大兴安岭南部二叠系—三叠系界线及沉积环境演化

北方陆相二叠系—三叠系界线研究是界线地层研究的重要问题之一,对采自内蒙古巴林右旗幸福之路组二段建组剖面的安山质岩屑晶屑凝灰岩开展锆石LA-ICP-MS U-Pb测年,结果为(255.5±1.1)Ma,火山岩上部多个层位粉砂质泥岩中发现孢粉化石,计17属20种,时代为晚二叠世—早三叠世,推断幸福之路组的时代为晚二叠世—早三叠世,大兴安岭地区二叠系—三叠系界线在幸福之路组内部,二者为整合接触。幸福之路组以红层做为划分标志,岩性为正常沉积碎屑岩夹火山岩,见大量泥灰岩夹层、钙质结核及干裂现象,反映了炎热、干燥及强氧化条件的气候环境,大兴安岭地区晚二叠世晚期(幸福之路组二段下部)就已出现反映干旱气候的红层沉积,与相邻的华北及新疆准噶尔盆地相似。

The sedimentary environment and boundary of Permian-Triassic strata in the southern Da Hinggan Mountains, Inner Mongolia

The continental Permian-Triassic boundary is one of the most important problems in the study of the global PermianTriassic boundary. Based on collecting and analyzing tuff samples, the authors conducted LA-ICP-MS U-Pb dating of zircon and obtained an age of (255.5±1.1)Ma in the stratotype section of the middle part of Xingfuzhilu Formation. A total of 20 species of 17 genera sporopollen fossils were discovered on the top of the tuff in the stratotype section. The age of Xingfuzhilu Formation is late Late Permian-early Early Triassic according to the sporopollen fossils. It is considered that the middle part of Xingfuzhilu Formation should be late Late Permian whereas the upper part belongs to early Early Triassic. The boundary of Permian and Triassic strata in Da Hinggan Mountains is actually located inside Xingfuzhilu Formation. Moreover, the Late Permian-Early Triassic strata are of continuous deposition in Da Hinggan Mountains. With red-red features as a dividing point, Xingfuzhilu Formation totally reflects the thermal, arid and strong oxidation climate. It can be dated back to late Late Permian-early Early Triassic. The red beds came into existence from the late Late Permian in Da Hinggan Mountains. In addition, the climate changed from warm and rainless to arid and thermal, which is in accord with features of neighbouring North China and Junggar basin in Xinjiang. These data prove that the thermal, arid and strong oxidation climate did exert an profound influence on Da Hinggan Mountains.