Ultradeep, or ultra-high-pressure (UHP), metamorphic rocks, formed from crustal protoliths within the stability field of coesite at pressures >2.5-3.0 GPa corresponding to depths >80-120 km, occur locally though regionally distributed in at least five continental areas. Their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high-pressure studies. Detailed petrographic and microprobe work, especially on mineral inclusions, in favorable cases allows the derivation of prograde PT paths during subduction and of retrograde ones during exhumation. Commonly, the gneisses adjacent to the • rocks do not exhibit signs of ultradeep metamorphism, apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete recxluilibrafion to shallower PT conditions during the retrograde path. It is also possible, however, that UHP equilibria were not attained throughout the rock volumes subducted, but only along zones of shearing and fluid introduction. If it is true that not all UHP metamorphic rocks return to the crustal oregenie belts, but some continue to be subducted to greater mantle depths, the classical geochemical pattern of a one-way mass transfer from mantle to crest throughout the Earth's history is at stake. The assumed gradual growth of continents may have had a counterpart of continent destruction during collision events. Most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous (Mg)Al-silicates exist that were not found in nature thus far, but may be characteristic minerals in the cold portions of old subducfion zones, thus extending the water retenfivity of subducting slabs to greater, and hitherto unexpected, depths.
Introduction and DefinitionsMetamorphism is the process by which a rock changes its mineral content toward a new state of chemical equilibrium in the course of changing environmental conditions which it is being subjected to within Earth. The main variables are pressure and temperature, both increasing with depth. In his pioneering work, Eskola [1921] defined specific mineral facies, now called metamorphic facies, to be characteristic for various combinations of these two variables. Depending on whether the tectonic movement of a rock unit is downward or upward, its metamorphism is prograde or retrograde, respectively. The rocks present prior to any metamorphism, the protoliths, may be sediments or igneous rocks formed upon or within Earth's crust. An example is basalt, formed on the ocean floor or on the continent, which may be converted by subduction metamorphism to eclogite. However, eclogites may also form at mantle depths directly from crystallizing mafic magmas, thus being igneous rocks of the mantle. Mantle eclogites are therefore inherently of ambiguous origin. crustal rocks at mantle depths" [Schreyer, 1985, p. 227]. On the other hand, the two well-known metamorphic facies of ultrabasic manfie material, spinel-peridotite and gamet-peridotite, do n...
