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Depositional Age of the Huashan Group on the Northern Margin of the Yangtze Plate and Its Constraints on Breakup of the Rodinia Supercontinent

TIAN Hui;LI Huaikun;ZHOU Hongying;ZHANG Jian;ZHANG Kuo;GENG Jianzhen;XIANG Zhenqun;QU Lesheng;Tianjin Center,China Geological Survey;Hubei Institute of Regional Geology and Mineral Resources Survey;  
Outcropped in the Dahongshan area on the northern margin of the Yangtze Plate,the Huashan Group consists of a suite of coarse-grained clastic rocks dominated by conglomerates and a suite of fine-grained detrital sedimentary rocks characterized by sandy slate in an ascending order;with the deposition accompanied by tholiitic basaltic magmatism.The Huashan Group was formed in a complex tectonic background and underwent low-grade metamorphism.There have been long disputes about the tectonic attribute of the Huashan Group and the tempo-spatial relationship with the so-called Huanshan‘Ophiolite Complex'.However,they probably provide an important constraint on Rodinia supercontinent assemblybreakup events on the northern margin of Yangtze Plate.One sample of basaltic tuff interlayered in finegrained sandstone in the lower Liufangzui Formation of the Huashan Group was collected for SHRIMP zircon U-Pb method dating,and three samples of clastic rocks(two from the Liufangzui Formation and one from the overlying Liantuo Formation of Nanhua System)were collected for LA-MC-ICPMS zircon U-Pb dating.We obtaind a zircon U-Pb age of 814.7±7.3 Ma for the basaltic tuff.Detrital zircon U-Pb ages of clastic rocks of the Huashan Group have three peaks respectively at~900 Ma,~2050 Ma and~2650 Ma,among which the most distinct one is ~2650 Ma.While the detrital zircon U-Pb ages of the clastic rock from the Liantuo Fotrmation also have three peaks at~900 Ma,~2050 Ma and~2500 Ma,with the most distinct peak at ~2050 Ma.The three samples of detrital zircons are similar to the clastic rocks of the Yangtze Plate in U-Pb age statistic peaks.The source for the clastic rocks of the Huashan Group might include the underlying Mesoproterozoic Dagushi Group, Archean Yudongzi Complex and Kongling Complex.Combined with the previous dating materials and regional tectonic research achievements,it can be concluded that the Huashan Group was probably formed in the time range of 820~815 Ma in an extensional environment,indicating that it is different from the Huashan"Ophiolite Complex"in terms of time and tectonic background.Huashan"Ophiolite Complex"and Huashan Group might reflect a tectonic regime transition from compression to extension respectively.Clastic sediments of the Huashan Group,along with mafic volcanic rocks and volcanoclastic rocks,are coeval sedimentary and volcanic formation formed in an extensional tectonic background.Based on previous discovery of a large volume of~820 Ma felsic-mafic magmatic activity records and the contemporary strata(820~800 Ma)around the Yangtze Plate margins,it can be presumed that the Huashan Group deposited in the initial breakup stage of Rodinia supercontinent related to a super mantle plume.
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