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 experiencedHP/LTmetamorphism during the period of 489—450Ma 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 Alxa blocks during the Early to Middle Ordovician.