SOME GEOLOGICAL FEATURES OF THE OPHIOLITES IN XINJIANG
Zhang Chi Institute of Geology,Xinjiang Geological Bureau
The ophiolites in Xinjiang mainly occur linearly in the Variscan geosynclinalfold belt and are closely related to deep fractures. Geographically, 11 ophiolite beltsare recognized from north to south. Most of them are composed of metamor-phic ultrabasic rocks, gabbro-diabase, basic lavas and pelagic sediments. Transitionalcomplexes only appear in the upper parts of a few ultrabasic masses or betweenultrabasic rocks and gabbro. The above mentioned rock types show no regular se-quence and the contact relationships between most of them are intrusive. The orderof emplacement is often such that the basic lava eruption came first, followed bygabbro-diabase, transitional complexes and ultrabasic rocks. In view of the widerange of SiO_2 content, the low TiO_2 contont and the (FeO)/MgO ratio greater th-an 1.7 in the basic lavas as well as of the paleogeographic environment in whichthey were formed, it is suggested that the ophiolite belts in the eastern and west-ern Junggar and the North Tianshan were formed in an arc-island environment. Most of the ophiolite belts in Xinjiang are found along the accreting marginsof the modern earth's crust. Presumably their formation is related to the deep fr-actures that cut deep into the upper mantle. These deep fractures not only causedthe formation of graben basins--geosynclines--by pulling apart the lithosphere, butalso brought about the remelting of mantle-derived rocks and the upward intrusi-on of the melts. Thus the ophiolites were formed by intrustion of the remelted m-antle malerials of different compositions by stages. The ophiolites in Xinjiang, in asense, mainly resulted from primary emplacement.