Shenglikou and Zedang peridotite massifs, Tibet (China): upper mantle processes and geodynamic significance

Kilometer-scale peridotite massifs in collision or suture zones provide optimal targetsfor investigating the structure, composition, origin and evolution of the Earth’s mantle. In thisstudy, I have investigated the Shenglikou garnet-facies peridotite body in the North Qaidamorogen of NE Tibet, and the Zedang spinel-facies peridotite massif in the Yarlung Zangbosuture of south Tibet, using petrographic, geochemical and multi-isotopic techniques toconstrain their origins and geodynamic processes. The Shenglikou peridotite massif is enclosed by ultrahigh-pressure (UHP) crustal rocks. Zircon U-Pb ages and Hf-isotope compositions show that the UHP crustal rockscontain Neoarchean (~2.8-2.5 Ga) components and experienced magmatic rejuvenation at~2.1-2.0 Ga and ~1.1-0.8 Ga. The rejuvenation made the Qaidam continent more mafic inbulk composition, which facilitated its subduction to reach UHP mantle depths. The Shenglikou peridotite massif mainly consists of pyroxene-rich and olivine-rich ultramafic rocks, and is locally crosscut by phlogopite-bearing pyroxenite dykes. Chemicaland isotopic data suggest that the Shenglikou massif originated as Archean continental lithospheric mantle, which experienced melt refertilization events (at ~1.5-1.4 Ga and ~0.7Ga) and fluid metasomatism during early Paleozoic time. The pyroxenite dykes were formedas cumulates within the lithospheric mantle wedge at ~500 Ma, and the parental magmascame from fluid-metasomatized convective mantle wedge beneath an Andean-type convergent margin. The subsequent continental subduction slab entrained the mantle-wedge fragments,undergoing UHP metamorphism (~440 Ma) and later exhumation (~400 Ma). The Zedang peridotite massif mainly comprises lherzolite (spinel Cr# = 0.17-0.30) inthe west and harzburgite (spinel Cr# = 0.33-0.62) in the east. The lherzolitic domain showsstronger plastic deformation with equilibration temperatures (up to 1260 °C) ~200-300 Khigher than the harzburgitic domain. The harzburgites experienced melt metasomatism andcontain chromitite-dunite associations, but the lherzolites did not. The mineral compositionsshow that the disseminated chromitites and dunites were derived by reaction betweenharzburgites and island-arc tholeiitic magmas. Grains of pure SiC, SiC + K-rich glasses ± zircon, SiC + Si + SiOX, SiC + Si + Fe-V-Ti-Mn alloys and some glasses have been found inthe Zedang harzburgitic domain. The super-reduced assemblages may have formed in themantle by local infiltration by CH₄ and/or H₂ fluids from subducting slabs or the deep mantle.The coexistence of the super-reduced phases (fO₂ of 5-7 log units lower than the IW buffer)with oxidized glasses and silica phases suggests their very short residence time in the hightemperatureambient mantle. Combining the new data from this study with those from previous studies, all evidencesuggests that the Zedang harzburgites were emplaced at shallow levels (after their deepsubduction) at ~200-150 Ma in a forearc spreading center. They ascended rapidly from themantle Transition Zone in a process triggered by slab rollback and driven by their buoyancy.The super-reduced phases formed rapidly and locally in the upwelling channel, as didthe diamonds in the appropriate pressure range. The harzburgites cooled in the Tethyanoceanic basin, and were then underplated by lherzolites, due to transient upwelling of fertile asthenosphere, triggered by a later subduction at ~130-120 Ma. After this underplating, theperidotites were affected locally by reaction with seawater until they were exposed as part ofthe Yarlung Zangbo ophiolite belt in the late Cretaceous. The Shenglikou and Zedang peridotite massifs and related rocks reveal two styles ofmantle geodynamic evolution: the first massif records SCLM processes from Archean to earlyPaleozoic, while the second one reflects upper-mantle processes during the evolution of theTethyan oceans and the formation of Himalayan-Tibetan orogenic system in Phanerozoictime.