贺兰山地区三叠系延长组沉积时限及形成环境:来自凝灰岩与玄武岩锆石U-Pb年龄的约束OA

The Helan Mountains lie along the northwestern margin of the Ordos Basin,where the Triassic Yanchang Formation is widely exposed but still lacks tight geochronologic control.This uncertainty limits regional correlation and hinders reconstruction of depositional systems and basin paleogeography.Here we bracket the depositional interval of the Yanchang Formation using paired zircon U-Pb ages from two thin tuff beds(lower and upper Yanchang)and the basalt that caps the succession.We integrate petrography,whole-rock major and trace element geochemistry,and LA-ICP-MS zircon U-Pb dating,and we use tectonic discrimination diagrams to evaluate magma provenance and tectonic setting.Zircons from the lower and upper tuffs yield weighted mean ages of 244.1±1.7 Ma and 201.9±2.2 Ma,respectively,whereas the youngest magmatic zircons from the capping basalt give ages of 201.5～194.1 Ma.Geochemical signatures indicate that the tuffs were sourced from arc-related,calc-alkaline volcanism,and that the basalt is tholeiitic,derived from an enriched mantle source overprinted by subduction-related modification,yet emplaced during intracontinental extension.Together,these ages constrain deposition of the Yanchang Formation in the Helan Mountains to the Middle-Late Triassic,broadly comparable to the depositional timing of the Yanchang Formation in the basin interior.The combined geochronologic and geochemical data further point to a regional tectonic shift from a subduction-related active continental margin arc setting to post-orogenic intracontinental extension.These results provide a firm temporal framework for stratigraphic correlation and for reconstructing depositional architecture and tectonic evolution along the northwestern Ordos Basin margin.