浙江长兴煤山二叠—三叠系界线年龄测定  

AGE DETERMINATON OF THE PERMIAN-TRIASSIC BOUNDARY AT MEISHAN,CHANGXING,ZHEJIANG PROVINCE

作　　者：张自超[1] J.C.Claouè-Long 马国干[1] 杜绍华[1] 

机构地区：[1]中国地质科学院宜昌地质矿产研究所 [2]澳大利亚国立大学

出　　处：《地质论评》1992年第4期372-381,T001,共11页Geological Review

基　　金：国家自然科学基金

摘　　要：本文报道了用高灵敏度高分辨率离子探针,测定的浙江长兴煤山二叠—三叠系临界层位,斑脱岩中锆石的U-Pb年龄,结果为351.3±3.4Ma(2σ)。这是国际上首次直接从界线层位准确测定的二叠—三叠系界线的同位素年龄。长兴煤山二叠—三叠系界线剖面是迄今所发现的地层剖面连续,所含化石门类众多,数量丰富,且在临界层位夹有火山岩层的少有剖面之一,它不仅对国际显生宙地质年表的研究具有重要意义,而且也具有作为二叠—三叠系界线层型剖面的优越条件。This Paper reports U--Pb age data measured by using SHRIMP ion micropobeon zircons from a 5 cm bentonite bed occurring within the Permian--Triassicboundary section at Meishan, Changxing, Zhejiang province. According to the fossilsin the horizons below and above this bed and the principle for biostratigraphicaldivision, this bentonite bed is the bed marking the Permian--Triassic boundary. Zircons from the bentonite are clearly of volcanic genesig. They are unsuitedfor conventional U--pb analyses as almost all the zircon crystals are rich in inclu-sions. But using the SHRIMP ion microprobe was able to avoid inclusions andtarget pure zircon for analyses. Of 35 zircons analysed(37 analyses), one was foundto be a Silurian xenocryst, two have areas that have leaked radiogenic Pb, andthe remained 34 analyses give within the error range a^(206)Pb/^(238)U age of 251. 2±3.4Ma (2σ), which is in good agreement with the ^(207)Pb/^(235)U age. This age is inte-rpreted as the age of eruption of the volcanic ash. Owing to the unique biostrati-graphic position of the volcanic layer, the age of the boundary is exactly the ageof the bentonite and needs no extrapolation. The Presence of a volcanic layer in the fossil--rich marine Upper Permianand Lower Triassic in Changxing, Zhejiang, offers a unique opportunity to corre-late the biostratigraphic and numerical time scales in the Phanerozoic. It is belie-ved that the Changxing Permian--Triassic section has a strong potential for con-tending for the international stratotype.

关 键 词：二叠-三叠纪 斑脱岩 界线年龄 

分 类 号：P534.46[天文地球—古生物学与地层学]