柴北缘阿木尼克地区晚古生代牦牛山组火山岩地质地球化学特征及其地质意义

Geological and Geochemistry Characteristics of Volcanics in the Late Paleozoic Maoniushan Formation in Amunike Area of Northern Qaidam Basin

柴北缘阿木尼克山地区晚古生代火山岩赋存于牦牛山组内,岩性以酸性火山岩为主,有少量的中性火山岩,在火山岩层间,夹薄板状(陆源碎屑)白云岩、薄层状钙质泥质粉砂岩等沉积岩。火山岩相包括爆发相、溢流相、爆发沉积相,柱状节理发育,具陆相喷发特征。岩石主量元素、稀土元素、微量元素特征与岛弧火山岩相似,综合宏观岩石组合特征,可以判断牦牛山组火山岩产于板内环境。在该套火山岩夹层中采得中泥盆统标准化石Lepidodendropsis sp.(拟鳞木),在英安岩中获得的锆石^(206)Pb/^(238)U年龄为(369.2±3.3)Ma、(392.4±3.3)Ma,据此可以确定火山岩形成时代为中泥盆世—晚泥盆世,柴北缘阿木尼克山地区牦牛山组火山岩形成的地质时代明显晚于东昆仑地区,由此也说明东昆仑、柴北缘的火山岩浆活动及大构造演化不是同步的。

The volcanics in Amunike area of Northern Qaidam Basin distribute in late Paleozoic Maoniushan Formation.It was composed of large numbers of acidic volcanic rocks and a small amount of intermediate volcanic rocks with some thin-bedded sedimentary rocks mezzanine such as dolomite and calcareous argillaceous siltstone. The volcanic facies can be divided into eruptive facies, effusive facies and volcanic sedimentary facies with Columnar Joint characteristics. Results of major elements shows a feature of S-type granites, while the REE and trace elements both illustrate a similarity with island arc volcanic rocks which formed in intraplate setting. The discovery of Lepidodendropsis sp., finding in the sedimentary rocks mezzanine,was one kind of standard fossils for Middle Devonian, and obtaining of zircon 206Pb/238U isotopic age of （369.2±3.3）Ma、（392.4±3.3）Ma in dacites can define the age of these volcanic rocks are Middle-Late Devonian, that significantly later than the East Kunlun area. We can concluded that magmatic activity and geotectonic evolution between the East Kunlun area and northern margin of Qaidam basin are not synchronized.