From subduction to collision in the northern Tibetan plateau: evidence from the early Silurian clastic rocks, northwestern China

The Qilian Orogen records early Paleozoic collisional suturing of the Qaidam Block and the Central Qilian Block to the North China Craton. The composition and U-Pb age of detrital zircons and the composition of Cr-spinels from the Early Silurian Lujiaogou and Angzanggou formations in the northern part of orogen indicate derivation from evolving oceanic and continental source terranes. Heavy-mineral chemistry indicates the incorporation of suprasubduction zone-type ophiolitic detritus in addition to continent-derived material. Integrating these chemical and age data with regional data on the duration of subduction-related magmatic activity, syn- and postcollisional granitic rocks, and high-pressure metamorphic rocks constrains the transformation from oceanic subduction to continental collision to 450-440 Ma. The collision resulted in a flood of detritus into the northern part of the orogen from the Central Qilian Block, which masked input from the intervening magmatic arc, implying rapid exposure of the block.