Petrogenesis of peridotites from the Purang ophiolite in the western part of Yarlung Zangbo suture zone, southern Tibet: A new perspective

Abstract: The ophiolitic peridotites constitute an important part of the ophiolite in that they not only record the experienced plate tectonic events but also offer help to discuss the composition of the mantle and the effect of the deep mantle. The Purang ophiolite located in the western part of Yarlung Zangbo suture zone is characterized by containing a large mantle peridotite massif ca.600km2 in area. In order to discuss the petrogenesis and tectonic significance of the Purang peridotite massif, the authors sampled a section of about 10 km across the massif in the eastern part of the Purang mantle peridotite massif. Based on a detailed field survey and mineralogical and geochemical analysis of systematically selected mantle peridotite samples, the authors have achieved some results: (1) The lithofacies distribution of the Purang ophiolite peridotites has been determined. The profile across the massif shows that harzburgites form the main part of the Purang peridotite massif, the harzburgites are mainly located within the peridotite massif, the banded lherzolites are mainly distributed at the edge of the peridotite massif, and the ratio of the exposed area of harzburgites to that of lherzolites is about 4:1; (2) Based on olivine-spinel associations and the formulae based on the reaction of 6Fe2SiO4+O2= 3Fe2Si2O6+2Fe3O4, the oxygen fugacity values have been calculated (?log(fo2)FMQ range from-1.11 to+0.45), which are significantly higher than the fO2 value obtained using the V content, indicating a reducing tectonic setting; (3) The Purang peridotites are not the simple residues of partial melting and might have experienced later melts metasomatism. According to the whole rock geochemistry and oxygen fugacity values, this paper argues that the later metasomatic melts were probably derived from the lower partial melting magma formed in a mid-ocean ridge setting; (4) The Purang ophiolite peridotites show similar bulk geochemical features and oxygen fugacity characteristics to the modern mid-ocean ridge abyssal peridotites. It is thus held that the Purang ophiolite peridotites formed in a mid-ocean ridge tectonic setting, without imprints of supra-subduction zone setting. The two stage evolution model (MOR→SSZ) of the peridotites from the Yarlung Zangbo suture zone needs to be reexamined.