DEEP UPPER MANTLE RECYCLING OF CRUSTAL MATERIALS: EVIDENCE FROM UHP MINERALS IN COLLISIONAL OROGENS, OPHIOLITES, AND XENOLITHS

UHP minerals present in deep-crustal UHT felsic granulites and in upper-plate protoliths of collision-type orogens, in mantle-derived chromitites spatially associated with ophiolites, and in eclogitic diamonds from kimberlites suggest the important recycling of supracrustal rocks through deep subduction, mantle upwelling and return to the Earth’s surface. Crust-mantle circulation is supported by (1) LP crust-derived mineral inclusions in UHP zircons separated from collision-type orogens and in mantle-derived chromitites of some Alpine-Himalayan-Ural ophiolites, (2) identification of early to mid-Proterozoic Sino-Korean protoliths as UHP rocks in the North Sulu orogen, (3) occurrences of hydrous ringwoodite as well as LP mineral inclusions in diamonds of mantle eclogitic xenoliths, and (4) light-carbon (i.e., organic) isotopes in diamond ± moissanite from these tectonic settings. Newly recognized in-situ micro-diamond ± moissanite inclusions are present in mantle-derived chromitites spatially associated with ophiolites from Tibet, the Polar Urals, and Myanmar. Some chromitites contain possible coesite pseudomorphs after stishovite, Fe-Ti alloys, TiO₂ II (α-PbO₂-type TiO₂), and several highly reduced mineral inclusions. Exsolution lamellae of coesite + diopside and Cpx inclusions in chromite suggest crystallization at P > 9–10 GPa and depths of > 250–300 km. Rare minerals of undoubted crustal origin, including Zrn, corundum, Fsp, Grt, Ky, And, Qz, and Rt occur in podiform chromitites and host peridotites; the zircons possess much older U-Pb ages than the formation age of spatially associated ophiolites. Such UHP mineral-bearing chromitites had a deep-seated history prior to mantle upwelling and partial melting at shallow depths, forming the overlying ophiolite complexes. Unraveling the mixing of UHP + LP mineral inclusions in chromitites will require mm- to nano-scale geochemical and isotopic characterizations of both solid and fluid inclusions in diamonds. Such investigations should allow a more quantitative understanding of the geodynamics of deep subduction, mantle upwelling, and crustal recycling.