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Zircon U-Pb geochronology and geochemistry of mafic dyke swarms in Dengxiangying Group on west margin of Yangtze Block,China

REN Guang-ming1,PANG Wei-hua1,SUN Zhi-ming1,YIN Fu-guang1,XIAO Qi-liang2 1.Chengdu Institute of Geology and Mineral Resources,China Geological Survey,Chengdu 610081,China; 2.Panxi Geological Party,Sichuan Bureau of Geology and Mineral Exploration,Xichang 615000,China  
In the Dengxiangying Group,there are a large number of mafic dyke swarms,indicating an important tectonic-magmatic event during the Neoproterozoic period on the west margin of Yangtze Block in China.Based on the SHRIMP U-Th-Pb dating for the single-grain zircon and the whole-rock geochemical analysis and test,the mafic dyke swarms in the Dengxiangying Group have the intrusion age of 744±10 Ma(corresponding to the early stage of Nanhua).And people also obtain one metamorphic zircon with the metamorphic age of 141±3 Ma(corresponding to the Late Jurassic to Early Cretaceous)which indicates one tectonic-magmatic thermal event in the Mesozoic.The geochemical characteristics show that the mafic dykes belong to the subalkaline basaltic series,whose mass fraction of Al2O3 is 14.40%~23.61%,high mass fraction of TiO2 is 1.73%~3.23%,and low mass fraction of P2O5 is 0.13%~0.44%.The REE results show that all of the samples have lower total REE(61.0×10-6~169.8×10-6) with the REE lines gentle or slightly to right,the(La/Yb)N value of 0.74~2.28 and(Ce/Yb)N of 0.69~2.21,and a weak or no Eu and Ce anomaly.The trace elements show that in the mafic dyke,the LILE and incompatible elements Rb,K,and Ba are concentrated with various degrees,Pb,Zr,and Hf do relatively,Nb,Ta,and Ti are weak loss,and Zr/Y(3.09~ 8.70) and Zr/Sm(33.5~74.83) ratios are high,characterized by the geochemistry of intra-plate basalt.Combined with the regional geological data,the authors consider that the Neoproterozoic(~770 Ma B.P.) mafic dyke formed in the intra-plate extensional environment after Jinning orogeny on the west margin of Yangtze Block,sourced from the depleted asthenosphere mantle(DM),and contaminated by the crustal materials when the mantle upwell.This process is synchronous with the breakup of Rodinia supercontinent.
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