THE MAIN GEOLOGICAL CHARACTERISTICS OF THE NORTHERN QILIAN MOUNTAINS AREA

The Qilian Mountains ars one of the largest mountain systems in Eurasia, and tectonically are a typical Caledonian geosyncline in the Kunlun-Qinling geosynclinal fold system. From north to south five tectonic units may be dis- tinguished there: （1）the Hexi Corridor marginal geosynclinal fold belt, （2）the Northern Qilian eugeosynclinal fold beld, （3）the Middle Qilian uplift belt, （4） the Lajishan taphrogeosynclinal fold belt, and（5） the Southern Qilian miogeo- synclunal fold belt. The Northern Qilian fold belt is situated between the Hexi Corridor fold belt and the middle Qilian uplift belt. In the Northern Qilian Mountains area, the exposed old basement rocks are limited. The pre-Changchengian rocks are distributed as NW-trending fault blocks in the western part of the area, re- presented by biotite-plagioclase gneiss, migmatitic gneiss, plagioclase amphibolite and sericite-quartz schist intercalated with minor intermediate-basic volcanic rocks and iron ore layers, greater than 6,000 m thick. The middle-upper Proterozoic, distributed in a near-NW direction in the Jingtieshan-Golmud area, is close to eugeosynclinal deposits, with a total thickness grater than 15,000 m. From the end of the Changcheng Period to early Sinian there occurred a lot of structural movements, which led to the formation of a ser- ies of disconformities and unconformities between the Daliugou Group and the Zhulongquan and Baiyanggou Groups and between the Daliugou Group and the Jintieshan Group. The lower Paleozoic is a main component part of the North- ern Qilian Mountains, marked mainly by clastic rocks, carbonate rocks, sili- ceous volcanic rocks and intermediate-basic volcanic rocks, with a total thick- ness of 25,000m. The upper Paleozoic is platform-type cover deposits, which overlie the old strata unconformably, the total thickness being 5,500m or more, The regional tectonic evolution in the Northern Qilian Mountains is as follows. 1. Precambrian stage The Northern Qilian Mountains represent a paleo-sea trough, consisting of volcano-sedimentary associations of a rift-type island-arc sea. The speading of the oceanic-crustal rift in the early phase resulted in the formation a quasi-eugeosynclinal sedimentary formation of the Chengcheng to Qingbaikou Periods and an ophiolite-like suite in the Northern Qilian paleo-trough of the Changcheng Period. At the end of the Qingbaikou Period, the paleo-sea trough was broadly folded, forming unconformities or diconformities between the Qing- baikou System and the Sinian System, and at the same time the Sinian tillite（megaconglomerate or breccia）. 2. Early Paleozoic No early Cambrian deposits have been found in the Northern Qilian Moun- tains, but this area again became a big sea trough between the middle- southern Qilian-Qaidam micro-oldland and the Tarim-Alxa platform in early Middle Cambrian time. The Middle Cambrian to Early Ordovician was mainly a spreading stage. On its southern side was developed a polyphase lithospheric deep fault（which represents an important suture line）, accompanied by the development of a polyphase ophiolite-like suite. By the Late Ordovician, the area had entered the late stage of a eugeosyncline, and a molasse-like for- mation originated in the Gulong Gorge. The Silurian Period was a closure stage of the geosyncline, and only small amounts of volcanic rocks, such as hornb- lende andesite and andesite basalt, occurred. At the end of the Caledonian the area was strongly folded and became mountains. The upper Paleozoic is represented by the cover of the platform, and the Devonian is of continental facies, which overlie the pre-Devonian or granite unconformably. The Carboniferous is of paralic facies, but the Permian and Meso-Cenozoic are all of continental facies.