An introduction to ultrahigh‐pressure metamorphism

Ultrahigh‐pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic‐ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80‐90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic‐ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite‐bearing eclogitic terranes and three diamond‐bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high‐P blueschist belts. This paper defines the P‐T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su‐Lu region of east‐central China, and the Kokchetav massif of the former USSR. Prograde P‐T paths for coesite‐bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust‐capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).