中国科学院南京地质古生物研究所机构知识库

本文选取了产自贵州兴义富含海生爬行动物及鱼类化石的法郎组竹杆坡段上部凝灰岩夹层中的锆石,进行了LA-ICP-MS U-Pb年代学测定。测得兴义动物群化石赋存层位年龄为240.8±1.8 Ma,化石赋存层位之上21 m处年龄为239.8±1.7 Ma。该年龄数据进一步验证了兴义动物群的时代为中三叠世拉丁期,而非晚三叠世卡尼期,这一结论可与此前菊石生物地层研究、碳同位素分析的结果相印证。结合兴义动物群中生物组合面貌的变化,推测三叠纪生态系统可能在拉丁期就已开始由辐射发展阶段转变为完全复苏阶段。

Objective：The Triassic Xingyi Fauna,from the Zhuganpo Member,Falang Formation,Xingyi City,Guizhou Province,southwest China,is famous for high-diversity well-preserved marine reptiles. The geological age of this fauna has been controversial for a long time. In this study,we focus on the isotopic dating of Xingyi Fauna.Methods：The volcanic tuffs sample PW1 and PW2（ approximately 15 kg in total） were collected from the Nimaigu Section, Wusha Town, Xingyi City, where yield aboundant fossils of marine reptiles, fishes and invertebrate. Sample PW1 was collected from the vertebrate fossil layer,sample PW2 from the position 21 m higher than the vertebrate fossil layer. The stuffs were concentrated and separated by standard techniques of density and magnetic separation at the Yuneng Mineral Separation Service Company at Langfang,Hebei Province,China.The LA-ICP-MS U-Pb analysis was did at the Inductively Coupled Plasma Mass Spectrum Laboratory,China University of Geosciences（ Beijing）. Zircons were mounted together with grains of the standard zircon 91500（1062Ma） and Temora（417Ma）. The internal structure of the zircons was examined using cathodoluminescence（CL）,transmitted light,and reflected light imaging prior to isotopic analyses. 29 zircons from sample PW1 and 26 zircons from PW2 are choosed for the analyses. Uncertainties for each individual analysis are reported at the 1σlevel; weighted mean ages of pooled analyses are quoted at the 95 % confidence interval. Data reduction and U-Pb concordia diagram were carried by Isoplot 3. 0.Result： We obtained an age of 240. 8 ± 1. 8 Ma（1σ,MSWD = 0. 043） for the vertebrate fossil layer of Xingyi Fauna,and an age of 239. 8 ± 1. 7 Ma（ 1σ,MSWD = 0. 22） for the position 21 m higher than the vertebrate fossil layer.Conclusion： The result of this research reconfirms that the era of Xingyi Fauna is Ladinian,the Middle Triassic,rather than Carnian,the Late Triassic,which show agreement with the previously result of ammonoid biostratigraphic and carbon isotope research focus on the same section. And considering the change of fossil assemblage in the Xingyi Fauna,it is supposed that the shift of Triassic ecosystem from radiation to fully recovery as early as Ladinian.