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

• Published in: Acta geologica Sinica (Beijing) 2018-04, Vol.92 (2), p.442-461
• Main Authors: LAI, Shengmin, YANG, Jingsui, DILEK, Yildirim, XIONG, Fahui, JIANG, Rui, CHEN, Yanhong
• Format: Article
• Language: English
• Subjects: Aluminum; Deformation; Dunite; Earth mantle; Gabbros; Iron; Isotopes; Magma; Melting; mid–ocean ridge (MOR); Minerals; Olivine; Ophiolites; Os isotopes; Osmium isotopes; Palladium; Plastic deformation; Plastics; Platinum; Subduction; Subduction zones; supra‐subduction zone (SSZ); Ultramafic rocks; Upper mantle; Volcanic rocks; Yarlung‐Zangbo suture zone; Zedong peridotites
• ISSN: 1000-9515; 1755-6724
• DOI: 10.1111/1755-6724.13536

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)=18.8–76.5] 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 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.