The Research Of The Charateristcs Of Lito-geochemistry And The Formation Background Of Gabbro In Jiacha And Lang

From Jiacha County to Lang County, there is a ultrabasic-basic complex belt that sort of runs east-west along the the southern side of Yarlung Zangbo suture zone occurs. Existing in parallel with Yarlung Zangbo ophiolitic melange zone, it mainly contains gabbro and diabase and largely alteration can be found there. Outcrops occruing around from Jiacha to Lang was observed detailedly and typical samples of the gabbro were collected. Gabbro of the whole researching area has almost the same chemical feature.It is presented by the systematical petrology and geochemical analysises that the contents of SiO2 of the gabbro are all between 45.80%-50.89%,which are similar to those of the normal mid-ocean ridge basalts(48.77%).The contents of MgO are 4.23%-7.80% and the average level is 5.74%,while those Mg# are 41.31-63.55 with an average level of 48.85 lower than the range of protomagma so that the hybridism may happen during the porecess of magma evolution.The fact that Eu negative anomaly, no Eu anomaly and inexistence of obvious fractional crystallization can be proved by the ranges of LREE/HREE,2.08-8.09%and an average level of 3.4 and of σEu,0.72-1.15 and an average level of 0.98.It can be seen from the primitive mantle normalized spider diagrams,the locations and trends of the lines of the samples are similar to those of E-MORB.It is suggested that the magma may root from enriched mantle or nomal depleted mantle and be affected by other solutions during the processes of formation and evolution,which leads to the enrichment of light rare earth.Based on the SHRIMP results of the ages of zircons,the gabbro may form 55.5-56Ma,which is in accordance with the suture time of Yajiang Ocean. As a result, it can be concluded that at the end of subduction because of the gravity and other factors the oceanic plate broke from the southern Himalaya block and partly fused in asthenosphere forming ultrabasic-basic magma.With the continuously effect of subduction and the suture of Yajiang Ocean,the magma emplacements rised and mixed with the crust to different extends.The baisc dyke formed in the background.