Mineralogy and PGE features of Zedang peridotites in eastern Yarlung Zangbo suture, Tibet

Abstract: The Zedang mafic-ultramafic body crops out in the eastern Yarlung Zangbo suture zone of Tibet. This massif consists of mantle peridotite, gabbro, diabase and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite and minor harzburgite with chromite. A dike-like bodies of dunite are also present. The Fo values of olivine vary in the range of 89.62-91.84 and all olivines belong to forsterite. The orthopyroxenes are enstatite, and the En values vary from 87.82 to 90.32. Clinopyroxenes are mainly endiopside and diopside, and the En values of clinopyroxene vary in the range of 44.12-50.02. The spinels have Cr# values from 17.00 to 93.59. The aluminum-rich spinels are of harzburgite and lherzolite. The PGE content of Zedang peridotites (16.67×10-9-32.59×10-9) is similar to that of primary mantle. The mineral-chemistry indicates that the lherzolite is abyssal peridotite and the harzburgite is fore-arc peridotite. The Cr# of spinel, Mg# of olivine and high Os（3.50×10-9-7.75×10-9）imply that Zedang peridotites were formed by partial-melting. The positive slope of PGE patterns and high Pd/Ir are consistent with the opinion that the Zedang peridotites were influenced by melts/fluids which were enriched with PPGE. The mineralogy and PGE geochemistry of the Zedang peridotites suggest that they formed originally at a mid-ocean ridge（MOR) and were later modified by suprasubduction zone（SSZ）melts.