北祁连造山带老虎山奥陶系硅质岩地球化学特征及古地理意义

Geochemistry characteristics and palaeogeographic significance of the Ordovician siliceous rocks from Laohushan area,North Qilian orogenic belt

对北祁连造山带老虎山地区下奥陶统和中、上奥陶统硅质岩的沉积学和地球化学研究表明:下奥陶统硅质岩为生物化学作用成因,沉积于被动大陆边缘深海环境;中、上奥陶统下部与玄武岩共生的硅质岩显示热液成因,沉积于洋脊环境;中、上奥陶统上部硅质岩指示生物化学成因,形成于大陆边缘环境。上述特征表明老虎山地区在早奥陶世为相对稳定的被动大陆边缘构造环境,所含硅质岩和陆缘碎屑岩为大陆斜坡相浊流沉积。中、晚奥陶世柴达木板块向华北板块俯冲在弧后产生离散型活动大陆边缘,形成弧后盆地,硅质岩及其共生的枕状玄武岩和浊积岩应属于扩张弧后盆地的产物。

The studies of sedimentology and geochemistry of two sorts of siliceous rocks, i.e. the Lower （O1 ） and Upper （O2-3 ） Ordovician siliceous rocks from the Laohushan area, North Qilian orogenic belt, show the following different features： the Lower Ordovician siliceous rocks suggest a bio-chemical origin, and were formed in a deep sea setting of the passive continental margin; the siliceous rocks intercalated with basalts in the lower part of Middle-Upper Ordovician suggest a hydrothermal origin, and were deposited in an oceanic ridge setting; and the siliceous rocks in the upper part of Middle-Upper Ordovician suggest a bio-chemical origin, and were deposited in an active continental margin in a deep sea environment. Geochemical and sedimentological characteristics of the siliceous rocks imply the existence of a relatively steady setting of the passive continental margin in the Early Ordovician in Laohushan area, North Qilian orogenic belt. The siliceous rocks and terrestrial elastic rocks belong to the continental slope facies. In the Middle-Late Ordovician, the Qaidam Plate was subducted under the North China Plate. As a result, an abyssal basin of the continental margin was formed in Laohushan area, and then a back-arc basin was formed. The Middle-Late Ordovician siliceous rocks together with pillow basalts as well as turbidites, are of the back-arc basin origin.