Time constraints on orogenesis from oceanic subduction to continental subduction, collision, and exhumation: An example from North Qilian and North Qaidam HP-UHP belts
大陆造山运动：从大洋俯冲到大陆俯冲、碰撞、折返的时限——以北祁连山、柴北缘为例

Two end-member subduction zones of the Early Paleozoic are preserved in the Qilian-Qaidam mountain systems at the northern edge of the Tibetan Plateau. One is the North Qilian oceanic-type suture zone and the other is the North Qaidam continental-type subduction zone. The North Qilian suture zone records a series of temporal events from seafloor spreading in the Neo-Proterozoic/Cambrian, to the cold oceanic subduction in the Ordovician, and to the strong mountain building in the Early Devonian. Metamorphic ages of the high-pressure rocks range from 490 to 440Ma, suggesting that the Qilian seafloor subduction continued for at least 50m.y. The North Qaidam UHP belt is the product of continental subduction. The petrology, geochemistry and geochronology reveal that protoliths of the eclogites are of both continental and oceanic origins. The HP-UHP metamorphosed ophiolite assemblage of 517±11Ma is coeval with the ophiolite suites in the North Qilian suture zone. The early stage of eclogite-facies metamorphism took place at 443~473Ma, the same age as that of the HP rocks in the North Qilian oceanic suture zone, representing the early stage of seafloor subduction. Coesite-bearing zircons from the metapelite and diamond-bearing zircons from garnet peridotites constrain the UHP metamorphic ages to be ca. 420~426Ma, which represents the timing of continental deep subduction and is ~20m.y. younger than the early stage of the Qilian seafloor subduction. At ~420Ma, the UHPM rocks started to exhume with strong orogenic movement in the North Qilian-Qaidam region. These two end-member subduction belts illustrate a complete mountain-building progression from seafloor spreading/subduction to continental subduction, to continental collision, and to the ultimate uplift with the accompanied exhumation in the Early Paleozoic.