北祁连山及其邻区古生代以来的大地构造演化初探

TECTONIC EVOLUTION OF NORTH QILIAN MOUNTAINS AND ITS NEIGHBOURHOOD SINCE PALEOZOIC

北祁连山出露有中寒武世中晚期和早中奥陶世两期蛇绿岩。同位素年龄分别为335.5 Ma和440-460 Ma的两期高压变质带赋存于中寒武统和下中奥陶统中。大量地质记录揭示北祁连山是介于阿拉善地块和中祁连地块间的一个早古生代缝合带。北祁连山及其邻区古生代以来大地构造演化是复杂的。中寒武世早期统一的中国古陆经陆内裂谷作用发生裂解,于中寒武世中晚期形成北祁连洋。到晚寒武世洋盆转化成残留海盆。早奥陶世北祁连地区再次拉张,遭受第二次大洋化,到中奥陶世形成具沟弧盆体系的成熟大洋。晚奥陶世洋盆转化成残留海盆。于晚奥陶世末碰撞成山,志留纪在新生褶皱山系前形成前陆盆地,盆地底部的下志留统下部层位的磨拉斯可视为碰撞造山的标志。到泥盆纪进入碰撞期后造山阶段,泥盆纪磨拉斯则作为碰撞期后造山作用的标志。北祁连山及其邻区经历了石炭-三叠纪上叠盆地发展阶段,到侏罗纪进入陆内造山阶段。陆内造山作用的主要特征是山脉抬升、盆地沉降,形成盆-岭构造。这个作用一直持续到现在。笔者还对碰撞作用和造山作用的类型进行了讨论。认为软碰撞(soft collision)引起洋盆闭合,但不造山,硬碰撞(rigid collision)使残留海盆闭合并形成新生褶皱山系。在北祁连可以辨认出碰撞造山、碰撞期后造山及陆内造山这3种类型各具特征的造山作用。

Two periods of ophiolites dating the middle-late of middle Cambrian, and early-middle Ordovician outcrop along North Qilian mountains. Two periods of high-pressure metamorphic belts, which have been dated 335.5 Ma and 440-460 Ma seperately by isotopic methods, have been discovered in the mid Cambrian and low-middle Ordovician strata. A lot of geologic records reveal that North Qilian mountains are an early Paleozooic suture zone between the Alashan massif and the central Qilian massif. Tectonic evolution of north Qilian Mts and its neighbourhood is very complex. During the begining of middle Cambrian the united paleo-China continent had been broken up by inland rifting, up to the middle late of middle Cambrian the rift de- veloped to the North Qilian ocean basin. A remnant sea basin replaced the ocean basin in late Cambrian. In early Ordovician North Qilian region again stretched and suffered second oceanization, up to middle Ordovician a mature ocean basin with a trough-arc-basin system already formed. The ocean basin became a remnant sea basin in late Ordovician period. At the end of Ordovician the collision took place and formed a new folded mountains. A fore-land basin, before the new folded mountains, generated in Silurian. Molass eguivalent to the tow level of the low Silurian seguences in the bottle of the fore-land basin has been regarded as a mark of the collisional orogeny. Up to Devonian the studied area came into a stake of post-collisional orogeny, the Devonian molass has been also regarded as a mark of the post-collisional orogeny. From Carboniferous to Triassic, the North Qilian and its neighbourhood underwent evolutional stage of subseqnent basin,and up to Jurassic came into an inland orogenic stage,which is characterized by lifting of mountains, subsiding of basins, and forming of abasin-range structure. This orogeny has been lasting to recent. Types of collision and orogeny have been discussed in this paper. Collision causing the close of an ocean basin is called as a soft collision, which didn't build mountains. However, the close of a remnant sea basin and formation of a new folded mountains resulted from collision that is named as a rigid collision. In North Qilian mountains the collisional orogeny,post-collisional orogeny, and inland orogeny can be distingushed. The three types of orogeny have their respective characteristics.