鄂尔多斯盆地西南部早—中二叠世源—汇充填过程及构造—沉积格局

The Early-Middle Permian source to sink filling process and its tectonic-sedimentary response in the southwestern Ordos Basin

鄂尔多斯盆地上古生界是主要的含气层,但其西南部早—中二叠世不同物源体系及构造—沉积格局的争议制约了进一步的勘探与开发。基于鄂尔多斯盆地西南部下—中二叠统85件样品的主、微量和稀土元素测试结果及16件锆石分析测试数据,多方法综合分析鄂尔多斯盆地太原—石盒子期沉积物源及其控制下的充填过程。通过微量稀土元素物源背景分析,认为太原组大地构造背景以被动大陆边缘为主,山西组和石盒子组表现为主动大陆边缘和大陆岛弧沉积。太原组沉积时期物源体系单一,母岩成分复杂;到山西组、石盒子组沉积时期,存在多个物源体系,母岩成分趋于稳定。太原组样品的锆石U-Pb年龄主要分布在400~500 Ma之间,且呈单峰分布;山西组样品则大致可以分为3段:280~500 Ma、1824~1873 Ma和2440~2569 Ma;石盒子组样品的锆石U-Pb年龄介于254~2769.2 Ma之间,年龄区间大,且不同区域锆石年龄谱存在较大的差异。综合多种物源分析结果,认为研究区太原组物源主要来自北祁连,而北部中亚造山带的影响较为有限;山西组沉积时期,祁连构造带抬升加剧、供源增强,但低于同时期华北古老基底;石盒子组沉积期,华北克拉通古老基底供源持续增强,同时中亚造山带的影响逐渐增强并成为第二大物源,北秦岭、北祁连局部供源,研究区此时属于鄂尔多斯盆地南北源—汇系统交汇区。

The Upper Paleozoic in Ordos Basin is the main gas-bearing reservoir,but what restricts the basin’s further exploration is the dispute of different provenance systems and tectonic-sedimentary pattern in the early-middle Permian in the southwest.Based on the main,trace and rare earth elements test results of 85 Lower and Middle Permian samples and 16 zircon analysis data,the sediment sources and their controlled filling processes of the Taiyuan-Shihezi Stage in the southwestern Ordos Basin are comprehensively analyzed by multiple methods.According to the provenance background of trace and rare earth elements,it is considered that the tectonic background of Taiyuan Formation is mainly passive continental margin,and Shanxi Formation and Shihezi Formation are active continental margin and continental island arc.The provenance system of Taiyuan Formation is single and the parent rock composition is complex.In the sedimentary period of Shihezi Formation in Shanxi Province,there were many provenance systems,and the parent rock composition tended to be stable.The zircon U-Pb age of Taiyuan Formation is mainly distributed in 400-500 Ma and shows a single peak distribution.The samples of Shanxi Formation can be roughly divided into three sections:280-500 Ma,1824-1873 Ma and 2440-2569 Ma;the zircon U-Pb age ranges from 254 Ma to 2769.2 Ma and the zircon age spectrum varies greatly in different regions.Based on the results of various provenances,it is considered that the provenance of Taiyuan Formation in the study area mainly comes from North Qilian,while the influence of northern Central Asian orogenic belt is limited;during the sedimentary period of Shanxi Formation,the uplift of Qilian structural belt intensified and the source was enhanced,but it was lower than the ancient basement in North China in the same period.During the sedimentary period of Shihezi Formation,the source supply of ancient basement in North China Craton increased,while the influence of Central Asian orogenic belt gradually increased and became the second largest source,and the study area of local source supply in North Qinling and North Qilian belongs to the intersection area of north-south source and sink systems in Ordos Basin.