华北克拉通北缘侏罗纪造山过程及关键时限的沉积证据

Sedimentary Evidences of Jurassic Orogenic Process and Key Time Limit on the Northern Margin of North China Craton

宁武—静乐盆地与浑源盆地位于华北克拉通中北部,侏罗纪地层序列完整,物源指向于阴山—燕山造山带中段,通过对盆地内侏罗纪沉积特征的研究,可以演绎华北克拉通北缘侏罗纪的造山过程。在盆地沉积分析的基础上,通过宁武—静乐盆地中侏罗统云岗组顶部的凝灰质泥晶碳酸盐岩及浑源盆地上侏罗统髫髻山组玄武-安山岩锆石U-Pb同位素测年,结合国际地层年代表推荐年龄及中侏罗统的沉积速率,对整个侏罗纪沉积序列转换的关键时限进行了限定。研究认为：中侏罗统云岗组底部砾岩沉积时期,侏罗纪沉积演化序列经历了早期湖进至晚期湖退的转换过程,暗示着区域应力场由早期的拉张向晚期的挤压转换,孕育着侏罗纪造山运动的开始,具体时限大约为168Ma;中侏罗统云岗组顶部凝灰质泥晶碳酸盐岩沉积时期,沉积地层的颜色由其下的灰绿色突变为其上的紫红色,孕育着湖盆地形的突然抬升,区域氧化性增强,气候环境的突变,侏罗纪造山运动进入了快速发展阶段,具体时限为161.0~159.0Ma;中/晚侏罗世沉积序列具有继承性的发育特征,随着湖盆地形的进一步抬升,侏罗纪沉积范围迅速退至造山带前缘,以砾岩沉积为主,为同期造山运动的产物,侏罗纪造山运动进入了高峰期,具体时限为159.0~153.0Ma;晚侏罗世晚期,区域上以玄武-安山岩及凝灰质角砾岩沉积为主,侏罗纪造山运动进入了造山期后的调整阶段。

Ningwu-Jingle basin and Hunyuan basin are located within the north-central of North China craton,and the Jurassic sedimentary sequences are complete from Early Jurassic to Late Jurassic,whose provenance came from the middle of Yinshan-Yanshan orogenic belt.Therefore,we may infer theorogenic process by the research of sedimentary sequences in these basin.According to U-Pb dating for the tuffaceous micritic limestone at the top of the Yungang Formation in the Middle Jurassic and the basalt-andesite at the bottom of the Tiaojiashan Formation,the recommended age of the Middle Jurassic in the international stratigraphic chart and the sedimentary rate of it,the key limit of the change of Jurassic sedimentary sequences was determined.The Jurassic sedimentary sequences experienced a conversion from the early lacustrine transgressive sedimentation to the late lacustrine regressive one at the bottom of the Middle Jurassic Yungang Formation（J2yg）.It was indicated that the regional tectonic stress field converted from extensional environment to compressional one,giving birth to the beginning of the Jurassic orogenic movement,and the time limit is about 168 Ma.The colors of the tuffaceous micritic limestone on the top of the Yungang Formation in the Middle Jurassic varied from celadon to fuchsia,indicating a suddenly uplift of the lake basin topography and a quick strengthen of regional oxidizing.The Jurassic orogenic movement experienced a stage of quick development,and the time limit is about 161.0-159.0 Ma.The sedimentary sequence from the Middle Jurassic to the Late Jurassic is continuous and inherited.With the further uplift of the lake basin topography,the sedimentary scope of Jurassic retreated quickly the leading edge of the orogenic belt.It is characterized by the synorogenic conglomerates,while Jurassic orogenic movement reached to the peak period,and the time limit is about159.0-153.0Ma.In the Late Jurassic,the regional sedimentary sequences consisted of basalt-andesite and the tuffaceous breccia,the Jurassic orogenic movement came into a post-orogenic adjustment stage.