内蒙古林西地区上二叠统-中三叠统沉积序列的碎屑锆石记录及对古亚洲洋(东段)闭合时间的制约

Detrital Zircon Records of Upper Permian-Middle Triassic Sedimentary Sequence in the Linxi Area,Inner Mongolia and Constraints on Timing of Final Closure of the Paleo-Asian Ocean(Eastern Segment)

晚二叠世-中三叠世在中亚造山带东段构造演化过程中是重要的转折期,它涉及古亚洲洋的最终闭合、沉积环境剧变等重大地质问题,而上二叠统林西组和中-下三叠统幸福之路组沉积序列记录了相应的构造演化过程。笔者对内蒙古东南部林西地区出露良好的林西组与幸福之路组,采取砂岩样品进行碎屑锆石U-Pb年代学和Lu-Hf同位素研究,解析年代学、沉积物源等信息。研究显示,被认为是三叠系标志层的陆相红色砂泥岩始现于晚二叠世。幸福之路组细砾岩样品最年轻碎屑锆石年龄为241Ma,从而将地层沉积时代上延至中三叠世后期。它与下伏林西组为整合接触关系,但在幸福之路组下部地层中发现微角度不整合,沉积时代存在间断(早、中三叠世之间)。林西组砂岩碎屑锆石U-Pb年龄组成分为4组:254~336Ma、372~528Ma、669~1000Ma和1300~2534Ma,εHf(t)值变化范围较大(-25.6^+17.2)。幸福之路组砂岩碎屑锆石U-Pb年龄集中于241~278Ma,εHf(t)值=+8.6^+16.5,其石炭纪和古元古代碎屑锆石少量。对比研究显示,林西组砂岩除兴蒙造山带物源外,还含有大量华北克拉通物源。与之相反,幸福之路组砂岩物源主要来自林西周边岩浆岩。结合区域地质特征,笔者认为古亚洲洋闭合于晚石炭世之前,晚古生代末是残余陆表浅海环境。内蒙古东南部地区在晚石炭世以来,表现为西伯利亚和华北克拉通联为一体后的陆内裂陷沉积活动及岩浆活动。

It is an important period for the tectonic evolution of the eastern segment of the Central Asia Orogenic Belt during the Late Permian to Middle Triassic, which involved the final closure of the Paleo-Asian Ocean and the sharp changes of sedimentary environment. The well exposed Upper Permian Linxi Formation and Lower-Middle Triassic Xingfuzhilu Formation record the evolution history of southeastern Inner Mongolia. Then, sandstone samples were chosen to carry out U-Pb and Lu-Hf isotope test, which is used to analyze the geochronology and extract sediment provenance information. The new U-Pb data show the red stratum which was considered as the symbol of Triassic before is also discovered in Late Permian, southeast of Linxi county. In addition, the previous Early Triassic Xingfuzhilu Formation is probably an Early to Middle Triassic sedimentary sequence, which is conformable contact with lower Linxi Formation. Furthermore, an age gap between Early and Middle Triassic was found base on both geochronological data and field observation. The geochronological results also show that sandstones from Linxi Formation have four groups of U-Pb age at 254-336Ma, 372-528Ma, 669-1000Ma, and 1300-2534Ma. The εHf（t） values range from -25.6-＋17.2. In contrast, samples from Xingfuzhilu Formation only have one group of U-Pb age at 241-278Ma, while the εHf （t） values range from ＋8. 6-＋16. 5, and few Carboniferous and Paleoproterozoic detrital zircons were found. In view of different characteristics of U-Pb and Hf isotope, the sandstones from Linxi Formation probably sourced from Xing-Meng Orogenic Belt and north of the North China craton, while the sandstones from Xingfuzhilu Formation mainly sourced from surrounding magmatic rocks. These results, combined with regional data, indicate that the final closure of the Paleo- Asian Ocean is before Late Carboniferous. Since then, the sediments and magmatism happened in an intracontinental environment.