RECYCLING OF UHP-UHT METAMORPHIC MINERALS FROM SUBDUCTED OCEANIC/CONTINENTAL CRUST

Newly recognized occurrences of UHP minerals in UHT felsic granulites and in chromitites associated with ophiolitic complexes lead to speculation about supracrustal materials being recycled through deep subduction, mantle upwelling and return to the Earth’s surface. This suggestion is supported by crust-derived mineral inclusions in zircon separates from collision-type orogens and in ultramafics of some Alpine-Himalayan-Ural ophiolites. Possible correlation of ‘organic’ light carbon isotopes of diamond and moissanite in ophiolitic chromitite and in kimberlite xenoliths further suggest that UHP and “Superdeep” diamonds and moissanite may have been crustal origin. For examples, In-situ microdiamond (± moissanite) inclusions in chromite grains occur 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. The precursor phase most likely had a Ca-ferrite or Ca-titanite structure; both are polymorphs of Cr-spinel at P >12.5 and 20 GPa (at 2000°C) respectively. Some of these chromitities and associated peridotites also contain rare supracrustal zircon, corundum, felspar, Grt, Ky, Sil, Qtz and rutile, and have much older U-Pb zircon ages than the formation ages of ophiolites. These UHP mineral-bearing chromitites had a deep-seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes. These new findings together with the isotopic and inclusion characteristics of kimberlitic diamonds provide compelling evidence for deep subduction of oceanic/continental lithosphere, recycling of surface ‘organic’ carbon into the lower mantle, and exhumation to the Earth’s surface through deep mantle plume and upwelling.