Geochronology and Hf Isotopes of Granite Gravel from Fanjingshan, South China: Implication for the Precambrian Tectonic Evolution of Western Jiangnan Orogen

The debate of assembly time between the Yangtze and Cathaysia blocks has aroused the disputation on the formation mechanism of Jiangnan orogen, South China. Therefore widespread magmatism of 830–750 Ma in South China is interpreted as the product of either plume or arc magmatism, which results in distinctive depositional background in the Neoproterozoic. Granite gravel located at the unconformity between the Banxi Group and Fanjingshan Group of the western Jiangnan orogen was collected, which gave a new age limit to the deposition of the Banxi Group and Nanhu rift. Zircons from the granite gravel crystallized yield a weighted mean 206Pb/238U age of 789±11 Ma, which probably represents the end of the Jiangnan orogeny and marks the onset of Nanhua rift. These zircons have negative εHf(t) of-2.1 to-6.0, with TDM of 1.38–1.52 Ga and Tcrust of 1.81–1.98 Ga indicating an old continental crust origin for the granite.