Characteristics And Geological Significance Of Shigatse Mantle Peridotite In Tibet

The Shigatse Ophiolite outcrops in the middle section of the Yarlung-Zangbo suture zone,Southwest Tibet.The massifs mainly comprise mantle peridotite,gabbro-diabase and basic volcanic rocks.Among these,the mantle peridotite mostly consists of harzburgite with strong plastic deformation.The Yarlung-Zangbo suture zone is the youngest one in the Tibet Plateau,hence study on the Shigatse Ophiolite is of great significance to revealing the palaeocontinental tectonic movement and the evolution and development of ocean lithosphere.Based on the field investigations,representative rocks were sampled for the further indoor researches.Mineral identification and rock naming were conducted on all the samples under the microscope,and then the most typical samples were picked out to carry on detailed study about lithology,geochemistry and mineralogy.The mainly mineral composition of harzburgite in this study includes olivine,orthopyroxene,spinel and ophiolite,and bits of chnopyroxene also appear in some samples.Electronic probe analyses on spinels suggests that they can be distinctly divided into two groups with Cr#mean values of 0.75 and 0.47,respectively.Therefore,One of them is classified as the chromium-rich spinels while the other one as aluminum-rich spinels.The depleted major elements composition together with the REE content(0.31×10-6?1.25×10-6)below the original mantle indicate that the mantle peridotite in this study comes from the depleted residual mantle rocks which suffered partial melting and melt extraction.All of the REE patterns present MREE-depleted "U" type or "V" type with the primary mantle-normalized values(La/Sm)N ranging from 1.29 to 2.63,suggesting that the mantle peridotite experienced partial melting.Mineralogy and geochemistry studies for the Shigatse ophiolite indicate that the mantle peridotite in study area formed in two kinds of tectonic settings,the mid-ocean ridge(MOR)and fore-arc(ARC)settings.