Early Palaezoic grranitoid magmatism and tectonic evolution in North Qilian, NW China
北祁连早古生代花岗质岩浆作用及构造演化

Zircon SHRIMP dating of granitoids from the middle segment of North Qilian showed that they are 512 Ma and 501 Ma for plagiogranite and quartz diorite from Kokoli, 508Ma for Yemazhui granite, and 424Ma for Jinfusi granite, respectively. Based on petrogeochemistry of granites and occurrence in tectonics, combined with the dating of other granites and regional geological data, we think that North Qilian oceanic plate was subducted southward under the Middle Qilian terrane, which caused two episodes of magmatism, one was forming Kokoli plagiogranite (512Ma), Yemazui granite (508Ma) and Kokoli quartz diorite (501Ma), another forming Niuxinshan granite (477Ma). Lately, subduction polarity for the North Qilian basin changed from southward to northward because it was blocked by the deep subduction of Qaidam continental plate northward on the south of Qilian, which formed Minleyaogou granite (463Ma). After about 440 Ma, the Qilian Ocean was closed because of collision between Qaidam and Alax terrane, forming the North Qilian orogen. The post-collision granites such as Jinfusi granite (424Ma) and Niuxinshan quartz diorite (435Ma), and so on, were formed in the North Qilian Orogen where different blocks stretched and collapsed because of lithosphere delamination below the orogen.