Nature of the Late Ordovician-Early Silurian Xiaohe section, Hunan-Hubei area, South China: implications for the Kwangsian Orogeny

The intraplate Kwangsian Orogeny is a key orogenic event in South China in the mid-Paleozoic. We re-examined the evidence for the Yichang Uplift, an inferred geographic feature during the Kwangsian Orogeny, to evaluate its timing and nature. Field, sedimentological, mineralogical and geochronological data were collected from the Late Ordovician-Early Silurian Xiaohe section, Hunan-Hubei area. Results suggest that the Xiaohe section is composed of the Late Ordovician Wufeng Formation black shale in the lower part and the Early Silurian Longmaxi Formation black shale in the upper part. We found that the clay layers interbedded in the Wufeng Formation are altered rhyolitic tuffs instead of parts of a subaerial wreathing crust. LA-ICP-MS U-Pb dating of zircons in the top tuff layer of the Wufeng Formation yielded an age of 447.0 + 1.4/- 2.2 Ma, consistent with biostratigraphic data, providing a radiometric constraint for the sedimentary break existed between the Wufeng and Longmaxi formations and confirming the absence of the Hirnantian (latest Ordovician) Guanyinqiao Formation in the study area. Our data support that the Yichang Uplift was a submarine highland possibly initiated by the reactivation of the inherited Jianshi-Enshi Fault in the Hunan-Hubei area during the Kwangsian Orogeny.

板内广西造山运动（Kwangsian Orogeny）是中古生代华南地区的关键造山事件。我们重新考察了广西造山运动期间被推断存在的地理构造——宜昌隆起（Yichang Uplift）的相关证据，以评估其形成时间与本质属性。研究团队从湘鄂地区晚奥陶世-早志留世小河剖面（Xiaohe section）采集了野外地质、沉积学、矿物学及年代学数据。结果表明，该剖面下部为晚奥陶世五峰组（Wufeng Formation）黑色页岩，上部为早志留世龙马溪组（Longmaxi Formation）黑色页岩。我们发现，五峰组内互层的黏土层实为蚀变流纹质凝灰岩，而非陆表风化壳的组成部分。对五峰组顶部凝灰岩夹层中的锆石（zircons）进行LA-ICP-MS U-Pb定年，获得了447.0 +1.4/-2.2 Ma的年龄数据，该结果与生物地层学数据一致，为五峰组与龙马溪组之间的沉积间断提供了放射性同位素约束，并证实研究区域内缺失赫南特阶（Hirnantian，晚奥陶世最末期）观音桥组（Guanyinqiao Formation）。本研究数据支持宜昌隆起为水下高地，其可能是在广西造山运动期间，由湘鄂地区继承性建始-恩施断裂（Jianshi-Enshi Fault）活化所形成。