Neoproterozoic Crustal Composition and Orogenic Process of the Zhanggongshan Area,Anhui—Jiangxi

Abstract

Zhanggongshan area of Anhui-Jiangxi border,located in the northern margin of the Jiangnan orogen (South Yangtze River),contains very thick low greenschist-facies metamorphic flysch rocks dominated by argillo calcareous fine-grained clastic rocks with minor volcanic materials. A mass of high precise isotopic dating shows that it may form in early Neoproterozoic between 821~840 Ma,bordered by the Jingdezhen-Zhangyuan-Fuchuan regional faulting belt. The Xikou Group in the southern part resulted from extension of continental margin,while the volcanic-terrestrial clastic rocks of Shuangqiaoshan Group in the northern part is the sediments in basins related to island arc. The basic rocks such as altered pillow-lump basic volcanic lava,gabbro and dolerite distributed from northeast to southwest on the northern slope of Zhanggong Shan (mountain),occur as structural fragments in flysch matrix consisting of Xikou Group volcanic-margin fine-grained clastic rocks. The geological feature and petrochemical analysis show that they resulted from low-speed spreading of small oceanic basins in continental margin and characterized with initial features of oceanic crust basic rock,indicating that they belong to part of the westward extension of the Fuchuan ophiolite in southern Anhui. Large amount of vitric crystal tuff and minor andesite and rhyolite occurring in the Shuangqiaoshan Group strata as interlayer formation is similar with geochemical features,suggesting that they belong to potassium-calcium-alkali rock series,and formed in a tectonic setting of the volcanic arc in the continental margin. Combined with sedimentary characteristic,magmatic and tectonic occurrences in the area,this study proposed that the Neoproterozoic Anhui-Jiangxi border underwent a series of tectonic development processes from the splitting of the southeastern margin of ancient Yangzi plate,limited subduction of small oceanic basins in continental margin,arc-continent collision orogeny and finally to post-orogenic rifting sedimentation of continental crust.