The Geochemical Characteristics And Geological Significance Of Ultrabasic Rocks In Doma,Tibet

The Doma area in northern Tibet is located in the middle part of the Bangong co-Nujiang suture zone.Two types of ultrabasic rocks are found in the area through field regional geological survey.As an important unit of ophiolite,ultrabasic rocks can be used as"carriers"of information of upper mantle source,inversion of tectonic environment and evolution process of upper mantle magma.In this paper,the ultrabasic rocks in Doma area are studied in petrology,mineralogy and geochemistry.The origin and tectonic background of ultrabasic rocks and their significance to mantle evolution are discussed,which will provide a new constraint for the tectonic evolution of Bangong co-Nujiang Ocean.Ultra-basic rocks in Doma area can be divided into serpentinites and serpentinized clinopyroxene peridotites.The two types of samples show different degrees of serpentinization.Serpentinization of serpentine samples is very high,with the average loss of ignition?LOI?as high as 12.13%,the average content of MgO as 44.83%,and Mg^#=0.90⁰.92,which is higher than that of the original mantle?Mg^#=0.89?.The content of fusible components?TiO₂,Al₂O₃,TFeO,CaO,etc.?is negatively correlated with the content of MgO.The average amount of rare earth elements is 0.54×10^-6,which is much lower than that of chondrite.The above characteristics indicate that serpentine is the remnant of upper mantle melting with strong depletion and high melting degree.The partial melting degree of serpentinites are estimated to be as high as 25-30%by Yb-Ti diagram and rare earth partition model diagram.Through MgO correlation diagrams,Yb-Ti diagrams and chromium spinel composition correlation diagrams,the original rocks are recovered to be mantle wedge peridotite with pre-arc properties,which was formed in SSZ tectonic environment.serpentinized clinopyroxene peridotites has a relatively low serpentinization degree.Olivine and clinopyroxene can also be observed under the microscope.Olivine is of low Mg-rich Fe type with Fo=81-85.The distribution pattern of rare earth elements is similar to that of OIB.Light rare earth elements are relatively enriched.The average value of LREE/HREE is 4.55,and the average value of total amount is 39.97×10^-6,which is close to E-MORB.The chromium spinel was formed in an intraplate oceanic island environment according to its composition diagrams.In addition,some high field strength elements?Rb,Pb,U,Th,etc.?and large ion lithophile elements are relatively enriched,which indicate that serpentinized clinopyroxene peridotites samples may not be remnants of mantle peridotite,but products of mantle metasomatism.Based on the previous data,it is found that serpentinites may be formed in the intra-oceanic subduction background of 190¹60 Ma and serpentinized clinopyroxene peridotites may be formed mantle metasomatism after 160 Ma.Two different ultrabasic rocks in the Doma area are the products of intraoceanic subduction and mantle metasomatism during the mature oceanic period,which provides a new perspective for the evolution of the Bangong co-Nujiang Ocean.