安徽泾县地区PTB界线粘土岩特征及其年代学意义

Characteristics of Clay Rock and Its Chronological Characteristics Near PTB in Jingxian Area of Anhui Province

全球,尤其是华南二叠—三叠纪界线(PTB)附近普遍发育火山成因的粘土岩,对分析生物大灭绝具有重要意义。报道了在安徽泾县昌桥PTB发现2套粘土岩,对其岩石学、全岩地球化学和LA-ICP-MS锆石U-Pb等进行了较为细致的工作,填补了区域的研究空白。研究表明:粘土岩具有烧失量含量偏高、Al_2O_3和K_2O含量较高、Na_2O、Mn O、Ti O_2含量偏低、Si O_2含量较为稳定、存在一定含量的钙质胶结物。同时,见典型的岩浆成因锆石与自形黄铁矿,具有明显火山作用的特征;粘土岩中锆石U-Pb同位素年龄主要集中于251~253 Ma,加权平均年龄为252.3±3.2 Ma,这与最新的地质年代表,P/T界线的年龄为252.17±0.06 Ma基本一致;安徽泾县地区作为华南的一部分,PTB附近存在着火山活动,具有大尺度乃至全球性,与峨眉山玄武岩顶部凝灰岩层发生时间相一致,可能指示了受峨眉山玄武岩影响。此外,火山活动与PTB生物大灭绝相一致,火山活动对当时的陆地和海洋生态系统恶化产生了极大的影响,加剧了生物绝灭事件。

Volcanic-generous clay rocks are commonly developed around the world, especially in the South China Pemlian-Triassic boundal7 （PTB）. It is important for analyzing biological extinctions. In this paper, we conducted a more detailed work in petrology, whole rock geochemistl7 and LA-ICP-MS zircon U-Pb for two sets of clay rocks found in Changqiao PTB, Jingxian area of Anhui Province, which filled the research gap of this aera. A research shows that clay rocks has high loss on ignition, including high content of A1203 and K20, low content of Na2O, MnO and TiO2, stable content of SiO2, and certain amount of calcium cement. At the same time, more characteristic product of volcanism like typical magmatic origin of zircon and euhedral pyrite are found. The zircon U-Pb isotope age of clay rocks is mainly concentrated in 251 - 253 Ma, with a weighted average age of 252.3 ±3.2 Ma, as is basically the same with the age 252. 17±0.06 Ma of the P/T boundary in line with the latest geologic age series. As a part of southern China, there is volcanic activity near PTB in Jingxian area of Anhui Province which is large-scale and globality. It is consistent with the time of tuff fommtion at the top of the Emei basahs, which may indicate an impact by the Emei mountain in basahs. In addition, volcanic activity is consistent with the mass extinction of PTB, and volcanic activity has had a great impact on the deterioration of the terrestrial and marine ecosystems at that time in exacerbating the biological extinction event.