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Is the Jiangnan Orogenic Belt a Grenvillian Orogenic Belt:Some Problems about the Precambrian Geology of South China

Zhou Jin-cheng   

  1. Department of Earth Sciences, Nanjing University, Najing 210093, China
  • Received:2008-03-20 Revised:2008-03-20 Online:2008-03-20 Published:2008-03-20

Abstract: This discussion is made on the basis of age data reported in recent years of 21 century, which have been obtained by SHRIMP, LA-ICP-MS zircon U-Pb and other new dating methods. The assembly of the Yangtze and Cathysia blocks took place between 860 and 820 Ma. The continent-continent collision event forming Grenvillian orogenic belt spanned a period from 1 190 to 980 Ma. Therefore, the collision between the Yangtze and Cathysia blocks occurred about 320~160 Ma later. The basement strata in the Jiangnan orogenic belt underwent thoroughly greenschist facies metamorphism. High grade metamorphism, such as granular facies existed widely in the Grenvillian orogenic belt, has never been found in the metamorphic sedimentary-igneous rocks in the Jiangnan orogenic belt. Because of above two reasons, the Jiangnan orogenic belt should not be considered as a Grenvillian orogenic belt. It is proposed that the Jiangnan orogenic belt might be a product of collision between two neighborhood blocks during the transformation from the Rodinia to Gondwana supercontinent. The Sibao, Lengjiaxi and Fanjingshan groups in the western part of the Jiangnan orogenic belt, as basement strata, were previously believed to be Mesoproterozoic. Recent dating results of detrital zircons from the sandstones of the basement strata indicate that these strata have the maximum depositional ages of 870~860 Ma, i.e. Neoproterozoic. The S-type granites distributed along the Jiangnan orogenic belt may be considered as a petrological record of the continent-continent collisional event between the Yangtze and Cathaysia blocks rather than the products of plume magmatism. Some Neoproterozoic mafic volcanic-intrusive rocks (~760 Ma) from the western part of Jiangnan orogenic belt and the mafic rocks from the bimodal volcanic rocks (803~818 Ma) distributed around or within the Yangtze block or within Cathaysia block show some geochemical signatures of OIB. However, their exposed volumes are very small. They might have been generated in the post-orogenic extensional stage due to local upwelling of asthenospheric mantle. It is believed that they shouldn't be considered as petrological evidence of breakup of the Rodinia.