Petrogenesis Of Peridotites From The Purang Ophiolite In Western Part Of Yarlung-Zangbo Suture Zone, Southern Tibet

The ophiolitic peridotites are important part of the ophiolite, 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 Yarlung-Zangbo suture zone extends more than 2000 km along southern Tibet, which is the biggest suture zone in china. The western segment includes the Dajiweng-Saga ophiolite sub-belt in the north and the Daba-Xiugugabu ophiolite sub-belt in the south. There are manly large scale and planar mantle peridotite massifs in the southern sub-belt, however, the mantle peridotite massifs in the northern sub-belt and the other segments of the Yarlung-Zangbo suture zone appear in narrow sheet. In order to discuss the petrogenesis and tectonic significance of the peridotite massif, this dissertation choose the Purang ophiolite containing a large mantle peridotite massif ca.600km2 in area in the southern sub-belt to carry out petrological study.In this dissertation, a profile about 10 km which cross the massif is sampled in the eastern part of the Purang mantle peridotite massif. Basing on detailed field survey and sellecting systematic mantle peridotite samples, then carried out systematic mineralogy and geochemical composition study on mantle peridotites, various proceedings and recognitions have been achieved:(1)The lithofacies distribution characteristics of the Purang ophiolite peridotites have been found out. The profile which cross the massif shows that the harzburgites are the main part of the Purang peridotite massif, the harzburgites mainly distribute within the peridotite massif, the banded lherzolites mainly distribute in the slide of the peridotite massif, and the ratio of the exposure area of harzburgites and lherzolites is about 4:1.(2)The partial melting degree of the Purang ophiolite peridotites has been analyzed and estimated. On the basis of the mineral chemistry (eg.Cr#-TiO2 in spinel, Cr# in spinel-Ol in olivine diagram) and geochemistry (eg.MgO-Al2O3, V-Yb diagram and HREE contents) characteristics, we calculate that the lherzolites are the residues of 9-15% partial melting of the mantle source, whereas the harzburgites are formed by 15-25% partial melting.(3)The oxygen fugacity of the Purang ophiolite peridotites has been calculated qualitatively and quantificationally. On the V-Yb oxygen fugacity diagram of the mantle peridotites, all of the peridotite samples plot between the FMQ and FMQ-1 trends. Basing on olivine-spinel associations and the formulas that are based on the reaction of 6Fe2SiO4+O2=3Fe2Si2O6+2Fe3O4, the oxygen fugacity values have been calculated (Alog(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.(4)The Purang peridotites are not the simple residues of partial melting and maybe experienced later melts metasomatism. The mineral chemistry of the Purang peridotites shows refactory signature.The Fo of olivine range from 89.5 to 92.8, the Mg s of orthopyroxene range from 87.6 to 92.0, the Mg#s of clinopyroxene range from 89.5 to 94.5, the Cr#s of spinel range from 18.5 to 71.7. The mineralogical structure and geochemistry characteristics of the Purang peridotites indicate that they maybe experienced melts metasomatism. Furtheremore, the mineralogical and textural characteristics indicative of melt-rock interactions have been identified within our samples, such as:1) high-temperature recrystallization of olivine neoblasts; 2) olivine embayments partly corroding the orthopyroxene porphyroclast; 3) orthoproxene and spinel symplectite; and 4) olivine as inclusions in the spinels. The whole rock REE contents of the peridotites show that a part peridotites exhibit LREE enrichment to MREE, demonstrating that the Purang peridotites maybe experienced later melts metasomatism. According to the whole rock geochemistry and oxygen fugacity values, this dissertation argues that the latter metasomatic melts maybe come from the lower partial melting magma which formed in mid-ocean ridge setting.(5)The tectonic setting of the Purang ophiolite peridotites has been restricted. The Purang ophiolite peridotites show similar bulk geochemistry and oxygen fugacity characteristics with the modern mid-ocean ridge abyssal peridotites. Therefore, this dissertation argues that the Purang ophiolite peridotites are formed in the mid-ocean ridge tectonic setting, without imprints of supra-subduction zone setting. The two stage evolution modal (MORâ†'SSZ) of the peridotites from the Yarlung-Zangbo suture zone needs to be reexamined.