The Characteristics And Significance Of Penghu Peridotites In The Middle Segment Of Bangong Co-Nujiang Suture In Tibet

The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet,which belongs to the Baila-Yilashan sub-belt of the the middle Bangong Co-Nujiang ophiolitic belt.The Peng Co ophiolite is mainly composed of mantle peridotites,cumulate,gabbro and diabase.About 70 percent peridotites are harzburgites and 30 percent are lherzolites.Mineral chemistry of the Penghu peridotites is characterized by low Fo contents?88.85⁹0.33?of olivine and high Al₂O₃ content?4.26%⁷.25%?in pyroxenes.Compared to the primitive mantle,the Penghu peridotites have relatively higher MgO contents,lower CaO,Al₂O₃ and TiO₂contents.The total rare-earth element?REE?contents of the lherzolites are1.11x10^-61.53x10^-6,which are lower than the primitive mantle.The chondrite-normalized REE patterns of the Peng Co peridotites display slight loss in LREE.In the primitive mantle-normalized spider diagram,the Peng Co peridotites exhibit negative Rb and Zr anomalies and intensively positive U,Ta,Sr anomalies.The PGE contents of Peng Co lherzolites are between 22.9x10^-927x10^-9.The chondrite-normalized PGE patterns of the Peng Co lherzolites are consistent with that of the primitive mantle.Mineral and whole-rock geochemistry characteristics of the Peng Co lherzolites show an affinity to abyssal peridotites,indicating that it may have formed in the mid-ocean ridge setting.Through quantitative modeling,we conclude that the Peng Co lherzolites formed after 5%¹0%degree of partial melting of the spinel-phase lherzolite mantle source.The sharp increase of Cr^#?56.74⁶0.84?in Spinel and relatively high Pd/Ir and Rh/Ir ratios suggest that they have experienced melt-rock reaction.The crystallization sequence of Peng Co cumulate is olivine-clinopyroxene-plagioclase.The Mg^#value of clinopyroxene in cumulate peridotite ranges from 86.92 to 89.93,and the mean value of Fo is 84.45,which is obviously higher than MOR-type ophiolite cumulates.The mineral composition,sequence of magmatic crystallization and mineral components of Peng Co cumulate are similar to those of the cumulate formed by the SSZ-type ophiolite in the subduction zone.Besides,the tectonic discrimination diagram also indicates the subduction environment.Therefore,we can draw a preliminary conclusion that Peng Co lherzolites were formed in an environment of mid oceanic ridge and were remnants of the spinel lherzolite zone which experienced a partial melting of no more than 10%.In the later period,due to the intra-oceanic subduction,it experienced the rock-melt interaction,and thus formed the SSZ-type cumulate and harzburgite of high Cr value.