大兴安岭南部幸福之路组的时代及二叠—三叠系界线研究——来自凝灰岩LA-ICP-MS锆石U-Pb年龄的证据

Age of Xingfuzhilu Formation and Contact Relationship between Permian and Triassic strata in southern Da Hinggan Mountains: Constraints from the tuff zircon U-Pb ages

多数研究者认为西拉木伦河—长春—延吉断裂带为华北与西伯利亚两大板块的最后缝合带,但最后拼合时间仍存在分歧,原因之一是对晚二叠世—早三叠世地层研究不足,尤其是对上二叠统与下三叠统接触关系研究更为欠缺。幸福之路组分布于大兴安岭南部,为一套以红层为主要特征的杂色碎屑岩组合,含有叶肢介、介形虫、双壳、植物及孢粉化石。笔者对采自巴林右旗幸福之路苏木幸福之路组上段的岩屑晶屑凝灰岩样品进行了LA-ICP-MS锆石U-Pb测年分析,结果为(254.3±2.6)Ma。结合已有的火山岩测年及化石资料认为,幸福之路组上段下部时代为晚二叠世,上部为早三叠世早期,大兴安岭地区上二叠统与下三叠统为连续沉积,晚二叠世晚期两大板块发生碰撞拼合,古亚洲洋消失,发生南北生物群的混生,早三叠世西伯利亚板块和华北板块碰撞结束。

The final suture position between North China plate and Siberia plate remains a topic of much controversy. Lots of researchers prefer describing West Lamulun River- Changchun- Yanji fault as the final suture zone of the two plates, but dispute still exists concerning the final closure time. One of the reasons is that there exists controversy concerning the Late Permian-Early Triassic stratigraphy in the Da Hinggan Mountains due to insufficient research; in particular, the study of the contact relationship between Late Permian and Lower Triassic remains very insufficient. The Early Triassic Xingfuzhilu Formation is of fluvial-lacustrine facies with red bed features and contains fossils such as ostracoda, sporopollen, conchostracan, bivalves and megaplants,which are located in southern Da Hinggan Mountains. The authors collected tuff samples and made analysis. LA-ICP-MS U-Pb dating of zircon yielded an age of（254.3±2.6） Ma. Combined with the existing volcanic dating and fossils in the upper part of the volcanic rocks, it is considered that the lower part of the third member of Xingfuzhilu Formation should be of Late Permian whereas the upper part belongs to Early Triassic, and that the Late Permian-Early Triassic strata are of continuous deposition. The PaleoAsian Ocean experienced collision suturing and the biota mixture in late Late Permian, and the closing time of the two plates should be Early Triassic.