The Characteristics And Significance Of Zedang Peridotites In Eastern Yarlung-Zangbo Suture In Tibet

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 a few harzburgite with chromite. A dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The Fo values of olivine vary in the range of 89.6~91.8 and all olivines are forsterite. The orthopyroxenes are enstatite, the En values vary from 87.8 to 90.3.Clinopyroxenes are mainly endiopside and diopside and the En values of clinopyroxene vary in the rang of 44.1~50.0. The spinels have Cr# values from 17.0 to93.6. The aluminum-rich spinels are from harzburgite, lherzolite and the chromium-rich are from dunite. The degree of partial melting of mantle peridotite is17%~34%, indicating that Zedang mantle peridotite may experience a multi-stage process. The depleted major element compositions and low REE content(0.15×10-6~0.61×10-6), which is lower than that of the primary mantle, indicating that the Zedang ultramafic rocks belonged to depleted residual mantle rocks after a significant degree of partial melting and melt extraction. Their primary mantle-normalized REE patterns are all U- or V-shaped with(La/Sm)N being 0.5~8.0(primary mantle-normalized values). These features are consistent with the opinion that the Zedang ultramafic rocks were formed by the interaction of depleted residual mantle rocks with melts/fluids. The peridotites are characterized by high abundances of Os(3.50~7.75ppb),indicating that Zedang ultramafic rocks were formed by the progress of partial-melting.The PGE content of Zedang peridotites(16.67ppb~32.59ppb) is similar with primary mantle. The 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 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.