摘要
一种与多类撞击记录共存的灰黑色碎屑，发现于黄土高原南缘段家坡黄土地层剖面午城黄土底部Ｇａ／Ｍ边界（２．４８ＭａＢ．Ｐ．）附近，通过对此碎屑的矿物学和地球化学特征研究，表明它们最初是源自青藏高原北部及其邻近地区碱性玄武质火山喷发的碎屑物，后来又在表生条件下与黄土层一起经历了包括水解、成土化和氧化等作用在内的风化改造。此外，本文更为强调的是，在火山碎屑形成过程中存在具有富Ｎｉ和Ｃｅ，高Ｓｍ／Ｎｄ比以及低Ｅｕ／Ｓｍ比等特点的地外物质的参与，据以论证这种火山活动确系Ｎ／Ｑ界面撞击事件引发。在黄土地层中，深入研究各种类型的火山碎屑，无疑对全面探讨第四纪环境变迁有着十分重要的意义。关键词午城黄土底部松山－高斯边界火山碎屑风化作用地外物质新近的研究表明，第四纪气候和环境变迁往往与起因于撞击和火山事件的大量尘埃物质有关［１］，而地外物体的撞击又常能引发火山活动［２、３］，因此第四纪沉积物中的火山喷发空落堆积物研究，无疑应该引起更大的关注。火山学发展至今，对各种火山碎屑物质，已有十分详尽的研究［４、５］。从国内第四纪火山碎屑的研究现状来看，虽然丁国瑜等早在１９６４年就曾在河北平原及太行山东麓的黄土地层中，首次发现了火山碎屑堆?
A great deal of gray black debris, together with the impact records of all kinds, has been discovered around the Gauss / Matuyama boundary in the bottom of Wucheng Loess of Duanjiapo section located at the south margin of Loess Plateau in the northern China. Their characteristics of mineralogy and geochemistry indicate that they are, at first, a type of pyroclasts generated from the alkaline basaltic volcanic activity in north Qinghai Xizang plateau and adjacent region, and then with the loess strata, they are reformed by weathering, such as the hydrolysis action, the argillation and the oxidation in the surface condition. It is emphasized in this paper that the pyroclasts are mixed with the extraterrestrial material characterized by the high Ir and Ni contents and a high ratio of Sm / Nd, a low ratio of Eu / Sm in the process of their forming. These imply that the volcanic activity producing the pyroclasts is indeed induced by the impact event around the boundary of Neogene / Quaternary. Thus, it is very important to study various pyroclasts in the loess strata for dealing with the Quaternary environmental evolution.
基金
国家自然科学基金
中国科学院开放实验室资助
关键词
黄土高原
火山碎屑
风化作用
地外物质
来源
bottom of Wucheng Loess Gauss / Matsuyama boundary pyroclasts weathering extraterrestrial material
