Two contrasting accretion v. collision orogenies: insights from Early Paleozoic polyphase metamorphism in the Altun–Qilian–North Qaidam orogenic system, NW China

The Altun–Qilian–North Qaidam (AQQ) orogenic system in northern Tibet is considered to be the northernmost orogenic collage of the Prototethyan domain. It is regarded as resulting from collisions between various continental terranes derived from the northern margin of Gondwana, although the AQQ orogenic system also includes abundant ophiolites, arc magmatic rocks and subduction–accretion complexes. Some researchers regard the orogenic system of north Tibet as a typical accretionary orogen built by the development of an evolving arc–accretion complex growing southwards along the margin of the Tarim and North China cratons during the Paleozoic. We propose, based on both published data and our new data, that two distinct accretion and collision orogenies developed in the AQQ during the early Paleozoic. The diagnostic marks are HP–LT metamorphic rocks in the North Altun–North Qilian Mountains and UHP metamorphic rocks in the South Altun–North Qaidam Mountains. A review of metamorphic, geochronological, geochemical and structural data indicates that the North Altun–North Qilian HP–LT metamorphic belt is related to early Paleozoic subduction– accretion and, together with ophiolite mélanges and arc metamorphic–magmatic complexes, forms an early Paleozoic accretionary orogen. By contrast, the South Altun–North Qaidam UHP metamorphic belt is associated with continental subduction and collision, accompanied by Barrovian-type metamorphic overprinting and collision-related magmatism, reflecting an early Paleozoic collisional orogeny.