Study On The Geochronology And Geochemistry Of Nanlao Unit Granite In Laojunshan Area,Southeastern Yunnan

Late Cretaceous granites are widely distributed in southeastern Yunnan,including Laojunshan granite,Gejiu granite and Bozhushan granite.There are many W-Sn deposits have been explored in the periphery of the granite,and the deposit is genetically related to the granite.Due to special tectonic location,it is still controversial what tectonic units or factors influence the diagenetic and metallogenic background in southeastern Yunnan.In this paper,the Laojunshan granite in southeastern Yunnan area is selected as the research object.We reported many new datas such as U-Pb geochronology,zircon trace elements,zircon Lu-Hf isotopes and geochemistry of major and trace elements of Nanlao unit granite,which is relatively weak in the western part of Laojunshan granite.And attempted to provide new information for the genesis,tectonic setting and crustal evolution of Late Cretaceous granites in southeastern Yunnan.The major and trace elements data of whole rock shows,the Nanlao unit granite in Laojunshan belongs to peraluminous-strongly peraluminous granite.The rock mass is enriched with light rare earth elements,heavy rare earth elements are depleted,Eu negative anomaly is obvious,Rb,Th,U,K and other large ion lithophile elements are enriched,Ba,Sr and high field strength elements Nb,P,Ti are depleted.The genetic type of Nanlao granite belongs to S-type granite and has undergone a high degree of evolution.It belongs to highly fractionated S-type granite.The rocks show negative?_Hf(t)(-12.88?-1.19)and old Hf(1226?1967Ma,Average is 1532Ma)crustal model age.Zircon Hf isotope and trace element characteristics of rocks indicate that the Nanlao unit granite in Laojunshan has the characteristics of crustal source.After systematic sampling of four granite samples,obtained four Late Cretaceous Ages,85.6±0.8Ma,86.3±0.46Ma,89.9±1.4Ma,90.37±0.77Ma,respectively.These ages can represent the diagenetic age of Nanlao unit granite in Laojunshan area.Based on the diagenetic and metallogenic chronological data of Gejiu granite and Bozhushan granite in southeastern Yunnan and their surrounding polymetallic deposits,showed the intrusive ages of granites and metallogenic ages are concentrate distribution from77 to 95 Ma.Aimed at controversy on tectonic dynamic background in Southeastern Yunnan and after a comprehensive analysis,it is preliminarily considered that the large-scale acid magmatism in Southeastern Yunnan is a response to the collision of the South China Block and the Indosinian Block in Late Triassic.The widely exposed S-type granite represents the tectonic environment is transformation to post collision extension.