摘要
用放射性化学和仪器中子活化分析,研究了宜昌分乡502号奥陶系—志留系界线剖面40多种元素丰度的分布。结果表明,在奥陶系—志留系界线层中铱的丰度突然增至0.64 ppb,显示出明显的铱异常。而且铱和一些亲铁和亲硫元素丰度的变化呈正相关性。在宜昌黄花场剖面的相同界线层中,还存在δ^(13)C偏离。上述地球化学的信号有利于奥陶纪末期生物集群绝灭可能与地外事件有关的设想,并为奥陶系—志留系界线应划在Hirnantia-Kinnella和G.persculptus带之间提供了新的证据。
The Terminal Ordovician extinction event showing an abrupt mass extinction of the Hirnantia-Dalmanitina fauna and eventual disappearance of dicellograptids and most diplograprids is clearly recorded between the Wufeng and Longmaxi Formations (i. e. between the Hirnantia-Kinnella and G. persculptus Zones) in South China. The boundary age has been dated at 439±18 Ma by the Rb/Sr method (whole rock) (Qin et al., 1986). More then 40 elemental abundance variations across the No 502 Ordovician/Silurian (O/S) boundary section of Fenxiang, Yichang are investigated by INAA and RNAA. The results show that a distinctive lr anomaly is present in the boundary bed. The lr concentration ranges from<0.05—0.08—0.18 ppb in the Guanyinqiao Member of the latest Wufengian (=Hirnantian) to a maximum of 0.64 ppb at the base of the G. persculptus Zone of the Longmaxi Formation. Moreover, the It, Cr, Au, Ni, Co, As and Sb are each other interrelated in the abundance variation. The characters of the elemental abundances at the O/S boundary section are similar to those of the K/T boundary section at Stevns Klint, Denmark.Theδ^(13)C excursion exhibited across the O/S boundary section at Huanghuachang, Yichang fully coincides in horizon with the lr anomaly found in the Fenxiang section. The δ_(13)C value arises steadily in the Guanyiqiao Member. At the base of the G. persculptus Zone it decreases abruptly to -30‰. The perturbation amplitude of the δ^(13)C value may reach 8‰ PDB across the O/S boundary.The geochemical signatures mentioned above support the hypothesis that the latest Ordovician mass extinction is likely to be related to extraterrestrial causes, and further suggest that the O/S boundary should be defined at the base of the G. persculptus Zone, i. e. between the Hirnantia-Kinnella and G. persculptus Zones.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1989年第3期255-264,共10页
Acta Geologica Sinica
