Mid-late neoproterozoic to early paleozoic volcanism and tectonic evolution of the Qilianshan, NW China

Mid-Late Neoproterozoic to Early Paleozoic volcanism in the Qilianshan area, which shows systematic variations in space and time, is the volcanic response to the tectonic evolution of the Qilianshan. The volcanism gradually changed from continental rift-related and continental flood basaltic through MORB-type, island-arc and back-arc to post-collisional rift-related eruptions along with the tectonic evolution of the Qilianshan shifting from rifting and break-up of Rodinia through opening, spreading, subducting and closing of the Early Paleozoic oceans to arc-continent and continent-continent collision. The continental rift-related and flood lavas with ages of 850–604 Ma distribute mainly on the Qilian and Qiadam Blocks. The widespread MORB-type and “island-arc-backarc”-type lavas were generated from ∼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 ∼522 and 442 Ma. The Early Paleozoic oceanic basins were closed at the end of Ordovician (∼446 Ma). Subsequent post-collisional vocanism occurred on the northern margin of the Qilian Block from ∼445 to ∼428 Ma. Such spatial-temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Qilianshan. These processes involve: (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 (∼420 - ∼400 Ma) in response to the exhumation of the subducted crustal materials. Mountain collapse and lithosphere extension happened in the priod of ∼400–360 Ma and formed post-collisional granitic intrusions, which marked the end of a complete orogenic cycle.