奥陶纪-志留纪边界附近火山活动记录：来华南周缘钾质斑脱岩的信息

扬子地台内奥陶纪顶部的五峰组到志留纪底部的龙马溪组间存在着多个粘土岩层.前人研究结果认为这些粘土岩层为钾质斑脱岩,是火山喷发的凝灰质物质在海相环境沉积、蚀变的产物.这些斑脱岩广泛分布于扬子地台周缘,对理解奥陶纪-志留纪时华南所处的大地构造位置和相应的构造事件具有重要的意义.本文对采自扬子地台内湖北宜昌地区和贵州桐梓地区奥陶纪-志留纪界线剖面的斑脱岩层进行了矿物学及地球化学工作,旨在判定其形成的构造环境.矿物学研究表明上述岩石除了含有粘土矿物外,还含有石英、长石、黑云母、磷灰石、锆石等中酸性岩浆岩中的常见矿物,属典型的钾质斑脱岩.本文利用在风化过程中不活动元素对斑脱岩的原岩进行了恢复,结果也表明其原岩为中酸性火山岩,包括安山岩-英安岩-及流纹岩等.微量元素特征显示多数样品具有典型的岛弧火山岩的特征,很可能与北面早古生代秦岭洋的闭合过程中的板块俯冲有关,也可能是早古生代在华南板块东南缘外侧存在的一古老洋壳向华南板块的俯冲.有一个样品(YC0711)没有Nb的负异常,但是具有明显的Ti负异常,在Th/Yb-Nb/Yb判别图上落在岛弧区附近,其原岩可能是富铌玄武岩之类的特殊岛弧岩石.火山活动的峰期为晚奥陶世赫南特阶时代,与地质历史上第二大的生物大灭绝事件同时.前人认为该生物灭绝事件与冈瓦纳冰川有关.考虑到华南以及欧洲、北美等地均出现大规模晚奥陶世-早侏罗纪钾质斑脱岩,奥陶纪-志留纪边界的火山事件是具有全球规模的,所喷发的岩石多是富含挥发份的中酸性岩,对大气圈和生物圈具有十分重要的影响,本文认为火山活动很可能是造成晚奥陶世的生物大灭绝事件和冈瓦纳冰川的主要诱导因素.

Volcanic event at the Ordovician-Silurian boundary: The message front K-bentonite of Yangtze Block

There are several clay beds recognized from the Wufeng Formation, the top of the Ordovician and, Lungmachi Formation, the bottom of the Silurian in the Yangtze Block. These clay beds were taken as K-bentonites, which was produced by volcanic ash-fall deposits after diagenic metamorphism in marine environments. These K-bentonites are widely distributed along the margin of the Yangtze Block, such that can provide important constraints on the plate tectonic setting of the Yangtze Block as well as major geological events in the Early Paleozoic. In this contribution, we carried our mineralogical and geochemical studies on K-bentonite samples collected from two Ordovician-Silurian boundary sections, located in Yichang, Hubei Province and Tongzi, Gnizhou Province. The results show that in addition to clay minerals, the clay beds also content quartz, feldspar, biotite, apatite, zircon, etc. , which are all common minerals in intermediate to silicic volcanic rocks. These clay beds should be typical K-bentonite. Most of the K-bentonite samples show typical characteristics of arc volcanic rocks, which were probably related to the subduction and closure of the Qinling Ocean in the Early Paleozoic. Alternatively, there might have been another subduction to the southeast of the South China Block. Sample YC0711 have no obvious Nb anomaly, but show Ti negative anomaly. They fall close to arc fields in a Th/ Yb-Nb/Yb diagram, suggesting that the protoliths were likely to special types of arc rocks, e. g. , Nb-enriched basalt. The peak age of the volcanic activity was Hirnantia stage, which is very close to the mass extinction at Ordovician-Silurian boundary, the second largest mass extinction in the Earth' s history, which was generally attributed to the Gondwana glaciation. Considering that K-bentonite is widely distributed in South China, North America and Europe, the volcanic event at the Ordovician-Silurian boundary was a global event. In addition, most of these volcanic rocks are silicic, which are usually volatile enriched, and thus might have major effects on the atmosphere and biosphere. Here we propose that these volcanic events were the triggers of mass extinction at the Orodocician-Silurian boundary as well as the Gondwana glaciation.