内蒙古巴林左旗谭家湾剖面中三叠统幸福之路组厘定及其物源分析——来自碎屑锆石LA-ICP-MS U-Pb测年的证据

The stratigraphic age determination and provenance analyses of Xingfuzhilu Formation of Middle Triassic along Tanjiawan section in Bairin Left Banner,Inner Mongolia: evidence from detrital zircon LA-ICP-MS U-Pb dating

内蒙古巴林左旗谭家湾剖面位于大兴安岭南部,岩性上表现为一套复成分砾岩及其上覆的以紫灰、灰紫色砂岩为主的红层沉积。通过区域地层对比认为,该剖面地层相当于幸福之路组建组剖面的中、下段。本文对采自该剖面幸福之路组中段下部的砂岩样品开展碎屑锆石LA-ICP-MS U-Pb测年分析,测年结果显示出最年轻的锆石年龄为244 Ma,最年轻的峰值年龄为268 Ma,可知该剖面幸福之路组中段沉积时代应晚于244 Ma;结合同层位粉砂岩中孢粉化石鉴定结果,推测该剖面幸福之路组中段沉积时代为中三叠世中期。砂岩样品碎屑锆石谐和年龄的组成和分布特征说明该剖面幸福之路组沉积物来源多样,结合相关区域构造和前人锆石U-Pb测年资料,推测主要物源为苏尼特左旗—锡林浩特—西乌旗一带古生代岩浆岩、林西地区中二叠世—中三叠世花岗岩(花岗质片麻岩)、华北板块北缘早古生代岩浆岩。

Objectives: The Tanjiawan section in Bairin Left Banner of Inner Mongolia located in the Xing’an-Mongolian Orogenic belt,North China.Combined with petrological study,LA-ICP-MS U-Pb zircon age and palynological study of the section,we could determine the stratigraphic age and analyze the provenances of the red-layer detrital rocks with the typical features of sediment associations and discuss the possbile geological significances.Methods: Based on the data of the Tanjiawan section measuring,through the regional stratigraphic correlation,the LA-ICP-MS U-Pb detrital zircon dating and rare earth element test analysis of the sandstone sample from the lower part of the section,the identification of the sporopollen fossils within the siltstone from the same horizon as the sandstone sample,and the provenance analyses.Results: The strata of the section were corresponding to the middle and low parts of the Xingfuzhilu Formation,through the regional stratigraphic correlation. The LA-ICP-MS U-Pb detrital zircon dating gained the youngest age of244 Ma and the youngest peak ages of 268 Ma.The composition and distribution characteristics of concordant ages of detrital zicrons from the sandstone sample suggests the multiplicity of the provenance of the section.Conclusions: The deposition time of the middle part of Xingfuzhilu Formation of the section ought to be later than 244 Ma. Combining the identification of the sporopollen fossils,it could be speculated that the deposition time of the middle part of Xingfuzhilu Formation of the section was Middle stage of Middle Triassic. Combining the relevant data of the regional structure and Zircon U-Pb dating,it could be speculated that the provenances were consisted of the Paleozoic magmatic rocks in the area around Sonid zuoqi-Xilinhot- Xi Ujimqin Qi,the granites or granitic gneisses of Middle Permian to Middle Triassic in Linxi area,and the Early Paleozoic magmatic rocks in the northern margin of North China Plate.