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

• Published in: 地质学报（英文版） 2018, Vol.92 (2), p.442-461
• Main Authors: LAI Shengmin, YANG Jingsui, Yildirim DILEK, XIONG Fahui, JIANG Rui, CHEN Yanhong
• Format: Article
• Language: English
• ISSN: 1000-9515
• DOI: 10.3969/j.issn.1000-9515.2018.02.002

The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2.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 (Fo89.7-91.2),orthopyroxene (En88-92),clinopyroxene (En45-49Wo47 -51Fs2-4) and spinel [Mg#=100×Mg/(Mg+Fe)]=49.1-70.7;Cr#=(100×Cr/(Cr+Al)=18.8-76.51 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" primitivemantle 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 187Os/188Os=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.