Mineral Chemical Characteristics And Geological Significance Of Clinopyroxene In Zedang Ophiolite,Tibet

Zedang ophiolite is located in the eastern section of the Yarlung Zangbo suture zone,and between the Himalayan passive continental margin and Gangdise magma arc.Previous studies on Zedang ophiolites have found that Zedang ophiolites experienced MORB and SSZ tectonic environments,while ophiolites experienced SSZ environments were generally modified by subduction zone melts.However,there are few studies on the properties of melts.The existing understanding of Zedang ophiolite has experienced intra-ocean subduction,and it is not clear whether the subduction zone melt is carbonate melt or silicate melt.This paper discusses the source of metasomatism melt from the perspective of clinopyroxene.Through the study of mineralogy and geochemistry of clinopyroxene with different occurrences in Zedang ophiolite,the tectonic environment and geological significance of Zedang ophiolite are inverted.（1）The monoclinic pyroxene in Zedang ophiolite in the study area has various forms of occurrence.Clinopyroxene in lherzolite is mainly granular,the harzburgite are granular,exsolution stripes and residual,and the dunite is mainly residual,and the chromite is mainly in the form of inclusion.The composition of clinopyroxene obtained by electron probe analyzer shows that the characteristics of high Ca O,Mg O,Al₂O₃,low Cr₂O₃,Na₂O and Ni O.The Mg^#[100×Mg/（Mg+Fe）]（68.0～97.4）the Cr₂O₃contents of the clinopyroxene display the tendency of increase from lherzolite,through harzburgite to dunite,and the Al₂O₃content have a decrease tend.The Mg^#value and Cr content of clinopyroxene in lherzolite,harzburgite and dunite increased in turn,reflecting the melting degree of host rocks increased in turn.The geochemical bivariate diagrams show that the lherzolite has plotted in the deep-sea mantle peridotite field,and the dunite in the fore-arc mantle peridotite field,indicate that the Zedang mantle peridotite may have been experienced two-stage tectonic environment of mid-ocean ridge（MORB）and supra-subduction zone（SSZ）.（2）The Ca/Al ratios of clinopyroxene can indicate the type of mantle metasomatism melts,and the Ca/Al ratios bigger than 5 usually indicate a carbonate-type melts.The monoclinic pyroxene modified by carbonate melt shows obvious high Zr/Hf ratio and low Ti/Eu ratio in the geochemical characteristics of trace elements in the in-situ micro-area.The Ca/Al ratio of clinopyroxene in mantle peridotite of Zedang area is 4.80～58.69,that of lherzolite is 4.80～28.42,that of harzburgite is 19.12～44.83,and that of dunite is 25.07～58.69,which reflects that the metasomatism melt may be carbonate melt（Ca/Al>5）.In addition,the trace elements of clinopyroxene in the Zedang ophiolite have high Zr/Hf ratio（>38,some MORB than 200）and low Ti/Eu ratio（<1500）,The clinopyroxene Ca/Al>5,Zr/Hf>38,Ti/Eu<1500 are the characteristics of carbonate melt metasomatism in the Zedang mantle peridotite samples.（3）The petrographic characteristics,mineral assemblage and mineral composition of clinopyroxene in Zedang mantle peridotite show that there have been experienced metasomatism by carbonated melt from SSZ.And the possible formation environment is the island arc environment in the ocean.Combined with the existing understanding,the Zedang ophiolite may have experienced two-stage evolution,from MORB to SSZ.The major/trace composition of the clinopyroxene record that the metasomatism melts is carbonate-type melts origin from the SSZ enrironment.