neoproterozoic, the original depth of the granitic magma on the southeast side was deeper, the denudation was more intense, and the magma was formed earlier than the magma on the northwest side and gradually migrated northwestward, i.e., the origin depth of magma became shallow and the formation time became younger.