Ultradeep metamorphic rocks: The retrospective viewpoint

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Abstract
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 UHP rocks do not exhibit signs of ultradeep metamorphism, apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration 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 orogenic belts, but some continue to be subducted to greater mantle depths, the classical geochemical pattern of a one‐way mass transfer from mantle to crust 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)A1‐silicates exist that were not found in nature thus far, but may be characteristic minerals in the cold portions of old subduction zones, thus extending the water retentivity of subducting slabs to greater, and hitherto unexpected, depths.