The Research Of Geochemical And Formation Background Of Intermediate-acid Intrusive Rocks In Renacuo Area,Gaize,Tibet

Tibet is always the hot point of geologic research of the world and the research area of this paper is located between Bangong Co-Nu Jiang suture zone and Longmu Co-Shuanghu suture zone.Bangong Co-Nu Jiang suture zone is the boundary of Lhasa block and Qiangtang block so it is the key to the research of Tethys and receives much concern.It is hold by some researchers that the oceanic plate of Bannu Ocean subducted from south to north while the reserve direction of subduction is approved by a group of researchers and two directions of subduction is also supported by some others.A set of intrusive rocks consisting of mica-quartz-diorite,quartz monzodiorite,quartz diorite and granodiorite can be found in the research area.Based on the characteristics of major elements,it is a set of medium acidic intrusion and is high-potassium calc-alkaline quasi-aluminous I type granite.A good linear correlation is presented by Harker diagram.The facts of slight Eu negative abnormity,enrichment of LREE and loss of HREE are showed by the right-inclined distribution lines of rare earth elements.The primitive mantle normalized diagram of rare earth elements is characterized by right-inclined lines.Highly incompatible elements are relatively enriched and high field-strength elements such as Ta,Nb,Ti are strongly negative anomaly.The results of SHRIMP tests of zircons are: quartz monzodiorite 139±4Ma,MSWD = 1.6; quartz diorite 147.3±3.3Ma,MSWD = 0.45; granodiorite 134±4Ma,MSWD=5.6.It can be concluded that the set of intrusive rocks formed in late Jurassic epoch-early Cretaceous epoch.Based on the location of the research area,the results of geochemical analysises and age tests,the conclusion can be drawn that the intrusive rocks in northern Gaize are volcanic arc granite forming during the process of subdution.They are characterized by high-potassium calc-alkaline quasi-aluminous I type granite,which are the production of comgmatic evolution and may root from the same magma chamber,whose material souce are mante and crust and is also affected by the materials from subdution plate.Migmatization happened during the process of magma evolution as well.Taken the formation times of strata and structural style,it can be concluded that the subdution of Bannu Ocean began in late Jurassic epoch and the direction was from south to north.