Continental orogenesis from ocean subduction, continent collision/subduction, to orogen collapse, and orogen recycling: The example of the North Qaidam UHPM belt, NW China

Abstract

The North Qaidam ultra-high pressure metamorphic (UHPM) belt in the northern Tibetan Plateau records a complete history of the evolution of a continental orogen from prior seafloor subduction, to continental collision and subduction, and to the ultimate orogen collapse in the time period from the Neoproterozoic to the Paleozoic. Lithologies in this UHPM belt consist predominantly of felsic gneisses containing blocks of eclogite and peridotite.The 1120–900Ma granitic and psammitic/pelitic gneisses compose the majority of the UHPM belt and is genetically associated with the previous orogenic cycle of Grenville-age, whereas protoliths of the HUPM eclogites are of both the 850–820Ma continental flood basalts (CFBs) and the 540–500Ma oceanic crust (ophiolite). The early stage of quartz-stable eclogite-facies metamorphism took place at ~445–473Ma, the same age as that of the HP rocks in the North Qilian oceanic suture zone, representing the earliest subducting seafloor rocks exhumed and preserved. Coesite-bearing zircons from the metapelite and eclogite, diamond-bearing zircons from garnet peridotites constrain the UHP metamorphic age of ~438–420Ma, which represents the timing of continental subduction at depths of 100–200km, ~10–20m.y. younger than the early stage of the Qilian seafloor subduction. Therefore, deep subduction of continental crust should be the continuation of oceanic subduction that is pulled down by the sinking oceanic lithosphere or pushed down by the overriding upper plate, which is an expected and inevitable consequence for the scenario of passive continental margins. Partial melting of subducted ocean crust might occur in response to continental subduction at ~435Ma.The UHPM rocks started to exhume accompanied by mountain building and deposition of Early Devonian molasses in the North Qilian region at ~420Ma. Decoupling of oceanic subduction zone and continent UHPM terranes may be attributed to the different exhumation path and mechanism between the subducted oceanic and continent crusts, or rollback of subduction zone. Decompression melting of UHP metamorphosed slab and continental crust during exhumation is responsible for the generation of adakitic melts and S-type granite. Mountain collapse and lithosphere extension happened in the period of ~400–360Ma and formed diorite–granite intrusions in the UHPM belt, which marked the end of a complete orogenic cycle.This UHP metamorphic belt presents an example of multi-epoch tectonic recycles, represented by recombination of the Neoproterozoic Grenvillian orogenesis and the Early Paleozoic Caledonian orogenesis.