New U-Pb age constraints on the upper Banxi Group and synchrony of the Sturtian glaciation in South China

摘要

The Nanhua basin in South China hosts well-preserved middle–late Neoproterozoic sedimentary and volcanic rocks that are critical for studying the basin evolution, the breakup of the supercontinent Rodinia, the nature and dynamics of the “snowball” Earth and diversification of metazoans. Establishing a stratigraphic framework is crucial for better understanding the interactions between tectonic, paleoclimatic and biotic events recorded in the Nanhua basin, but existing stratigraphic correlations remain debated, particularly for pre-Ediacaran strata. Here we report new Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) U-Pb zircon ages from the middle and topmost Wuqiangxi Formation (the upper stratigraphic unit of the Banxi Group) in Siduping, Hunan Province, South China. Two samples show similar age distribution, with two major peaks at ca. 820?Ma and 780?Ma and one minor peak at ca. 910?Ma, suggesting that the Wuqiangxi sandstone was mainly sourced from Neoproterozoic rocks. Two major age peaks correspond to two phases of magmatic events associated with the rifting of the Nanhua basin, and the minor peak at ca. 910?Ma may correspond to the Shuangxiwu volcanic arc magmatism, which represents pre-collision/amalgamation subduction on the southeastern margin of the Yangtze Block. The youngest zircon group from the topmost Wuqiangxi Formation has a weighted mean age of 714.6?±?5.2?Ma, which is likely close to the depositional age of the uppermost Banxi Group. This age, along with the ages reported from other sections, constrains that the Banxi Group was deposited between ca. 820?Ma and ca. 715?Ma. The age of 714.6?±?5.2?Ma from the top of the Wuqiangxi Formation is indistinguishable with the SIMS U-Pb age of 715.9?±?2.8?Ma from the upper Gongdong Formation in the Sibao village section of northern Guangxi, South China. It is also, within uncertainties, overlapped with two TIMS U-Pb ages from pre-Sturtian strata in Oman and Canada. These ages indicate that the Jiangkou (Sturtian) glaciation in South China started at ca. 715?Ma instead of ca. 780?Ma and support a globally synchronous initiation of the Sturtian glaciation at ca. 715?Ma. Graphical abstract Display Omitted Highlights ? New ages are reported from the Wuqiangxi Formation in Nanhua basin. ? Deposition of the Banxi Group ended at ca. 715?Ma rather than ca. 780?Ma. ? The Jiangkou (Sturtian) glaciation initiated synchronously at ca. 715?Ma. ? The Wuqiangxi Formation was mainly sourced from Neoproterozoic rocks.