The Geological And Geochemical Characteristics For Caledonian Dacite In Nanjing Basin,southern Jiangxi Province,and Its Geological Implications

NanJing basin is located in the southern Jiangxi Nanjing and Zhongzhai town,shouth Qinzhou to Hangzhou belt which between the Yangzi ancient plate and the ancient Chinese plate,in shouth Luoxiao fold and Taoshan to Zhuguang Uranium metallogenic belt.It is one of the region which research tectonic environment,magmatic activity and mineralization geological problems in the south China.The basin consists of weak metamorphic light gray dacite and gray andesite,covering on the Cambrian-Ordovician weak metamorphic rocks.Predecessors consider its part of Late Mesozoic volcanic rocks through lithologic and lithofacies contrast.However,the SHRIMP zircon U-Pb dating results show the 206Pb/238 U ages of the dacite and andesite are(439.9±3.7)Ma and(442.1±3.9)Ma from Ding Hui(2016),indicating the geological age is Ordovician-Early Silurian.Therefore,studied in the volcanic rocks in the basin of field geology characteristics and petrological and petrological characteristics,discuss it tectonic setting and petrogenesis for understanding of the geologic background in Caledonian and Xiazhuang Uranium metallogenic make important senseField geology characteristics display,Nanjing group volcanic rocks unconformity under the Yunshan group sedimentary rocks,Yunshan group conglomerate contain Nanjing group lava,Huangzhudong group weak metamorphic clip lava layer under Nanjing group,Nanjing group dacite and fused tuff and tuff and volcanic breccia cross layer.Nanjing group dacite has weak metamorphic features and some mineral speckle are slightly deformed.Dacite is flow structure and patchy structure,the plagioclase porphyry ring structure is clear,amphibole and biotite mineral are darkened,the quartz spotted into a rounded shape,As aphase or vitreous partial degeneration into aphase and microcrystalline,local see secondary mineral magnetite,carbonate minerals and so on.These characteristics show that the Nanjing basin dacite is not only volcanic lava,but also with the HuangzhuDong group represented by metamorphic rock series experienced metamorphism.Dacite SiO2=64.3%~65.6%,K2O+Na2O=5.15%~6.17%,located in sub-alkaline series of dacite in TAS.K2O/Na2O=1.01~1.89,A/CNK=1.00~1.09,CaO=3.61%~4.83%,alkalinity is 1.66~1.98,TFeO=4.99%~6.13%,belonging to high potassium calc-alkaline weak aluminum series rock.The CaO,TFeO,MgO,TiO2,ALK and P2O5 shows positive coorelation against SiO2.?REE=153~191×10-6,?LREE =131~169×10-6,(La/Yb)N=6.03~9.17,?(Eu)=0.54~0.68,?(Ce)=0.91~9.94,in the rare earth element distribution curve for the right-type.large ion lithophile elements Rb,Th,U,K enrichment,high field elements Nb,Ta,P,Ti and Ba,Sr loss,and the geochemical characteristics are similar of typical island arc magmatic rocks.Zr=(178~215)×10-6,TZr(?)is between 786~810,the ratio of CaO/Na2O=1.5~2.19,Mg#=44.1~49.3 is higher,Rb/Sr =0.59~1.45 is lower,in chart?Eu-(K2O+Na2O)falls in the blend of crust and mantle,display existence crust and mantle magma mixing action.In Al2O3/(Al2O3+TFeO)-TFeO/TiO2,Al2O3/(Al2O3+TFeO)-(La/Ce)N,SiO2-K2O/Na2 O,Yb-Th/Ta,Ta/Yb-Th/Yb,Ta/Yb-Th/Ta tectonic environment,the projection points fall within the range of active continental margin magmatic rocks.The dacite shows high(87Sr/86Sr)i valeus(87Sr/86Sr)i=0.70857~0.70799,low ?Nd(t)values ?Nd(t)=-8.05~-7.03 and relatively older Nd-modle age 1745 to 1828,O isotope high,?18OV-SMOW‰=8.70~8.90.They exhibit hight radiogenic Pb isotopic compositions,(206Pb/204Pb)t=17.99~18.37,(207Pb/204Pb)t =15.77~16.41,(208Pb/204Pb)t= 38.54~38.92.In?Nd(t)-(87Sr/86Sr)i the projection point are located between the lower crust and depleted mantle,(87Sr/86Sr)i-?18O located in crust and mantle magma mixing,O isotope located between the lower crust and near mantle,Sr-Nd-Pb-O isotope characteristics display magma from lower crust and depleted mantle blendBased on the geologic of the field and the geochemical characteristics of the main and trace elements,Sr-Nd-Pb-O isotope,and zircon U-Pb age of the dacite,it is confirmed that the Nanjing basin dacite is Caledonian volcanic rock.It is at Latest Ordovician-Early Silurian was caused by the ocean subduction melting of the mantle and the Mesoproterozoic facies,and the crust-mantle mixing occurred,and the magma was formed that after the formation of differentiation.The Nanjing dacite shows the characteristics of the island arc,combined with the newly reported ophiolite and island arcs volcanic in the south Guangxi and north Guangdong,which exist Ocean basin and ocean subduction,in the south China of south Guangxi and north Guangdong and south Jiangxi.It also opened a new window for the metallogenic geological background of Xiazhuang uranium orefield.