SEVERAL PROBLEMS ON THE BLUESCHIST BELT IN NORTHERN HUBEI

The blueschist belt in northern Hubei mainly occurs in the upper Sinian Doushantuo Formation, Yaolinghe Group and Wudang Group （the upper part of which is the Shuangtai Formation）. It is localized in the rocks characterized by rich Na₂O （2-7%） and a higher Fe₂O₃/FeO ratio. The belt began to form in the middle Proterozoic. In the early stage there occurred a rift trough between the Yangtze platform and North China platform, in which very thick acid volcanic rocks were deposited. In the middle stage, extension and subsidence continued and thus there developed siliceous and pelitic rocks and tholeiite. By the terminal late Proterozoic, the trough closed and the northern and southern continents were assembled again, accompanied by strong compressive stress which brought about metamorphism. By that time a blueschist belt was formed.