Detrital zircon U-Pb geochronology and geochemistry of early Neoproterozoic sedimentary rocks from the Northwestern Zhejiang Basin, South China

The Jiangnan Orogen in the South China Block collapsed rapidly after the Neoproterozoic assembly of the Yangtze and Cathaysia blocks. Extensional collapse of orogens is generally triggered by post-orogenic extension and/or continental rifting. Detrital zircon U-Pb geochronology and trace elements geochemistry was conducted on sedimentary rocks across the Neoproterozoic angular unconformity in the Northwestern Zhejiang Basin. Detrital zircon U-Pb ages indicate that the Chentangwu Formation (upper Pingshui Group) and the Heshangzhen Group were deposited from 840 to 820 Ma and 810-780 Ma, respectively. The Trace elements geochemistry of early Neoproterozoic detrital zircons suggests that the source rocks of the Chentangwu Formation were derived from continental arcs and oceanic islands whereas those of the Heshangzhen Group (the Luojiamen and the Hongchicun formations) were likely related to continental arcs, oceanic islands and mid-oceanic ridges. This study reveals that the Chentangwu Formation was probably deposited in a Neoproterozoic back-arc basin which was filled by sediments from the Cathaysia Block, the Shuangxiwu Arc, and the Jiangnan Orogen. The source regions of the Hongchicun formations outcrops in the eastern and western Northwestern Zhejiang Basin are the Cathaysia Block and the Jiangnan Orogen, respectively. However, the Shangshu Formation in the west has the same source regions as the underlying Hongchicun Formation in the east. The variations in thickness and source rocks exhibited by the Hongchicun formations in the west and east are controlled by normal faulting along the Dexing-Huangshan fault zone, which is the western boundary fault of the Northwestern Zhejiang Basin. The growth fault resulted in the formation of a ridged rollover anticline in the middle of the Northwestern Zhejiang Basin and prevented ∼810 Ma zircons from being transported to the west during deposition of the Hongchicun Formation. Comprehensive analysis reveals that the extensional collapse of the Jiangnan Orogen resulted from 810 to 780 Ma post-orogenic extension.