RECYCLING OF UHP-UHT MINERALS — THE FATE OF SUBDUCTED CONTINENTAL CRUST

New occurrences of UHP minerals in UHT felsic granulites from NW Africa and the Bohemia Massif and earlier findings of microdiamond, coesite and nano-size α-PbO2–structured TiO2 polymorph in pelitic gneisses of the central Erzgebirge suggest that a big segment of crustal basement has been exhumed from mantle depths > 150 km. Inclusions of microdiamond and coesite occur in Grt, Ky and zircon of felsic granulites; those in Africa have much higher P-T estimates of P > 4.3 GPa and T > 1100°C, exhibit topotaxial overgrowths of diamond and coesite, lack palisade Qtz around relict coesite, and have intergrowths of coesite and phengite possibly after K-cymrite. These UHP–UHT granulites are closely associated with mantle-derived Grt peridotites that were interdigitated with deeply subducted continental crust rather than being tectonically emplaced at shallow depths.

UHP minerals have also been discovered in podiform chromitite associated with ophiolite. Diamond, moissanite, possible coesite pseudomorphs after stishovite, Fe-Ti alloys, osbornite, cubic boron nitride, TiO2 II, and zabonite occur as nano- to micro-scale inclusions in podiform Luobusa chromitite, Tibet. In-situ microdiamond (± moissanite) inclusions in chromite grains have been recognized in numerous ophiolitic massifs along the 1400-km-long Yarlung-Zangbo suture between India and Asia, and in the Polar Ural Mountains. These UHP minerals and chromite containing exsolution lamellae of coesite + diopside suggest that the chromitites formed at P > 9–10 GPa at depths of > 250–300 km. Thin lamellae of pyroxene in chromite also occur in chromitites of the northern Oman ophiolite. These chromitities and associated peridotites contain rare zircon, corundum, feldspar, Grt, Ky, sillimanite, Qtz and rutile, and have much older U-Pb zircon ages than the formation ages of ophiolites. Apparently, these UHP mineral-bearing chromitites had a deep-seated evolution prior to the formation of overlying ophiolite complexes at shallow depths. These findings lead to speculation about supracrustal materials being recycled through deep subduction, mantle upwelling and return to the Earth’s surface supported by crustal mineral inclusions in diamonds and possible ‘organic’ light carbon isotopes of diamond and moissanite in both UHP terranes and kimberlitic xenoliths.