Petrology And Geochemistry Characteristics And Tectonic Setting Of The Gongzhucuo Peridotite In The Western Yarlung Zangbo Ophiolitic Belt

Yarlung Zangbo ophiolitic belt represents the remnants of the Neo Tethys ocean, it is very hot and controversial ophiolite belt. The Gongzhucuo ophiolite, located to the south of Gongzhu lake in SW Tibet, occurs in the southern sub-belt of the western Yarlung Zangbo ophiolitic belt. It is is mainly composed of mantle peridotites, which are intruded by mafic dikes. The peridotites are composed by harzburgites(about 80 percent) and lherzolites(about 20 percent). Harzburgite contains about 75%-80% olivine(Fo=89.3-91.4), 10%-20% prthopyroxene(En=85.2-90.6), 1%-4% clinopyroxene(En=44.1-49.6), and 1%-2% spinel(Cr#=15.9-28.3); lherzolitic contains about 70%-80% olivine(Fo=89.3-91.2), 10%-20% prthopyroxene(En=84.6-89.6), 5%-7% clinopyroxene(En=43.8-51.2), and 1%-2% spinel(Cr#=12.7-20.4). The Gongzhucuo harzburgite have Mg O contents of 42.22 to 44.38 wt.%, Al2O3 contens of 1.49 to 1.82 wt.%, Ca O contens of 1.09 to 1.97 wt.%, Ti O2 contens of 0.02 to 0.04 wt.%. The Gongzhucuo lherzolites have Mg O contents of 40.84 to 42.20 wt.%, Al2O3 contens of 1.89 to 2.46 wt.%, Ca O contens of 2.17 to 2.46 wt.%,Ti O2 contens of 0.03 to 0.05 wt.%. Compared to the primitive mantle, the Gongzhucuo peridotites have relatively higher Mg O and relatively lower Ca O, Al2O3, and Ti O2 contents. The total rare-earth element contents of Gongzhucuo harzburgite and lherzolites are 0.66-1.10 ppm and 0.90-3.78 ppm respectively. The REE contents of both the harzburgites and lherzolites are lower than those of the primitive and depleted mantles. The chondrite-normalized REE patterns of the Gongzhucuo mantle peridotites display a slight enrichment in LREE. The primitive mantle-normalized spider diagrams of the Gongzhucuo peridotites exhibit intensively positive U anomalies, slightly positive Nd anomalies, and intensively negative Zr anomalies. The PGE contents of Gongzhucuo harzburgites and lherzolites are 15.26-25.23 ppb and 18.81-26.86 ppb respectively. The chondritenormalized PGE patterns of these peridotites are relatively flat pattern with narrow ranges of IPGE. The mineral chemistry and whole rock geochemistry characteristics of the Gongzhucuo peridotites show an affinity to abyssal peridotites, indicating that they may have formed in the middle ocean ridge setting. Through quantitative modeling, we conclude that the Gongzhucuo peridoties formed after at most 16% degree of partial melting of the spinel phase mantle source. The slight enrichments in LREE contents and relatively higher Pd/Ir and Rh/Ir ratios than those of C1 chondrite suggest that they may also experienced the process of melt-rock reaction.Thus, we conclude that the Gongzhucuo mantle peridotites have suffered low degrees of partial melting in the spinel stability field beneath a mid-ocean ridge environment, and were modified by melt-rock reaction in the same tectonic setting.