大兴安岭中段林西组砂岩LA-ICP-MS碎屑锆石U-Pb年代学及其地质意义

大兴安岭中段扎赉特旗西北部分布大面积的晚二叠世林西组沉积地层，该沉积地层主要为长石砂岩、粉砂岩、长石岩屑砂岩夹不同规模的含砾砂岩及灰岩，中部含有较为丰富的双壳类、植物化石。本次研究针对林西组下部的长石岩屑砂岩进行LA-ICP-MS碎屑锆石U-Pb测年工作，共获得98个有效年龄，年龄分布范围为（2 552.3±22.5）~（227.6±5.4）Ma，大部分年龄位于谐和曲线上，部分结果偏离曲线，根据测试数据按年龄值和频率分布特征大致分为以下几组：上泥盆世—晚二叠世（380.0~246.1 Ma）、志留纪—寒武纪晚期（489.0~413.8 Ma）、中元古代—新元古代（1 239.5～808.1 Ma）、古元古代（2 554.0～1 784.3 Ma）。上述年龄与区域上的岩浆事件基本吻合。综合研究表明研究区内林西组沉积下限为（253.4±1.5） Ma，林西组的物源主体应来自于西拉木伦河—延吉一线，其余锆石来源主要为兴蒙造山带内部、华北板块北缘和东北各地块，1.8和2.5 Ga的碎屑锆石更有可能来自华北板块北缘，华北板块和西伯利亚板块在（253.4±1.5） Ma之前已经完成碰撞、拼合，古亚洲洋已经闭合，兴蒙海槽形成。林西组海相核形石的发现表明晚二叠世大兴安岭中段地区应以海相环境为主，直到三叠纪早期才逐渐转变为陆相沉积环境。

LA-ICP-MS U-Pb Dating of Detrital Zircons from LinxiFormation in Jalaid Banner Region，Middle Part of the Greater#br# Khingan Mountains and Its Geological Implications

The Late Permian Linxi Formation is widely distributed in the northwest of Jalaid Banner, Middle Part of the Greater Khingan Mountains. The formation is mainly composed of arkose，siltstone，feldspar lithic sandstone containing pebbled sandstone, and limestone of different scales, and the middle part contains abundant bivalves and plant fossils. In this study, LA-ICP-MS U-Pb dating was carried out for the arkose litharenite sandstone in the lower part of the Linxi Formation, and a total of 98 effective ages were obtained. Based on the test data, they are roughly divided into the following groups according to the characteristics of age value and frequency distribution: Upper Devonian-Late Permian（380.0-246.1 Ma）,Silurian-Late Cambrian（489.0-413.8 Ma）,  Mesoproterozoic-Neoproterozoic（1 239.5-808.1 Ma）, Paleoproterozoic（2 554.0-1 784.3 Ma）. The above six groups of age peaks are basically consistent with the time of the regional magmatic activities. The comprehensive study shows that the lower deposition limit of the Linxi Formation in the study area is in（253.4±1.5） Ma, the remaining zircons are mainly from the Xingmeng-Mongolia orogenic belt, the Northern Margin of the North China plate, and the various blocks in Northeast China. The detrital zircons with 1.8 and 2.5 Ga ages are more likely to have come from the northern margin of the North China plate. The collision of the North China plate and the Siberian plate was completed before（253.4±1.5）Ma, the ancient Asian Ocean was closed, and the Xingmeng Trough was formed. The discovery of the marine  oncolytic in the Linxi Formation also confirms that the marine environment should still be dominant in the Middle Part of the Greater Khingan Mountains in the Late Permian, then it gradually changed into the continental sedimentary environment.