祁连山新元古代中—晚期至早古生代火山作用与构造演化

祁连山地区的新元古代中—晚期至早古生代火山作用显示系统地时、空变化,其乃是祁连山构造演化的火山响应。随着祁连山构造演化从Rodinia超大陆裂谷化—裂解,经早古生代大洋打开、扩张、洋壳俯冲和弧后伸展,直至洋盆闭合、弧-陆碰撞和陆-陆碰撞,火山作用也逐渐从裂谷和大陆溢流玄武质喷发,经大洋中脊型、岛弧和弧后盆地火山活动,转变为碰撞后裂谷式喷发。850~604 Ma的大陆裂谷和大陆溢流熔岩主要分布于祁连和柴达木陆块。从大约550 Ma至446 Ma,在北祁连和南祁连洋-沟-弧-盆系中广泛发育大洋中脊型、岛弧和弧后盆地型熔岩。与此同时,在祁连陆块中部,发育约522~442 Ma的陆内裂谷火山作用。早古生代洋盆于奥陶纪末(约446 Ma)闭合。随后,从约445 Ma至约428 Ma,于祁连陆块北缘发育碰撞后火山活动。此种时-空变异对形成祁连山的深部地球动力学过程提供了重要约束。该过程包括:(1)地幔柱或超级地幔柱上涌,导致Rodinia超大陆发生裂谷化、裂解、早古生代大洋打开、扩张、俯冲,并伴随岛弧形成;(2)俯冲的大洋板片回转,致使弧后伸展,进而形成弧后盆地;(3)洋盆闭合、板片断离,继而发生软流圈上涌,诱发碰撞后火山活动。晚志留世至早泥盆世(420~400 Ma),先期俯冲的地壳物质折返,发生强烈的造山活动。400 Ma后,山体垮塌、岩石圈伸展,相应发生碰撞后花岗质侵入活动。

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.