Abstract
Lying in the northwestern border region,Xinjiang has a vast territory,covering a total area of 1.65 million sq km.The strata,magmatic rocks,metamorphic rocks and geological structure of various ages in the region are very complex and the mineral resources are rich.The stratigraphy,intrusive rocks and geological structure are briefly introduced as follows. 1.Regional stratigraphy The strata from the Archean to Quaternary are all developed in the region.The Archean,about 3278 m thick,consists of hypers- thene granulite,granulitite,gneiss and migmatite.The Lower Proterozoic,up to 30000 m in thickness,is composed of schist,gneiss,migmatite,quartzite and marble.The middle Upper Proterozoic （Jixian System-Qingbaikou System） is represented by elastic rocks and carbonate rocks,containing five stromatolite assemblages.The Sinian consists of sandstone,mudstone,moraine limestone and conglomerate,comprising three glacial stages.The Cambrian,ranging in thick- ness from 35 to 608 m,is marked by clastic rocks,siliceous rocks and limestone, whose base generally contains phosphatic horizons,and 13 trilobite zones have been set up.The Ordovician,having a total thickness greater than 500 m,is dominated by siltstone,shale and limestone,and 7 graptolite zones has been established.The Silurian is represented by clastic rocks and limestone with local intermediate volcanic rocks,yielding the Tuwajian fossil assemblage in the Altay area.The Devonian-Carboniferous are widespread,including continental clastic rocks and volcanic rocks,yielding mainly plant fossils;in marine clastic rocks and limestone,various groups of fossils are very abundant.The Permian, ranging in thickness from 3000 to 6000 m,consists mainly of continental and paralic clastic rocks and volcanic rocks,with Early Permian cold-water faunas and Gondwanic facies sediments newly found on the northern slope of the Karakorum Mountains.In the Triassic-Tertiary,there are developed continental and marine sediments.Of these two types of sediments,the continental sediments,occurring mainly in the Northern Tianshan-Junggar area,are composed of mudstone,sand- stone and conglomerate,yielding plant and vertebrate animal fossils and containing such useful minerals as coal,iron,salt and gypsum,and the marine sediments,occurring mainly in the Karakorum Mountains,consist of marls, mudstone and siltstone,yielding ammonoid,coral and brachiopod fossils and containing gypsum in places.For the Quaternary,in the Altay,Tianshan and Kunlun mountains,glacial sediments are very well developed;in the basins and intermontane depressions,there occur mainly fluvial sandstone and conglomerate and locally lacustrine mudstone and conglomerate. 2.Intrusive rocks Intrusive rocks of various ages are exposed in the region, covering a total area of up to 110000 sq km.The proportions of various types of the intrusive rocks are as follows:basic-ultrabasie 2.8%,diorite-granite 95%, syenite 2.2%.The proportions of the intrusive rocks of various ages are as follows:Proterozoic rocks 13.6%,Caledonian rocks 7.9% and Variscan rocks 69.5% and Indosinian and Yanshanian rocks 4% and 4.9%,respectively.There are a total of 24 ophiolite belts in the region,of which the better-developed ones are composed of peridotite,cumulate rocks,pillow lavas and radiolarian siliceous rocks,dike swamps being generally absent.The sequence of the ophiolites is-often not complete owing to structural destruction. 3.Regional structure With the Tarim platform as the boundary,Xinjiang may be tectonically divided into two parts:the northern part is represented by the Tianshan-Khingan fold region （including the Altay,Junngar-Northern Tianshan and Tianshan fold systems）,while the southern part consists of the Kunlun-Qinling fold region （including the Eastern and Western Kunlun fold systems） and Yunnan-Tibet fold region （including the Songpan-Garz（?）,Karakorum and Gangdise fold systems）.The basic characteristics of the crustal evolution are polycylic extension-closing and control by “X” shearing.From north to south the crust becomes younger and the structure becomes complex,thus forming the present—day rhombic tectonic framework.
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