Rectifying the Neoproterozoic Stratigraphic Framework of Eastern Jiangnan Orogen, Southeast China

摘要

Studies on the metamorphic and related magmatic rocks within the Jiangnan Orogen,southern China,are important to understand the formation and evolution of this Neoproterozoic orogen because they can provide evidence for revealing the tectonic evolution of the South China Block.Following on from earlier attempts at stratigraphic correlation and creating a framework for the low-grade metamorphosed basement,new first-hand information on the composition and deformation of the East Jiangnan Orogen basement indicates that the sedimentary rocks in the north are composed of the sequential Shangxi Group(Gp)and the overlying Likou Gp,with a clear regional unconformity in between.The Likou Gp includes the Zhentou Formation(Fm.)and the Dengjia Fm.,with the previously named Puling Fm.only basalt interbeds within the Dengjia Fm.The Xucun granodiorite pluton of^830 Ma intrudes the Shangxi Gp,thereby resulting in a 100 mwide hornfels zone within the wall rocks,indicating that the formation of the group occurred earlier than that of the pluton.By contrast,the southern metamorphic rocks of the Xikou Gp and the overlying Jingtan Fm.are flaky,disordered,and strongly deformed.The Jingtan Fm.is roughly equivalent to the Heshangzhen Gp and includes the Zhoujiacun conglomerates in the lower part and a rhyolite in the upper part with interbedded basalt.This assessment of Neoproterozoic stratigraphic sequences and magmatic rocks in the east of the Jiangnan Orogen differs from and rectifies previous studies,emphasizing that explanation of analytical results,particularly geochronology,should be consistent with facts gathered in the field.