The characteristics and significance of Zedang peridotite 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 similar to 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 range of 44.1 similar to 50.0. The spinels have Cr# values from 17.0 to 93.6. The aluminum-rich spinels are from harzburgite, lherzolite and the chromium-rich are from dunite. The degree of partial melting of mantle peridotite is 17% similar to 34%, indicating that Zedang mantle peridotite may experience a multi-stage process. The depleted major element compositions and low REE content (0.15 x 10(-6) similar to 0.61 x 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 similar to 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 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.