Detrital zircon geochronology of Precambrian basement sequences in the Jiangnan orogen: Dating the assembly of the Yangtze and Cathaysia Blocks

In the Jiangnan orogen, a clear angular unconformity between the Precambrian basement sequences and the overlying Neoproterozoic sedimentary strata (e.g. the Danzhou/Banxi Group, younger than ca. 800 Ma) marks the collisional orogenesis (the Jinning orogeny) between the Yangtze and Cathaysia Blocks. In contrast to the upright, open folds in the Danzhou/Banxi Group, the basement sequences were deformed into high-angle tight linear and isoclinal overturned folds. It has been previously accepted that the basement sequences are of Mesoproterozoic age. However, LA-ICP-MS U-Pb dating of detrital zircons suggests that the maximum depositional age of the basement sedimentary rocks in the western part of the Jiangnan orogen (i.e. the Sibao/Lengjiaxi Group) is ca. 860 Ma. This provides a lower limit for the assembly of the Yangtze and Cathaysia Blocks. Consequently, there may be no significant (ca. 200 Ma) early Neoproterozoic sedimentary hiatus in South China. These data, combined with published dates on orogeny-related igneous rocks in the Jiangnan orogen, indicate that the Jinning orogeny took place at 860-800 Ma, significantly younger than the typical Grenvillian orogeny at 1.3-1.0 Ga. The Sibao/Lengjiaxi Group may have been deposited in a foreland basin. The Yangtze Block and the arc terrains that resulted from the early subduction along the Jiangnan orogen might be the two main source regions for the sedimentary rocks.