大兴安岭南段阿鲁科尔沁旗坤都地区林西组碎屑锆石U-Pb年龄、地球化学特征及其地质意义

大兴安岭南段阿鲁科尔沁旗坤都地区乔洛吐山林西组剖面,岩性为一套黄绿色岩屑长石细砂岩、黄绿色岩屑粉砂岩、灰黑色泥质粉砂岩等细碎屑岩组合,含丰富的双壳类、植物化石,指示时代为晚二叠世。用LA-ICP-MS测得乔洛吐山林西组下部黄绿色岩屑长石细砂岩样品中的碎屑锆石~(206)Pb/~(238)U年龄主要集中于267Ma、299Ma、484Ma、726Ma和933Ma五个峰值,最年轻的~(206)Pb/~(238)U锆石年龄为256Ma,最年轻的峰值年龄为267±2Ma,表明该地区林西组的最大沉积年龄不早于晚二叠世吴家坪期。林西组砂岩地球化学特征显示,乔洛吐山林西组物源具有长英质物源特性,沉积时期的水体为陆相开阔的淡水环境。结合古生物学、岩石学、地球化学和同位素年代学研究,推测研究区内古亚洲洋在晚二叠世已经闭合,林西组沉积环境为陆相湖盆,沉积物源显示出复杂性和多样性特点,反映物源区主要来自于兴蒙造山带,少量可能来自于华北板块。

U-Pb dating of detrital zircons and geochemical characteristics of Linxi Formation in the Kundu area of Ar Horqin Banner, southern Da Hinggan Mountains, and their geological significance

The Qiaoluotu geological section of the Linxi Formation in the Kundu area of Ar Horqin Banner in Da Hinggan Moun-tains is a set of yellowish green debris feldspar fine sandstone, yellowish green lithic siltstone, grayish black siltstone crust rock assem-blage. The age of the section is the Late Permian according to rich bivalves and plant fossils. According to the study of LA-ICP-MS detrital zircon U-Pb in the lower sandstone of the Qiaoluotu geological section of the Linxi Formation, the authors hold that the ag-es of ²⁰⁶Pb/²³⁸U zircons are mainly concentrated at 267Ma, 299Ma, 484Ma, 726Ma and 933Ma, with the youngest ²⁰⁶Pb/²³⁸U zircon age being 256Ma, which indicates that the maximum deposition age of Linxi Formation in this area was not earlier than that of Late Permian Wujiaping period. Geochemical characteristics of sandstone in Linxi Formation show that the source of the Qiaoluotu geo-logical section was characterized by the felsic materials, and the water body during the sedimentary period was a freshwater environ-ment. Combined with paleontology, petrology, geochemistry and isotopic chronology research, it is inferred that the ancient Asian Ocean in the study area was closed in the Late Permian, the sedimentary environment of Linxi Formation was continental lake basin, and the sedimentary source had complex and diversity characteristics, suggesting that the main source area was from the Xingmeng orogenic belt, with a small amount probably from the North China plate.