大兴安岭南段阿鲁科尔沁旗地区林西组沉积环境特征及其时限的地质意义

Geological Significance of Sedimentary Environment Characteristics and Time Limit of Linxi Formation in Arhorchin Banner, Southern Great Xing’an Range

针对兴蒙-松嫩地区林西组沉积环境、沉积时限及古亚洲洋最终闭合时间等争议问题,通过对阿鲁科尔沁旗罕庙地区林西组的岩性、古生物、微量元素及碎屑锆石U-Pb测年等综合研究认为:林西组发育淡水双壳类和叶肢介化石,微量元素Sr/Ba值主要分布在0.05~0.18之间(平均值为0.12),B/Ga值主要分布在1.84~3.22之间(平均值为2.50),V/Cr值介于1.36~2.21之间(平均值为1.81),其沉积时期应为缺氧或贫氧的淡水湖泊环境。结合林西组砂岩碎屑锆石具有多时代物源特征、最年轻年龄为251 Ma、沉积下限年龄为晚二叠世等特征推测,古亚洲洋在研究区的最终闭合时限应为中二叠世末期,晚二叠世时期研究区即已转化为陆相沉积。

Aiming at the controversial issues such as the sedimentary environment, time-limit of Linxi Formation, and the final closing time of the Paleo-Asian Ocean in Xingmeng-Songnen area,a comprehensive analysis was conducted for the lithology and paleontology of Linxi Formation in Hanmiao area of Arhorchin Banner, in particular,the trace elements, and the U-Pb dating of detrital zircon. The ratio of trace elements Sr/Ba is mainly distributed between 0.05 and 0.18(average 0.12), the ratio of B/Ga is mainly distributed between 1.84 and 3.22(average 2.50), and the V/Cr value is between 1.36 and 2.21(average 1.81), which indicate that the sedimentary period of Linxi Formation was an anoxic freshwater lake environment. The freshwater bivalves and leaf-lime fossils are developed in Linxi Formation. The sandstone clastic zircons of Linxi Formation have multi-epoch provenance. The youngest age is 251 Ma, reflecting the lower sedimentary age in Late Permian. Based on the above characteristics, it can be inferred that the final closing time of the Paleo-Asian Ocean in the study area is the end of the Middle Permian, and the study area transformed into continental sediments during the Late Permian.