Geological Characteristics And Tectonic Significance Of Zhuobukabujihai Ophiolite In Shuanghu,Tibet

The Zhuobukabujihai area of Shuanghu in Tibet is located in the middle of Bangong Lake-Nujiang suture zone.The ultramafic components at the bottom of the ophiolite suite are found in this area through field geological survey,which are generally considered to be of great significance for understanding the deep information of the upper mantle.On the basis of field geological survey,through the means of petrology,petrogeochemistry and chronology,and combined with previous research results,this paper studies the magmatic rocks exposed in this area,and then discusses the origin and formation environment of magmatic rocks in this area.The ultrabasic rocks in Zhuobukabujihai area have been serpentinized intensely.According to their differences in color and mineral composition,they were initially classified as peridotite and gabbro by the field study.The microscopic observation and geochemical data show that the latter also belongs to ultrabasic rocks with strong serpentinization.Therefore,the sets of ultrabasic rocks are divided into Serpentinized Peridotite and Metamorphic pyroxene peridotite.The two types of ultrabasic rocks,outcropped in the Zhuobukabujihai area with nearly EW-trending,are produced in wall rock in the form of tectonic slices,and they are in fault contact with the surrounding strata.The standardized distribution maps of rare earth elements in chondrites of two types of ultrabasic rocks in Zhuobukabujihai area have similar distribution curves,showing similar characteristics of "U" type,showing relative enrichment of light rare earth,relative depletion of medium rare earth and relative enrichment of heavy rare earth.The total rare earth elements of the two groups of samples are significantly lower than that of chondrites(3.292 ?g/g),indicating that the area is characterized by relatively enrichment of light rare earth,relatively depletion of medium rare earth and relatively enrichment of heavy rare earth The exposed mantle peridotite is the remnant of upper mantle melting with strong depletion and high melting degree.Based on Yb-Ti diagrams,the partial melting degree of rocks is estimated.Both types of ultrabasic rocks are close to the spinel partial melting curve,which indicates that theultrabasic rocks in Zhuobukabujihai area are the products of the partial melting of spinel facies rocks.The partial melting degree of Serpentinized Peridotite is estimated to be 19%-20%,and that of Metamorphic pyroxene peridotite is 20%-22%.It can be seen from MgO correlation diagrams that both types of ultrabasic rocks in Zhuobukabujihai area have the characteristics of pre-arc peridotite,which is a typical SSZ type mantle peridotite.The two types of ultrabasic rocks have undergone serpentinization to varying degrees due to different environments,and are located in the Zhuobukabujihai area after tectonic movement.Zircon U-Pb dating results show that the weighted average age of the rocks is152.9±3.2 Ma,which represents the age of ultrabasic rocks in the area,indicating that they were formed in the middle Late Jurassic.