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

摘要

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.