PRECAMBRIAN TECTONIC EVALUTION OF DABIE SHAN
Yang Sennan Wu Jian (Wuhan College of Geology) (Ministry of Geolog and Minerals) Yang Xuezhong Lu Zhongdan (313 Geological Team of Anhui Province)
An important geological feature in the eastern part of China is of the remarkable disparities in many of geological aspects between North China and South China. Research of the interspacing Qilian Shan-Qin Ling-Dabie Shan folded mountain system, especiallyits evolutionary interrelation with the North China Massif and the Yangzi Massif, is the key to the problem of long-term disparity between North and South Massifs. In this paper. the Precambrian tectonic evolution of theDabie Shan will be discussed. The Dabie Shan-Beihuaiyang folded mountain system is separated from the North China Massif on the north by the Feizhong-Minggang Fault Zone and the Yangzi Massif on the south by the Guangji-Xiangfan Fault Zone. It can be divided into three tectonic zones, the Beihuaiyang ancient marginal sea, the Dabie ancient island arc and the North Yangzi continental margin zone, separated successively by the Mozitan-Tonbo Fault Zone and the Guanji Yinshan Fault Zone. The first two zones belong to the southern margin of the North China continental region. The Guangji Yingshan Fault Zone is a collisional zone of two continental margins. Two terrains with different tectonic characteristics and evolu- tionary histories are contacted each other along the zone. It is also an important line subdividing the northern and southern paleobio-gepgraphic regions. The extensive distribution of volcanic eruptions and basic and ultrabasic rocks have the characteristic features of ophiolites, moreover the glaucophane represeting high P.-low T. facies has been found between Dawu suixian yingshan and Huangpi counties in Hubei province. The Dabie Shan-Beihuaiyang folded mountain system under-went complicated tectonic evolution during Precambrian. It was a part of an ancient continental marginal zone on the south of the Hehuai Archean nuclei in Early Proterozoic, deposited by mainly clastic deposits of immense thickness. After the Dabie Movement at the end of Early Proterozoic, the volcanic arc appeared in the Dabie Shan in Middle Proterozoic, which developed into an island arc in Late Proterozoic. Beihuiayang, located on the north side of the island arc, was an aulacogen at first, then developed into a marginal sea,in which flysch deposited extensively.The South China Ocean was to the south of the island arc,and its ocean crust subduced underneath the Dabie Shan island arc along the Guangji-Yingshan consumption zone. At the end of Late Proterozoic, the Beihuaiyang marginal sea reduced and subduced anti-poleward beneath the island arc along the Mozitan-Tongbo Fault Zone. In this stage, the Yangzi Continent gradually approached the North China Continent due to subduction of the South China ocean crust. Two opposite continental margin zone had almost come to collision at the end of Late Proterozoic or at the beginning of Sinian. This postula-tion is confirmed by palaeomagnetic data obtained from the Lower Sinian of Beihuaiyang and the Yangzi Gorges. During that epoch orogeny became strengthened, with the result that the molasse and tillites appeared in adjacent continental regions. The trough in the North Yangzi continental marginal zone did not entirely disappear until the late stage of Early Pa-laeozoie. The collisional zone is an. unstable region in tectonics. This was clearly in later geological history after the collision of the North China Massif and the Yangzi Massif, and is the intermal cause of multi-phase remobilization of Dabie Shan-Beihuaiyang region.