鄂尔多斯盆地西南缘富县组凝灰岩夹层年代学及地球化学研究

The process of uplift, erosion, subsidence and deposition from the end of Late Triassic Yanchang period to Early Jurassic in Ordos basin, which are in response to the evolution of Qinling orogenic belt because of continental collision of North China and Yangtze block. Fuxian formation is the earliest sedimentary record of Jurassic with fluvial couplet clastic sediments,developing on the fluctuate palaeo-geomorphology due to the stress field changed at the end of Yanchang period in southwestern Ordos basin. It has an important geological significance of the precise age and formational mechanism of tuff interlayer within Fuxian formation to reveal the basin and regional dynamics environment. The tuff interlayer of Fuxian formation is recently found in southwestern Ordos basin, through the detailed sedimentological analysis to identify the spatial distritution of thickness; obtaining the exact age of tuff interlayer by zircon U-Pb chronology; analysing the nature and types of tuff interlayer by lithology, mineralogy, in situ zircon Hf isotope geochemistry and elements geochemistry; and then combining the regional geology background and synchronizing magmatic activities to discuss the origin and formational mechanism of tuff interlayer. The aim of the project is providing the temporal constraint and litho-geochemistry evidences to reveal the pre-Jurassic geodynamic setting of Ordos basin and adjacent areas.

鄂尔多斯盆地晚三叠世延长期末构造抬升、遭受剥蚀和侏罗纪早期又复沉降、接受沉积，是对华北-扬子大陆碰撞和其间秦岭造山带演化进入重要转折阶段的区域动力学响应。盆地西南缘富县组是在延长期末抬升剥蚀的侵蚀古地貌背景下发育的一套河流相粗-细碎屑沉积，代表着盆地构造环境转换进入侏罗纪湖盆鼎盛期前的最早沉积记录，对发育于其中的凝灰岩夹层时代的精确测定和形成环境的深入研究具有重要的盆地和区域动力学意义。本研究拟对新近在盆地西南缘发现的富县组凝灰岩夹层，进行详细的沉积学分析，以查明其空间分布特征；通过锆石U-Pb年代学测试获得其精确年龄；利用岩石矿物学、锆石原位Hf同位素及元素地球化学等综合分析凝灰岩夹层属性、类型；并结合区域地质背景和周邻同期岩浆活动特征等各种相关资料及研究成果，探讨凝灰岩夹层源区及其成因机制；为揭示侏罗系沉积前盆地及邻区地球动力学环境提供时间约束和岩石-地球化学证据。

鄂尔多斯盆地晚三叠世延长期末构造抬升、遭受剥蚀和侏罗纪早期又复沉降、接受沉积，是对华北-扬子大陆碰撞和其间秦岭造山带演化进入重要转折阶段的区域动力学响应。盆地西南缘富县组是在延长期末抬升剥蚀的侵蚀古地貌背景下发育的一套河流相粗-细碎屑沉积，代表着盆地构造环境转换进入侏罗纪湖盆鼎盛期前的最早沉积记录，对发育于其中的凝灰岩夹层时代的精确测定和形成环境的深入研究具有重要的盆地和区域动力学意义。. 本项目通过对鄂尔多斯盆地西南缘富县组凝灰岩夹层锆石U-Pb年代学研究，确定其年龄为195±4Ma；同时对富县组砂岩做了碎屑锆石定年，其中最年轻的一组年龄的峰值也为195Ma，说明富县组的沉积年龄为195Ma。这一年龄值表明富县组沉积年代为早侏罗世早期，而不是前人认为的早侏罗世晚期。. 通过对富县组及其上、下安口组与延长组碎屑锆石年代学、砂岩的岩石矿物学、元素地球化学物源综合分析，重塑了盆山耦合关系，指出在223Ma前后，秦岭造山带才开始抬升；223-204Ma为鄂尔多斯前陆盆地的重要发育时期；204-195Ma秦岭造山带与鄂尔多斯盆地处于准平原状态，盆地内不断遭受剥蚀，河流切割严重；195Ma秦岭造山带由碰撞造山开始向陆内造山转换。