New zircon U-Pb ages of the pre-Sturtian rift successions from the western Yangtze Block, South China and their geological significance

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ABSTRACTThe Neoproterozoic Kaijianqiao Formation is one of the most important pre-Sturtian rift successions in South China and there has long been a lack of reliable geochronological constraints for its minimum depositional age. In this study, new zircon U-Pb ages of volcaniclastic rocks from the topmost Kaijianqiao Formation are presented. The youngest SHRIMP and LA-ICP-MS zircon 206Pb/238U weighted mean ages of the tuff sample are 715.0 ± 9.8 and 718.8 ± 9.4 Ma, respectively. The youngest LA-ICP-MS zircon 206Pb/238U weighted mean age of the tuffaceous siltstone sample is 720.8 ± 7.4 Ma and represents the maximum depositional age of the topmost Kaijianqiao Formation. The results show that the minimum depositional age of the Kaijianqiao Formation in the western Yangtze Block should be ca. 715 Ma, consistent with other pre-Sturtian rift successions in South China, such as the Banxi Group, Chengjiang, and Liantuo formations. Together with the published zircon U-Pb ages, it is demonstrated that the Sturtian glaciation in South China (Jiangkou glaciation) most likely initiated around 715 Ma. In other Rodinia blocks, like Laurentia and Arabia, the Sturtian glaciation probably started between 712 and 717 Ma, thus our new results further support that the Sturtian glaciation was a rapid and globally synchronous event. Other 206Pb/238U zircon ages display five distinct peaks at ca. 751, 780, 799, 819, and 848 Ma, which corresponded to the tectonic-magmatic events related to the break-up of Rodinia.