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THE MIDDLE AND LATE PROTEROZOIC GEOLOGICAL EVOLUTION OF NORTH QINLING WITH DISCUSSION ON SOME RELATED PROBLEMS

Zhou Dingwu, Liu Liang, Hua Hong, Dong Yunpeng   

  1. Department of Geology, Northwest University.Xi’an, 710069
  • Received:1996-06-20 Revised:1996-06-20 Online:1996-06-20 Published:1996-06-20

Abstract: Lies across the central section of mainland China, the Qinling Orogenic Belt is a Phanerozoic continental orogenic belt.with the Shangdan belt as its suture zone, and to its north is generally called North Qinling.The study of geological evolutionary features of the well developed Middle and Late Proterozoic geological bodies in this district is of great significance to the investigation of early crustal formation and evolution of China continent. The Middle and Late Prorerozoic geological bodies in North Qinling are chiefly of pre-Sinian metamorphic rock series which had experienced polyphase reconstruction of complicated deformation and metamorphism, and can be divided into several diverse rock associations as follows from north to south based oi1 the present rock associations as well as their distribution in time and space. 1. Early Middle Proterozoic (1800 1400Ma) sedimentary-volcanic rock belt of rift. with the Xiong’er Group as an example; 2. Sedimentary belt of stable continental margin in tl1e late stage of Middle Proterozoic (1400-1000Ma).represented by a set of terrigenous clastics and carbonate of Gaoshanhe Group and Luonan Group in Shaanxi Province, Guandaokou Group and Ruyang Groups in Henan Province, which overlain upon the Xiong’er Group; 3. Middle Proterozoic (1800~1000Ma) sedimentary-volcanic belt of rift oceanic basin environment, exemplified by the Kuanping group; 4. Late stage of Middle Proterozoic to initial stage of late Proterozoic accreted volcanic; complex belt of,. various attributes, constituted by Songshugou ophiolite sheet and parts of the sedimentary volcanic rocks of Danfeng Group. The existence of tectonic facies in good time and space sequences indicate that, after the construction of North China plaeocontinental block through 1ateral accretion and vertical overgrowth of North China cratonic nucleus at the end of Early Proterozoic, its southern part splitted and developed into an ocean in the early Middle Proterozoic (1800Ma), with its flourish developing stage of oceanic basin to be 1400±Ma, and its only to the late Middle Proterozoic did the oceanic basin started to underthrust and subduct. A comprehensive study of the deformation, metamorphism, magmatism and isotopic age data of the Middle and Late Proterozoic geological bodies suggests that this district had undergone an important orogenic event in 1000Ma±. The fact that Songsbugou ophiolite sheet was exotic block tectonically emplaced in 983±140Ma (Sm-Nd Isochron age), accompanied with high (superhigh?)pressure metamorphism constituting a PTt evolutionary locus of clockwise relief of pressure of ITD type by means of its structural analysis, in addition to the occurrence o{collision granite of 1000Ma in this area, suggests that North Qinling had experienced an orogenic process of oceanic closure and continental collision in 1000Ma. As a whole, in the Middle and Late Proterozoic, the North Qinling was a paleo-accretion belt similar to modern plate tectonics, which includes splitting from the southern part of North China paleo-continent, gradually evolved to an ocean, and finally subducted.