CHARACTERISTICS OF THE STRUCTURES IN THE NORTHERN QINLING MOUNTAINS AREA IN SHAANXI PROVINCE AND THEIR EVOLUTIONAL HISTORY

The Northern Qinling Mountains area in central Shaanxi is located in a relatively peculiar tectonic position. Its northern part belongs to the SinoKorean platform, while its southern part the Qilian-Qinling geosyncline. It has undergone the effects of almost all previous tectonic movements. As a result, in the area the tectonism, magmatism and metamorphism are very strong, the tectonic movements are especially complex, and mineral resources of various kinds are quite rich. In view of such complex geological processes, the authors used the theory and method of polycyclic tectonics founded by Prof. Huang Jiqing, drew on the theory of plate tectonics and based themselves on such geological records as the stratigraphy, contact relationships, lithofacies and formations, biotic fossils, metamorphism, magmatism, tectonism, isotopic ages and mineralization to divide the histeory of tectonic evolution in the area into 12 tectonic cycles and 4 development stages. Such division is relatively objective. In elucidating the history of tectonic evolution, this paper deals with the evolution of uranium in more detail. In the Precambrian, uranium deposits were not formed and the uranium content was not high in the terranes, but the folding and uplift of the Qilian-Qinling geosyncline gave rise to the Caledonian Northern Qinling tectono-magmatic belt and uranium mineralization of pegmatite type. Since the Mesozoic, the area had been strongly affected by the tectonism of the Tethyan and circum-Pacific tectonic domains, thus forming the early Yanshanian E-W-trending tectono-magmatic belt and the late Yanshanian NNE-trending tectono-magmatic belt. Under the effects of such tectonism, uranium was continuously mobilized and migrated, and was precipitated in favourable structural positions, thus forming a number of uranium occurrences or mineralizations in the E-W-trending tectono-magmatic belt and the major 401 uranium deposits and some minor uranium deposits and occurrences in places where the two belts are superimposed. So the Yanshanian cycle is an important metallogenic epoch of uranium in the area. In summary, the use of the polycyclic tectonic theory may better reveal the history of the crustal evolution and uranium mineralization in the area and yields better results in the prediction of uranium deposits.