Geochemistry,chronological framework and tectonic implications of metamorphic volcanic-sedimentary rocks from the Dahongshan Group in southwestern Yunnan Province
The Dahongshan Group in Xinping County,Yunnan Province,is located in the southwestern Yangtze Block.It mainly consists of volcanic-sedimentary rocks with lower greenschist-amphibolite facies.The lack of sys-tematic research on the petrology,chronological framework and tectonic setting of the Dahongshan Group has ham-pered a comprehensive understanding of the tectonic evolution history in the southwestern Yangtze Block at＜～1.75 Ga.In this paper,meta-sedimentary rocks and intercalated meta-volcanic rocks from the Laochanghe Formation in the bottom of the Dahongshan Group were selected for an integrated study including petrographic observation,whole-rock geochemistry and zircon U-Pb dating.The geochemical data reveal that the meta-sedimentary rocks are chemi-cally close to that of the upper crust sediments.The meta-sedimentary rocks are derived from mudstone/shale that was formed in the tectonic setting of the passive continental margins with high maturity and low degree of sedimenta-ry recirculation.Chemical compositions of the meta-volcanic protolith equates to a calc-alkaline peraluminous A-type rhyolite which formed in the post-orogenic continental rift environment.Zircon U-Pb age data show that the detrital zircons from meta-sedimentary rocks of the Laochanghe Formation document two main age peaks of 2.3～2.2 Ga and 1.9～1.75 Ga and a secondary age peak of 2.7～2.6 Ga.Combined with the previous studies,we proposed that the sources of the Dahongshan Group mainly from the Paleoproterozoic and Archean basement rocks in the southwestern Yangtze Block.The zircon cores of the meta-volcanic rocks define the protolith formation age of the Laochanghe Formation as 1 713～1 711 Ma.Zircon rims of the meta-volcanic rocks constrain the timing of peak metamorphism at 843±5 Ma.Combined with the previous and studies,we conclude that the Dahongshan Group un-derwent the Neoproterozoic metamorphic event of 849～837 Ma.In summary,the Dahongshan Group well preserved an anorogenic magmatism is related to Columbia supercontinent Cracking event in the southwestern Yangtze Block.The Neoproterozoic metamorphic event may be closely related to the assemblage and break-up of Rodinia superconti-nent.
万方数据
维普
