The South Gongzhucuo peridotite massif: A typical MOR type peridotite in the western Yarlung Zangbo suture zone

The South Gongzhucuo ophiolite is located to the south of Gongzhu Lake, Ali area, Tibet, belonging to the southern sub belt of the western Yarlung-Zangbo ophiolitic belt. The South Gongzhucuo ophiolite is mainly composed of mantle peridotites and gabbro dikes. About 80 percent peridotites are harzburgites and 20 percent are lherzolites, few of them are dunites. Mineral chemistry of the South Gongzhucuo peridotites are characterized by low Fo contents (89. 3 similar to 91. 4) of olivine; high Al2O3, contents (1. 89% similar to 6. 06%) in pyroxenes; and low Cr-# values (12. 7 similar to 28. 3) of Cr-spinet. Compared to the primitive mantle, the South Gongzhucuo peridotites have relatively higher MgO contents, and lower CaO, Al2O3, and TiO2 contents. The total rare-earth element (REE) contents of the South Gongzhucuo harzburgites and lherzolites are 0. 66 x 10(-6) similar to 1. 10 X 10(-6) and 0. 90 x 10(-6) similar to 3.78 x 10(-6), respectively, which are significantly lower than those of the primitive mantle. The chondrite-normalized REE patterns of the South Gongzhucuo mantle peridotite display slight enrichments in LREE. In the primitive mantle-normalized spider diagrams, the South Gongzhucuo peridotites exhibit intensively positive U anomalies, slightly positive Nd anomalies, and intensively negative Zr anomalies. The PGE contents of South Gongzhucuo harzburgites and lherzolites are 15. 26 x 10(-9) similar to 25. 23 x 10(-9) and 18. 74 x 10(-9) similar to 26. 86 x 10(-9), respectively. The chondrite-normalized PGE patterns of the South Gongzhucuo peridotites are consistent with that of the primitive mantle. Mineral and whole-rock geochemical characteristics of the South Gongzhucuo peridotites 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 South Gongzhucuo peridoties formed after at most 16% degree of partial melting of the spinel-phase lherzolite mantle source. The LREE enrichments and relatively high Pd/Jr and Rh/Ir ratios suggest that they have experienced melt-rock reaction. Thus, we conclude that the South Gongzhucuo mantle peridotites have suffered low degrees of partial melting in the spinel-lherzolite stability field beneath a mid-ocean ridge environment, and were modified by melt-rock reaction in the same tectonic setting.