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Geochronology and petrogenesis of the Permian marine basalt in the southern Beishan region and their tectonic implications

XU Wei;XU Xueyi;NIU Yazhuo;SONG Bo;CHEN Gaochao;SHI Jizhong;ZHANG Yuxuan;LI Chenhao;Xi'an Center of China Geological Survey;School of Earthscience & Resources,Chang'an University;Centre for Orogenic Belt Geology CGS;China Geological Survey;Institute of Geomechanics CAGS;  
In order to clarify the tectonic evolution of the southern Beishan orogen during the Late Paleozoic,a comprehensive study was carried out on the Permian basalt,andesite,and dacite collected from the southern Beishan area,using whole rock geochemistry,Sr-Nd isotopes and zircon U-Pb age.Zircon LAICP MS U-Pb dating yields the ages of 289.5±2.3 Ma and 267.8±3.1 Ma for dacite and andesite,respectively.Combining with the isotopic ages of volcanic rocks and fossils in the profiles,we suggest that the eruptive age of volcanic rocks ranges from Arinskin to Wordian(290~265 Ma).This set of basaltic rocks occurring in marine sequence belongs to tholeiitic series with changeable contents of major elements and moderate concentrations of TiO_2(0.9% ~2.53),and show no distinct differentiation of LREE and HREE((La/Yb)N=1.6~6.2)with unobvious Eu anomalies(δEu=0.79~1.19).They exhibit similar geochemical characteristics including varying degrees of enrichment in La,Ce,Nd,Zr,and depleted in Nb,Ta,Ti,P,etc.TheεNd(t)and(~(87)Sr/~(86)Sr)i values for these basalts vary from +1.4 to+10.9,and from 0.703365 to 0.707285 respectively.The trace elements characteristics and Sr-Nd isotopic data suggest that the Permian basalts from southern Baishan area derived from asthenosphere hybridized by subduction material with different degree partial melting of garnet-bearing spinel Iherzolite.Discrimination diagram shows that the basalts formed neither in arc(island arc and continental arc)nor in plume-related settings,are similar to WPB(within-plate basalt)and MORB(mid ocean ridge basalt)in discrimination diagrams.Combined with other geological evidence,it can be suggested that the basalts were triggered by delamination of thickened lithosphere after collision.
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