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Permian Tectonic Evolution of the Middle Section of Northern Margin of the North China Plate:Constraints from Zircon U-Pb Geochronology and Geochemistry of the Volcanic Rocks

Wang Shijie;Xu Zhongyuan;Dong Xiaojie;Du Yang;Cui Weilong;Wang Yang;College of Earth Sciences,Jilin University;  
The LA-ICP-MS zircon U-Pb dating and geochemical data were obtained for the volcanic rocks of the Suzy Formation in the central region of Inner Mongolia,so as to reveal the tectonic nature of the middle section of the northern margin of North China plate in Early Permian.The zircons from the volcanic rocks in the Suzy Formation are euhedral-subhedral in shape,and display fine-scale oscillatory zoning with high Th/U ratios(0.61-1.06),implying their magmatic origin.The LA-ICP-MS zirconU-Pb age data indicate that the andesite from the Suzy Formation was formed in the Early Permian(279.0±2.1 Ma).The geochemical data indicate that the andesites belong chemically to medium-K calcalkaline series,poor in SiO2(49.0%-55.2%)and HFSEs,rich in Al2O3(15.7%-17.8%),Mg(Mg#=49.8-67.0),and LILEs,and display low REE abundance and flat REE pattern,similar to those from an island arc.Combined with its characteristics of High-Mg andesite,we consider that its primary magma were mainly derived from partial melting of the depleted mantle wedge alterated by the subduction of slab-derived fluid.It can be concluded that the andesite from Suzy Formation was formed under a tectonic subduction when the Paleo-Asian Ocean was not closed.
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