| The volcanic rocks of Yutian Group are located in Longnan,Baimianshi and Changpu basins along the Sannan?Quannan,Longnan and Dingnan?–Xunwu fault belt in southern Jiangxi Province,its rock association is “rhyolite and basalt” bimodal volcanic combination,and epoch belongs to the early Jurassic.This paper choose Yutian Group basalt of Dongkeng basins as object of study,according to systematic study of field geology investigation,microscopic identification,petrology,geochemistry,Sr–Nd–Pb isotope and monoclinic pyroxene composition,determinestratigraphic classification of rock of Yutian Group,ascertaingeochemical characteristics of basalt and monoclinic pyroxene,and discuss the nature of the source region and formation of the tectonic environment.Based on the analysis of field geological features and geological section,Yutian group can be divided into 3 parts.On the bottom,the rocksinclude mottled gravel,sandstone with siltsone and basalt,this partbelongs to the sedimentary eruption facies,can be called Shuitoujing formation;In the middle,the rock is composed of “rhyolite and basalt” bimodal volcanic combination,this part belongs to eruption facies,can be Changpu formation;On the upside,the rocksinclude yellow green or yellow conglomerate,pebbly sandstone,yellow green tuffaceous sandstone andamaranthpelitic,siltstone,this part belongs to lacustrine deposit,can be classified among Luoao formation.This evidence not only confirmed Yutian group is the product of a complete volcanic cycle,and confirmed that Luoao formation is concordancy on calamus group for the first time.The content of SiO2 of basalt of Dongkeng basin is 47.1%50.1%?average:48.9%?;the content of K2O is 0.52%1.59%?average:1.19%?,relatively lower than Na2O;the content of Na2O is 2.23%3.06%?average:2.51%?;the content of K2O and Na2O is 2.75% 4.65%?average:3.70%?;On the diagrammatize of TAS,most of points locate on the subalkaline basalt series area;the content of MgO is 4.16%5.83%?average:4.95%?;the content of TFeO is 14.3%17.1%?average:16.2%?,on the diagrammatize of A–F–M,points locate on tholeiitic series area,these data indicate that basalt belongs to tholeiitic.On the diagrammatize of Harker of major element,along with the reduction of content of MgO,the content of CaO is markedly increased,and the content of TiO2,Fe2O3,Na2O and MnO is increased,the change of content of SiO2,FeO and K2O is unconspicuous,indicates that there is crystal fractionation during the evolution ofmagma.The total content of REE of Dongkeng basin is relative low,variation range is relative small.?REE=113.9×10-6255.5× 10-6,the average value is 184.2×10-6,light REE enrichment,heavy rare earth loss,LREE/ HREE is 2.344.11,the content of 3.5.REE distribution graph is slightly to the right,Eu negative anomaly is not obvious,the LILE?Rb,Ba,Th,U?of basalt of Dongkeng basin is relatively enriched,the HFSE?Ce,Ta,Zr,Hf,Ti?is relatively depleted,on the discrimination diagrams of Zr/Al2O3–TiO2/Al2O3 and on the diagrammatize of 2Nb–Zr/4–Y,projection points are located on plate areasuggest that basalt formed in extensional tectonic environment.The electron probe analysis of monoclinic pyroxene of basalt show that the character of monoclinic pyroxene is low abundance of Ti,Al and Na,and high abundance of Si and Fe.SiO2=51.6%53.0%,Al2O3=2.04%2.62%,TiO2=0.23%0.38%,MgO=15.3%16.7%,MnO=0.16%0.34%,CaO=12.0%13.2%,Na2O=0.1%0.19%,TFeO=15.1%16.9%,Mg#=54.357.8,Wo=27.2%28.0%,En=43.5%47.9%,Fs=23.4%27.9%.The crystallization temperatureis 11391165?,the pressure is 0.0010.24 GPa.On the diagrammatize of SiO2–Al2O3,projection points are located onsubalkalic series.On the diagrammatize of Al2O3–TiO2,all projection points are located ontholeiite series,consistent with the classification of basalt magma series.On the diagrammatize of Wo–En–Fs,all projection points are located on Fe-augite.On the double factor discrimination diagram of Fe–F2,most of projection points are located onintraplate tholeiite of and suboceanic basaltregion,consistent with structure characteristics of basalt.The basalt of Dongkeng basin has higher Sr initial value?0.709260.72944,average value is 0.70933?,and lower Nd initial value?0.5125920.512625?.On the diagrammatize of t/Ga–Isr and t/Ma–?Nd,reflect that basalt has the characteristics of mantle derived material,and the ratio of Nb/U,Nb/Th and Ti/Yb prove that the magma is effected by source area.(206Pb/204Pb)t=18.0118.19?average value is 18.11?,(207Pb/204Pb)t=15.3115.45?average value is 15.36?,(208Pb/204Pb)t=37.8038.40?average value is 38.10?,and the diagrammatize of 208Pb/204Pb–206Pb/204 Pb and 207Pb/204Pb–206Pb/204 Pb alsoreflect that basalt has the characteristics of DMM and EM?mantle material.Nd isotopic two-phase model age?917949 Ma?,combing with the character of rocks formed in the intraplate extensional tectonic environment,these evidenceindicates that magma is likely to bethe mixtureproduct of continental lithospheric mantle and ashospheric mantle... |
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