Study on Source and Origin of Pyroclasts in the Bottom of Wucheng Loess in the South Margin of Loess Plateau
黄土高原南缘午城黄土底部火山碎屑的来源和成因

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.