摘要
下—中三叠统界线年龄的精确测定是进行区域地层年表和国际地层委员会全球标准年表研究中重要的组成部分。中国贵州望谟甘河桥剖面下—中三叠统地层为奥仑尼克阶—安尼阶界线层型的理想候选剖面 ,其临界层位的玻屑凝灰岩为下—中三叠统界线的岩性标志。应用高精度高灵敏度的二次离子探针质谱对玻屑凝灰岩中的锆石进行 U- Pb年代学分析 ,结果为 2 39.0± 2 .9Ma(2σ)。这是首次直接从界线层位准确测定的下—中三叠统界线的锆石 U - Pb同位素年龄。中国贵州望谟甘河桥剖面下—中三叠统界线剖面地层连续 ,牙形石分带清楚 ,临界层位发育区域上分布较为稳定的玻屑凝灰岩 ,是极好的区域等时标志层。这一剖面有望成为国际下—中三叠统界线层型剖面。
The Lower—Middle Triassic boundary is situated 0.5 m below the top of the Ziyun Formation, where the Chiosella timorensis zone first appears on the Ganheqiao section, Wangmo County, Guizhou Province. The 5 cm boundary glassy tuff bed is located 0.5 m above the location where the Chiosella timorensis zone first appears. The first appearance of Chiosella timorensis is the preferred index to define the boundary and regarded as a candidate Olenekian—Anisian boundary stratotype.Zircons of glassy tuff from the Lower—Middle Triassic boundary have been analyzed using the SHRIMP ionprobe. Zircons from glassy tuff within the Ziyun Formation, Ganheqiao section have a weighted mean 206Pb/ 238U age of 239.0±2.9Ma (2σ). This is the first direct constraint on the zircon U-Pb age of the Lower—Middle Triassic boundary. The SHRIMP zircon 206Pb/ 238U age is the same as the isochron age of 238.9±4.8 Ma obtained by 40Ar/ 39Ar dating of glassy tuff polished sections. This zircon U-Pb age is accurate and comparable with other well-measured Phanerozoic ages. It is interpreted as the age of eruption of volcanic ash. Owing to the unique biostratigraphic position of the volcanic layer, the age of the boundary is exactly the age of the glassy tuff and thus extrapolation is not necessary.The presence of volcanic layers in the fossil-rich marine of the Lower—Middle Triassic in Wangmo, Guizhou, offers a unique opportunity to match the biostratigraphic and numerical time scales in this section of the Phanerozoic. The Ganheqiao section is one of the best locations in the world to study the Lower—Middle Triassic and can be designate as the type area to define the Olenekian—Anisian boundary.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2004年第5期586-590,i003,共6页
Acta Geologica Sinica
基金
中国地质科学院地质研究所北京离子探针中心资助
科技部基础工作专项"中国典型地层剖面的立典研究"的下-中三叠统界线层型研究项目 (编号 2 0 0 1DEA2 0 0 2 0 -10 )
中国主要断代地层建阶研究项目 (编号 2 0 0 3 13 0 0 0 0 76)资助成果
