ULTRAHIGH-PRESSURE METAMORPHIC MINERALS AND “FORBIDDEN” P-T CONDITIONS

Ultrahigh-pressure (UHP) metamorphism refers to neoblastic mineral assemblages of continental + oceanic crustal protoliths ± associated mafic-ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere, but which subsequently have experienced P-T conditions within or above the coesite stability field (> ~ 2.7 GPa, ~ 700oC). Typical products include eclogite, garnet peridotite, and UHP varieties of metapelite, quartzite, marble, paragneiss, and orthogneiss. UHP phase assemblages require relatively cold lithospheric subduction to mantle depths; some recrystallization even occurs under “forbidden” P-T conditions characterized by a geotherm of < 5oC/km. In appropriate bulk compositions, UHP metamorphism produces coesite, microdiamond and other indicator phases such as majoritic garnet, TiO2 with α-PbO2 structure, supersilicic clinopyroxene, high-P clinoenstatite, stishovite and K-cymrite (KAlSi3O8.nH2O). The presence of majoritic garnet and stishovite pseudomorphs in supracrustal rocks suggests continental subduction to mantle depths > 300 km. Such UHP metamorphic terranes should be distinguished from deep-seated mantle xenoliths that contain UHP minerals. Coesite lamellae in chromite and coesite inferred as stishovite pseudomorphs have been reported from chromitite layers in harzburgite from the Luobusa ophiolite, east-central Tibet. Evidently, some oceanic crustal sections include UHP phases derived from deep mantle conditions. Cold subduction zones are sites of major recycling of H2O back into the mantle; high-P experiments on mafic-ultramafic bulk compositions reveal that many important hydrous and formally anhydrous phases are stable under UHP conditions. Alex Navrotsky and other mineral physicists have pioneered many UHP experiments under “forbidden” P-T conditions.