Abstract Numerous ultra-high-pressure minerals have been recovered from podiform chromities in the Luobusa ophiolite, Tibet. Recovered minerals include diamond, moissanite, Fe-silicides, wüstite, Ni–Fe–Cr–C alloys, PGE alloys and octahedral Mg–Fe silicates. These are accompanied by a variety of native elements, including Si, Fe, Ni, Cr and graphite. All of the minerals were hand-picked from heavy-mineral separates of the chromitites and care was taken to prevent natural or anthropogenic contamination of the samples. Many of the minerals and alloys are either enclosed in, or attached to, chromite grains, leaving no doubt as to their provenance. The ophiolite formed originally at a mid-ocean ridge (MOR) spreading centre at 177±33 Ma, and was later modified by suprasubduction zone magmatism at about 126 Ma. The chromitites were formed in the suprasubduction zone environment from boninitic melts reacting with the host peridotites. The UHP minerals are believed to have been transported from the lower mantle by a plume and incorporated in the ophiolite during seafloor spreading at 176 Ma. Blocks of the mantle containing the UHP minerals were presumably picked up by the later boninitic melts, transported to shallow depth and incorporated in the chromitites during crystallization.

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Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.