A Study Of M(?)ssbauer Spectrometry Of Podiform Chromitites And Mantle Peridotites In Middle And Eastern Sections Of Yarlung Zangbo Ophiolites

The tectonic setting and formation mechanism of podiform chromitites and mantle peridotites along Yarlung Zangbo Ophiolites(YZO)are still highly debated.The prospect for podiform chromitites in Xigaze ophiolites along middle section of YZO remains unclear.Oxygen fugacity(f _O2)is one of the most important physicochemical parameters and may be an essential controlling factor of formation of podiform chromitites.Meanwhile,the oxidation states of chromitites and mantle peridotites are closely related to tectonic settings and thus may indicate their tectonic environments during their formation.To solve the problems above,this study compared similarities and differences between Dazhuqu ophiolite,a representative of Xigaze ophiolites,and Luobusha podiform chromite ore deposit in geology,major elements of both whole rocks and minerals,M(?)ssbauer Spectrometry of chromites and their f _O2.In both Dazhuqu and Luobusha,the major elements of lherzolites and harzburgites are consistent with the trend of decompression partial melting,while the degrees of partial melting were estimated to be 9?18%.The major elements of olivines and chromites are similar to those of abyssal peridotites.These features suggest that they are residues after decompression partial melting.By contrast,the compositions of their pyroxenes,especially clinopyroxenes,show a similarity with those of forearc peridotites.Besides,they are eroded by olivines,and their Al₂O₃ and Cr₂O₃ contents has a positive correlation,which deviate from the partial melting trend.These phenomena all indicate both lherzolites and harzburgites have been influenced by melt-rock reaction to different extent.The lherzolites and harzburgites display similar values(-1.02?0.13 log units against the fayalite-magnetite-quartz buffer)to the oxidized end of abyssal peridotites but associate with some dunites with high f _O2(-0.42?2.19 log units).This peculiar associate is similar to forearc peridotites,so the lherzolite/harzburgite-dunite-chromitite associate in both Dazhuqu and Luobusha form in forearc setting.Although involved into a subduction zone,both lherzolites and harzburgites are obviously reduced than sub-arc peridotites,inferring that they are slightly affected by fluids originating from the subducting slab.The abyssal peridotites may be oxidized by the fluids derived from the subducting slab to the high f _O2 level similar to sub-arc peridotites.Hence,dunites and chromitites with high f _O2 values likely form during subduction initiation.The high f _O2 values(range from 0.04 to 2.25 log units,with a mean value of 1.39 log units)of chromitites indicate that their parental magma is relatively oxidized.However,magmas with high f _O2 values cannot form through partial melting of relatively reduced lherzolites or harzburgites.A reasonable explanation is that the parental magma for chromitites may assimilate oxidized components or mix with an oxidized magma originated from the subducting slab before crystallization of chromites.Although there are similar host harzburgites to Luobusha ore deposit,some dunites recording the activity of boninitic melts and some chromite mineralizing spots in Dazhuqu ophiolite,the dunites and chromitites are different from chromitites and dunite envelopes from Luobusha in their major elements and f _O2 values.Thus,based on the available information,Dazhuqu ophiolite shows no bright prospect for podiform chromitites.