摘要
本文通过对中国南方9条二叠—三叠系界线剖面样品的微量元素地球化学及其异常的研究,提出一个关于二叠—三叠系界线粘土层成因的混合模式。即认为界线粘土岩层是小行星撞击事件和中、酸性火山喷发事件的混合产物。在其物质来源上,撞击溅射产物与火山灰的比例大体为3:7。
A large mass extinction occurred at the end of the Permian. What caused the extinction? Extraterrestrial impact or terrestrial voleanicity? Some researchers think the mass extinction might be caused by a large-scale impact event. This hypothesis is based on Pt-group element anomaly and presence of microspherules and clastics of metamorphic rocks in boundary clays, which were thought to be related to impact event. However, some others considered the Permian/Triassic event as a voleanic one. Their reasons are as follows. (1) the volcanic rhyotaxitic structure was found in the boundary elaystone, (2) many high-temperature βquartzs formed in volcanic activity were discovered from the boundary claystone; and (3) there are 3 to 7 claystone layers near the P/T boundary, which are similar to the boundary claystone in elemental composition. In order to answer the question, the authors checked eleven sections of Permian/Triassic boundary in China and the Nammal section in Pakistan by NAA. A slight Ir anomaly has been detected in the black shale on the P/T boundary, and very clear siderophile elements anomalies were found in the Fusui section of China. Those anomalies may be caused by the enrichment process of the black shale, but Ir and Ni abundaces, 5 times higher than the background of the shale, may be too high to the normal enrichment of black shale. There must be another siderophile source in addition to the background of sea water. The REE abundances in claystone at and near the P/T boundary in different sections of South China were detected by INAA. And the REE patterns of boundary claystone differ from those of non-boundary claystone, which indicates their different sources. For comparison, the authors also determined the REE of the acidic-intermediate volcanic rocks of Triassic age and those of the Pre, D/C and C/T boundary claystones. From those REE patterns and some other evidence, the authors put forward a proposal that the P/T boundary claystone may be a mixed product consisting of acidic-intermediate ashes and the upper crust substances representing sputtering component (here the data of C/T boundary claystone are used). And the calculated mixing ratio of the former to latter is 7:3.
The mixed model of forming boundary claystone can better explain the experimental facts from two aspects, i. e.: the volcanic activity and impact event. A possible scenario is that at the end of the Permian, an large-scalc impact triggered severe activity of volcanos. Or these two interactions simultaneous occurred at that time.
出处
《地质论评》
CAS
CSCD
北大核心
1991年第1期51-63,共13页
Geological Review
