Mineral Geochemical And Oxygen Isotopic Characteristics Of The Darbute Mantle Peridotite In The West Junggar,Xinjiang

The Darbute ophiolites,located in the southwestern CAOB,are the products of the spreading,subduction,closure of the ancient Asian ocean basin in the CAOB and record the history of formation and tectonic evolution of the ocean basin.Scholars have not obtained a unified understanding of the tectonic background of the formation of Darbute ophiolites.To explore the tectonic evolution process represented by the ophiolite,this author presents a new sight on the genesis and tectonic setting of the Darbute mantle peridotite through detailed rock and mineral geochemistry and its oxygen isotope study using the mantle peridotite in the Darbute ophiolite.The loss of incompatible elements in the whole rock and minerals of the mantle peridotite reveal the partial melting and extraction melting during mantle evolution.From lherzolite,harzburgite to dunite with gradually increasing partial melting,the major element characteristics of the mantle peridotite are consistent with the trend of partial melt evolution.The HREE content for the harzburgite coincides with the trend line of melt evolution at 20%～30%partial melting level,which is consistent with the20%partial melting feature in the V-Yb diagram.In addition,with the characteristics of partial melting residues,olivine is enriched in compatible elements and depleted in incompatible elements with higher Ni/Co and Ni/Mn ratios than those of the original mantle.The spinel has Cr^#values of 47-52 and TiO₂content of 0.01%-0.04%,and the olivine has Fo values of 90.34%～90.98%in the partial melting diagram,which indicates that the harzburgite experienced>20%partial melting.Theδ¹⁸O_olivinevalues+5.1～+6.2‰for olivine andδ¹⁸O_cpxvalues+5.6～+6.9‰for clinopyroxene in the harzburgite vary extensively and are generally higher than theδ¹⁸O values for the olivine and the clinopyroxene in the normal mantle.The oxygen isotope fractionation coefficients between mineralsΔ¹⁸O_Opx-olivineaverage-0.3‰andΔ¹⁸O_Opx-cpxaverage-0.7‰and differs significantly from the oxygen isotope fractionation coefficients in the normal mantle equilibrium,with obvious metasomatism characteristics.It is believed that peridotite may have undergone the metasomatism of highδ¹⁸O fluids that are formed by the dehydration of the crustal sourced material during the subduction,combined with the characteristics of LREE enrichment and high oxygen fugacity for peridotite and LREE depletion for olivine.The major and trace elements characteristics of the whole-rock and minerals in the Darbute harzburgite have transitional-type characteristics between the fore-arc and abyssal mantle,which is similar to the mantle peridotite that is formed at the initial stage of fore-arc subduction.The differences of partial melting degree of different lithologies in mantle peridotites and the transitional type of both fore-arc and abyssal environments prove that mantle peridotites have experienced partial melting and metasomatism and enrichment in different evolutionary stages under the environment of composite features.The author believes that the Darbute mantle peridotite was formed in the initial subduction environment of the fore-arc,combined with the oxygen isotope anomaly in the minerals and the oxygen isotope imbalance between the minerals.Plate subduction leads to fore-arc expansion,thus forming new oceanic crust.The upwelling asthenosphere MORB-like melting and the melt/fluid formed by the melting of subducted crust source material interact with mantle peridotite,thus forming Darbute mantle peridotite.