二叠纪末期生物绝灭的地内原因

THE TERRESTRIAL CAUSE OF ORGANIC EXTINCTION BY THE END OF PERMIAN

湘西北地区各研究剖面的二叠—三叠系界线附近地层,夹有多层火山喷发物成因的粘土岩。地层内富含铁质微球粒,微球粒的铁元素含量与磁铁矿近乎相同,缺少陨石特征元素,其形态构造具有喷发物的特征。层内还含有以造岩元素为主的火山灰颗粒。所含大量的六方双锥石英不仅指示了高温热液的环境,而且晶体具有的菱形破碎纹理还证明当时存在强大的应力场。结合已发表的华南其它地区的资料,本文认为湘西北地区、乃至整个华南当时存在剧烈的火山喷发;并进一步指出火山喷发和古地理变迁等地内因素是促使二叠纪末期生物大规模绝灭的主要原因。

The evidences from northwestern Hunan Province have verified that the violent voleanie eruptions took places at the time of Permo-Triasic boundary.The iron mierospherules contain 70%-73% of ,Fe, which is nearly sa.me with tlie theoretical value (72.1%) of magnetite. The contents of their Ni, Cr, Co and Ti are much less than that of the meteorites. On the morphology, they can he divided into two sorts, globular and bomb-like ones. It has been inferred that the globulars were formed from melt iron magma in the sea and the bomb-like ones by condensing in the air.Besides a large amount of SO(?) (30%-50%), the volcanic ashes mainly consist of rock-making elements including A12O3 (about 15%), SiO2(about 15%), CaO( about 5% ). K2O(about 2%) and MgO (about 3%). β-quartzes are mostly found in the circumstances of high temperature. The rhombus crackles that the quartzes have show that there was a strong-stress field at that time, which may be brought by the crust-moving and volcanic eruption.The clay rocks around the boundary bed are basically composed of illite-montmorillonite, which are derived from the volcanic materials. Except for Eu, the abundances of REE and boron are very high at the boundary bed.In the above study, the terrestrial factors, i. e. volcanic eruption, paleo-geographic changes etc., have primarily been considered as the external cause of organic extinction by the end of Permian.