Provenance and Palaeogeomorphology study of the southwestern North China Craton during Middle-Late Triassic and its geological implication

The study of tectonic, provenance, and paleogeomorphologic systems is important for defining the basin evolution and it is also the basis for the accurate evaluation of oil and gas potential of a basin. The provenance and paleogeomorphology of the western Ordos Basin and the Hexi Corridor in the southwestern North China Craton(sw-NCC) have long been debated. In this study, detrital zircon geochronology, petrography and geochemistry methods are used to analyse the evolution of provenance and paleogeomorphology in the sw-NCC during the Middle–Late Triassic, and the provenance and paleogeomorphic relationships with tectonic events are discussed. The U–Pb detrital zircon geochronology of the Yaoshan samples in the western Ordos Basin are clear and consistent, and shows that there are three major age populations, 2700–2200 Ma, 2150–1650 Ma and 450–220 Ma. While the age spectrums of the samples from different formations in the Hexi Corridor are complex, showing significant differences. The whole results imply that the Hexi Corridor and the west margin of the Ordos Basin had a unified source during the Middle Triassic, which is the western NCC. However, in the Late Triassic, the sediment of the Hexi Corridor was mainly from the Qilian–Qaidam Terrane while the source of the western Ordos Basin remained unchanged. Besides the provenance analysis, the stratigraphic correlation, petrology and geochemistry analyses also support that the Hexi Corridor was temporarily connected with the Ordos Basin during the Middle Triassic but was separated from the Ordos Basin by a small-scale, underwater paleo-uplift during the Late Triassic. The evolution of the provenance and paleogeomorphology in the southwestern Ordos Basin was related to the closure of the shallow shelf of the Paleo-Tethys Ocean.构造物源与古地貌系统的研究，对于明确盆地演化历程至关重要，同时亦是精准评价盆地油气资源潜力的核心基础。华北克拉通西南部（southwestern North China Craton，sw-NCC）的鄂尔多斯盆地西部与河西走廊地区的物源及古地貌格局，长期以来存在诸多学术争议。本研究采用碎屑锆石年代学、岩石学与地球化学方法，对华北克拉通西南部中晚三叠世的物源与古地貌演化过程开展分析，并探讨了物源、古地貌与构造事件之间的关联机制。鄂尔多斯盆地西部尧山样品的铀-铅（U-Pb）碎屑锆石年代学结果清晰且一致，显示出三个主要年龄区间：2700–2200 Ma、2150–1650 Ma 以及 450–220 Ma。而河西走廊不同地层样品的年龄谱系则较为复杂，呈现出显著的差异性。综合所有研究结果可知，中三叠世时期河西走廊与鄂尔多斯盆地西缘具有统一的物源，即华北克拉通西部。但进入晚三叠世，河西走廊的沉积物主要源自祁连-柴达木地体，而鄂尔多斯盆地西部的物源则未发生明显改变。除物源分析外，地层对比、岩石学与地球化学分析结果亦佐证：中三叠世时期河西走廊与鄂尔多斯盆地曾短暂连通，而晚三叠世二者被一处小型水下古隆起所分隔。鄂尔多斯盆地西南部的物源与古地貌演化，与古特提斯洋浅海大陆架的闭合过程密切相关。