Petrological and Os Isotopic Characteristics of Zedong Peridotites in the Eastern Yarlung-Zangbo Suture in Tibet

The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km(2). This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine (Fo(89.7-91.2)), orthopyroxene (En(88-92)), clinopyroxene (En(45-49)Wo(47-51)Fs(2-4)) and spinel [Mg-#=100xMg/(Mg+Fe)]=49.1-70.7; Cr-#=(100xCr/(Cr+Al)=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content (0.08-0.62 ppm). Their "spoon-shaped" primitive-mantle normalized REE patterns with (La/Sm)(N) being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites (18.19-50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with Os-187/Os-188=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge (MOR) and were later modified in supra-subduction zone (SSZ) environment.