Geochronology and structural deformation of Precambrian metamorphic basement in the eastern Jiangnan orogenic belt: constraints on the assembly time of the Yangtze and Cathaysia blocks.

The Jiangnan Orogenic Belt in South China formed as a result of the amalgamation between the Yangtze Block and the Cathaysian Block and records the geological information of collision. However, the timing of the amalgamation of two blocks remains controversial. Here, structural deformation and zircon U-Pb geochronological results from Precambrian metamorphosed sedimentary-volcanic rock units at the eastern domain of the Jiangnan orogenic belt in southern Anhui Province are presented. The pre-Nanhua rocks in the study area were widely involved in the metamorphism and deformation to with NW or SE-dipping foliation, tight folds and local development of strike-slip ductile shear zones, which is induced by collision of Yangtze Block and the Cathaysia Block. In this study, a mylonitic granodiorite sample intruding in the Xikou Group, a mylonitic dacite sample from Jingtan Formation and two deformed quartz veins samples in the Shexian region were dated by LA-ICP-MS zircon U-Pb dating method. Zircons from the mylonitic granodiorite and dacite yield the weighted mean ages of 811.2 ± 7.5 Ma (MSWD = 0.51, n = 30) and 779 ± 7.4 Ma (MSWD = 0.42, n = 29), respectively, interpreted as the crystallization ages of the original rocks, whereas the zircons from deformed quartz veins yield the weighted mean ages of 779 ± 14 Ma (MSWD = 2.6, n = 22) and 769 ± 16 Ma (MSWD = 2.4, n = 16), respectively, representing the crystallization age of the quartz veins. Combined with previous data, our results show that the metamorphosis and deformation of Pre-Nanhua rocks took place during the period of 820–770 Ma. Therefore, we consider that the Yangtze Block and the Cathaysian Block the eastern Jiangnan Orogenic Belt don't amalgamate until 770 Ma. [ABSTRACT FROM AUTHOR]