Zicon U-Pb Geochronology, Geochemistry And Implication For Tectonic Setting Of Jiasha Complex In Gejiu District

The Gejiu tin-copper polymetallic ore field,located at the westernmost end of the Cathaysia Block in South China,is one of the largest tin polymetallic ore field in the world.It is associated with magmatic-hydrothermal ore-forming processes triggered by deeply seated structures and concealed granites.The genesis and tectonic setting of Jiasha complex is still unclear and full of argument.In this paper,new LA-ICP-MS U-Pb zircon dating results,the whole rock geochemical and Sr-Nd-Pb isotope characteristic were used to discuss the age,genesis and tectonic setting of Jiasha complex.Systematic geochemical results indicate that:?1?The complex rocks in the Jiasha field include gabbro,monzonite,quartz monzonite,mafic microgranular enclave?MME?-bearing granites,with the characteristic of rich alkali.The different rocks in the primitive mantle-normalized trace elements and chondrite-normailized REE variation diagrams,show the similarity.They all rich in large ion lithophile elements such as K,La;radioactive elements?Th,U?;and other elements such as high field strength elements?Nb,Ta,Ti?showed clear negative anomaly.?2?Systematic new LA-ICP-MS U-Pb zircon dating results indicate that all the intrusive rocks in the Gejiu district are coeval,ranging from 78 Ma to 85 Ma.The gabbro age is?83.2±0.4 Ma?;the monzonite age is 79.5±1.0 Ma—81.1±1.1 Ma);Quartz monzonite age is?77.21±0.89 Ma⁷8.04±0.91 Ma?.From gabbro to granite,the age of magma becomes more and more young.?3?Through the comparasion of Jia Sha monzonite and Quartz monzonite to Ching granitoid,JiaSha complex rocks are commonly enrich in ore-forming elements such as W,Sn,Cu,Pb,Mo;mineralizer elements;such as;Sb,B,F,Ba,As,Be,Bi.?4?Based on the analysis of Sr,Nd,Pb isotopes,combining with the age of Nd isotopic pattern age(T_DM2),showing that:in the process of the upwelling of mantle-derived magma,magma mingling and mixing between the crustal-and mantle-derived exist,followed by crystal fractionation.?5?the JiaSha complex was formed in late cretaceous,under the setting of subduction of the Pacific plate to Eurasia plate under active continental margin just like the formation of andes.