北祁連中段俯衝－增生雜岩／火山弧的時代探討

The subduction-accretionary complex/volcanic arcs in the middle sector of North Qilian may be subdivided into subduction-accretionary complex zone and the volcanic arc zone. The former mainly comprises the Early Ordovician ophiolites, ophiolitic melanges and deep-marine flysch with features of ocean crust, within which the Middle to Late Cambrian volcanic rocks and clastic rocks characterized by continental rifting and continent-ocean transitional environment occur as lenticular blocks. Isotopic data show that the deep unit of the subduction-accretionary complex experienced I-IP/ LT metamorphism during the period of 489-45OMa and the age of volcanic arcs is 495-466Ma, and that these geological bodies were formed as a result of northward subduction of Paleozoic Qilian oceanic crust beneath the Aixa blocks during the Early to Middle Ordovician.