Mid-Late Neoproterozoic to Early Paleozoic volcanism and tectonic evolution of the Qilian Mountain

Mid-Late Neoproterozoic to Early Paleozoic volcanism in the Qilian Mountain area, which shows systematic variations in space and time, seems to have been the volcanic response to the tectonic evolution of the Qilian Mountain. The volcanism gradually changed from rift-related and continental flood basaltic through MORB-type and island-arc and back-arc to post collisional rift-related eruptions along with the tectonic evolution of the Qilian Mountain shifting from rifting and break up of Rodinia through opening and spreading of the Early Paleozoic oceans, subduction of the oceanic slabs and back-arc extension and ocean closure to arc-continent and continent-continent collision. The continental rift-related and flood lavas with ages of 850-604 Ma are distributed mainly on the Qilian and Qaidam Blocks. The widespread MORB-type and "island-arc-backarc"-type lavas were generated from about 550 to 446 Ma in both the North Qilian and the South Qilian ocean-trench-arc-basin systems. In the meantime, the intracontinental rift related volcanism occurred in the central Qilian Block between about 522 and 442 Ma. The Early Paleozoic oceanic basins were closed at the end of Ordovician (about 446 Ma). Subsequent post-collisional volcanism occurred on the northern margin of the Qilian Block from about 445 to 428 Ma. Such spatial-temporal variations provide important constraints on the geodynamic processes that evolved at the depth to form the Qilian Mountain. These processes involved (1) upwelling of mantle plumes or a mantle superplume and subsequent rifting and break-up of Rodinia and subsequent opening, spreading and subduction of Early Paleozoic oceans followed by island-arc formation, (2) roll-back of the subducted oceanic slabs followed by back-arc extension and back-arc basin formation, (3) ocean closure and slab break-off followed by upwelling of asthenosphere and post-collisional volcanism. Intensive orogenic activities occurred in the Late Silurian and Early Devonian (about 420 to about 400 Ma) in response to the exhumation of the subducted crustal materials. Mountain collapse and lithosphere extension happened and formed post-collisional granitic intrusions at < 400 Ma.