Geochronology And Geochemistry Of Magmatic Rock In Xiongma At Shigatse,Tibet

Xiongma magmatic rock located in the south of the central part of Yarlung Zangbo Suture Zone(YZSZ),intruded Triassic flysch located in a village called Xiongma,Shigatse.The magmatic rocks are mainly composed of fine grained diabase,coarse grained diabase,Quartz diorite.The magmatic rocks all show the characteristics of high Ti O2(1.64%-3.64%),Al2O3(11.29%-14.33%),and low MgO(1.73%-7.93%).Besides,the magmatic rocks enrich large ion lithophile elements(LILE)(Rb,Ba,Th,U,Pb),the Pb has remarkable positive anomalies,and the enrichment of Nb and Ta shows that the rocks have the characteristics of the basalt rocks in the continental rift valley.The standardization diagram of the rare earth elements can be characterized by the right tilting model and the rich set of light rare earth elements.The heavy rare earth elements are strongly fractionated,LREE/HREE =5.29-7.87,(La/Yb)N=6.39-10.49,the average is 8.3,(La/Sm)N=2.17-3.26,and the average is 2.7.There are Eu negative anomalies,?Eu=0.69-0.94 with an average of 0.80,indicating the possibility of a separation of plagioclase.U-Pb dating of zircons by LA-ICP-MS show that the age of the fine grained diabase,coarse g grained diabase and quartz diorite are 136.4±2.2 Ma(MSWD=0.40),133.8±2.7 Ma(MSWD=1.4),138.8±1.5 Ma(MSWD=1.07),which indicates the dolerite fomed in lower Cretaceous.The mineralogy,petrology and geochemical characteristics of Xiongma magmatic rock show that the male magmatic rocks mantle source area may be garnet and spinel,indicating the enrichment mantle rock may be composed of deep(including the garnet phase)<10% partial melting of the form.Three kinds of lithology Mg# were significantly lower than native magma(Mg#=68-68),indicating the magma evolution in the early stages of the magnesium olivine and monoclinic pyroxene crystallization separation rate of iron ore,rare earth trace element characteristics suggest that male magmatic rocks may also suffer from the earth's crust material of hybrid,fine grained diabase,coarse grain of gabbro,quartz diorite may represent the same source area different degrees of partial melting.Xiongma magmatic rocks,the main elements of trace element geochemical study shows that with the geochemical characteristics of intracontinental rift basalt,preliminary thought formed in the intracontinental rift environment.Xiongma magmatic rock represent may be earlier than the yarlung zangbo river ophiolite belt is formed,and a little earlier than measures of large igneous province mantle plume magmatism extensional activities within a small plate.