The Petrological, Geochemical Characteristics And Tectonic Significance Of Gabbro Rock In Changsheshan District, Qiangtang, Tibet

Gabbro rock in Changsheshan district mainly distribute in Longmucuo, themiddle of Shuanghu suture zone, which is located in Mayer Kangri of Qiangtang,thenorth of Tibet. The Basic intrusive rocks in this zone exposed comparatively large,Accounted for more than sixty percent of the entire exposed area of magmatite,occurred more in miniaturized veins,which can be Divided into nearlyeast-westward and nearly south-northward, the lithology is mainly gabbro and moreoccurred alteration,the wall rock in this zone mainly consist of phyllite,Phyllitic sandstone and metamorphic quartz sandstone. the basic intrusive rocks andwall rocks are contacted by ductile fault in this zone. The rock structure,occurrence environment and geochemical characteristics of east-westward intrusiverocks and south-northward intrusive rocks in this zone have great difference, I havemade a detail analysis to the gabbro rock in this zone by petrology, mineralogy,geochemistry and chronology, The results show that:Total alkali （Na2O+K2O）of gabbro rock contents1.80%-4.16%,the average is2.71.the contents of P2O5are high up to0.11%-0.21%,the Ratio of MgO/(FeO+MgO) lies between0.44-0.64,is lower compared to original mantle,which liesbetween0.68-0.72.it proves that its primary magma has some difference withoriginal mantle’s, then we can infer that it had been melted with other materialduring the process of magmatic evolution.In the research zone, the mean ratio of∑LREE/∑HREE in Gabbro rock liesbetween4.16-5.85μg/g,the average is4.99, light and heavy REE show obviousfractionation and LREE enrichment. The Gabbro rock in research zone is closer tocontinental crust composition (enriched in Pb, depleted in Nb、Ta、P). then we caninfer that the gabbro in research zone formed in continental crust.According to the age of Zircon U-Pb (312±4Ma)and whole-rockSm-Nd(299±13Ma、314±5Ma),We can ensure the age of the basic dyke swarms inQiangnan formed in Late Carboniferous(Li Cai.ect,2006). According to the geochemical data of gabbro rock and the projection map inChangsheshan, we can make sure that the basic intrusive rocks there formed incontinental rift sitting, and is typical continental rift sitting, whose magma sourcecome from enriched mantle. In accordance with the research of tectonic evolutionhistory in this district by predecessors, we can infer that the basic dyke swarms inChangsheshan formed in the infancy of Wilson cycle, which formed in the earlycontinental crust of Tethys Ocean. Then we can ascertain the early environment whenthe optional period of Tethys Ocean, It has provided many references to the futurework.