Geochronology,Geochemical Characteristics And Petrogenesis Of Andesites In Maquan Basin,Gan Hang Uranium Metallogenic Belt

The Gan Hang uranium metallogenic belt starts from Shaoxing,Zhejiang in the east and Yongfeng,Jiangxi in the west.The ore-hosting wall rocks of the volcanic uranium ore basin in this metallogenic belt are basically dominated by acidic volcanic rocks,and the Maquan basin is one of the few uranium-bearing basins with neutral volcanic rocks as the ore-hosting wall rock.There are certain disputes on the rock type and rock series attribution,and there is a lack of systematic research on basic geological issues such as chronology,geochemical characteristics and petrogenesis.Therefore,this paper takes the neutral volcanic rocks in the Maquan basin as the research object,and studies its petrology,chronology and geochemical characteristics,and draws the following conclusions:The neutral volcanic rocks in the Maquan basin have a massive structure and a porphyritic structure.The phenocrysts are mainly plagioclase and contain a small amount of pyroxene,amphibole and biotite.The content of Si O₂ in the whole rock varies from 56.83%to 58.52%,with an average value of 57.81%;the content of total alkali（Na₂O+K₂O）is 6.89%to 7.35%,with an average value of 7.10%,showing the characteristics of rich alkali;The variation range of Na₂O is between 1.07 and 1.28,the average value is 1.16,showing the characteristics of potassium richness;the content of Fe is relatively low,the content of TFe is 5.96%to 7.63%,and the average value is 6.76%;the content of Ti O₂ is 0.986%to 1.03%,the average value is 1.01%,its content is less than 1.3%,and it has the characteristics of poor titanium;on the TAS and Nb/Y-Zr/Ti O₂ diagrams,the sample points in the study area all fall into the range of alkalescence series,and in Si O₂-K₂O/Na₂O and FAM diagrams,the sample points in the study area all fall into the range of the calc-alkaline series.Comprehensive analysis shows that the neutral volcanic rocks in the Maquan basin should be andesites,which belong to the high potassium calc-alkaline series.The zircon of the andesites in the Maquan basin has a clear oscillating ring structure,and the Th/U ratio（average value is 0.79）is a typical magmatic zircon.The SHRIMP zircon U-Pb age is 137.1±1.1Ma（N=15,MSWD=1.5）,which is the product of magmatic activity in the early Early Cretaceous.The total amount of rare earth elements in andesite is relatively high,ranging from230.88×10^-6 to 285.26×10^-6,with an average value of 247.89×10^-6;the fractionation degree of light and heavy rare earth elements is significant,and the LREE/HREE value is 8.85 to22.18（The average value is 15.52）,the enrichment of light rare earth elements is obvious.The large ion lithophile elements Rb,Ba,K,etc.show obvious enrichment characteristics,and the high-field strength elements Nb,Ta,Ti,etc.show relative depletion.(⁸⁷Sr/⁸⁶Sr)_i=0.70855～0.70882,εNd（t）=-9.77～-7.02,(²⁰⁶Pb/²⁰⁴Pb)_i=18.17～18.27,(²⁰⁷Pb/²⁰⁴Pb)_i=15.56～15.62,(²⁰⁸Pb/²⁰⁴Pb)_i=38.43～38.61;on(⁸⁷Sr/⁸⁶Sr)_i-(²⁰⁶Pb/²⁰⁴Pb)_i and(¹⁴³Nd/¹⁴⁴Nd)_i-(²⁰⁶Pb/²⁰⁴Pb)_i and(¹⁴³Nd/¹⁴⁴Nd)_i-(⁸⁷Sr/⁸⁶Sr)_i diagrams,both indicate horses The andesite source area in the Quan Basin has the characteristics of EM II enriched mantle.On the tectonic environment discrimination diagrams,it is indicated that the andesites in the Maquan basin were formed in an intraplate extensional environment.Comprehensive studies show that the andesites in the Maquan basin were partially melted by the EM II enriched mantle formed by the metasomatism of mantle fluids in the Early Cretaceous extensional tectonic background product from the surface.