Oceanization of the subcontinental lithospheric mantle recorded in the Yunzhug ophiolite, Central Tibetan Plateau

Abstract

The Yunzhug ophiolite, in the middle sector of the Shiquanhe-Namco ophiolite belt (SNOB), has generally been interpreted as a remnant of the Neo-Tethyan oceanic lithosphere, composed of mantle peridotites and mafic-ultramafic magmatic rocks. However, detailed study reveals that the mantle parts are ultra-depleted peridotites with affinities to subcontinental lithospheric mantle (SCLM), including major harzburgites and minor dunites. The olivine in dunite has the highest Fo values, up to 93.8, and the harzburgite is divided into three groups. Olivine included in Opx in Group I has Fo 92.4–93.3, close to the dunite. In Group II, olivine in the peridotites has Fo between 91.8 and 92.5. Group III olivine occurs in vein-like clusters with small grains of Cpx and Spl within Group II peridotites, and has the lowest Fo, mainly at 91.5 and the Cr# of Spl as low as 50 with MOR-like features. The chemical composition of Opx is consistent with that from on-craton garnet lherzolites, while the Cpx displays lower Na2O and Al2O3, and elevated Cr2O3 contents with an embryonic oceanic-crust affinity. The high Mg# values and low 187Os/188Os (0.11301–0.12374; TRD = 2.30 Ga − 0.54Ga) of whole rocks and the high Fo in Ol of Group I and II suggest that the peridotites originated from ancient SCLM. The fine-grained mineral clusters in Group III reflect modification of the peridotite massif, representing the oceanization process during the initial rifting of the Gondwana continent.