ULTRADEEP METAMORPHIC ROCKS - THE RETROSPECTIVE VIEWPOINT

Ultradeep, or ultra-high-pressure (UHP), metamorphic rocks, formed fro m crustal protoliths within the stability field of coesite at pressure s >2.5-3.0 GPa corresponding to depths >80-120 km, occur locally thoug h regionally distributed in at least five continental areas. Their rec ognition is solely based on characteristic minerals and mineral assemb lages calibrated by experimental high-pressure studies, Detailed petro graphic and microprobe work, especially on mineral inclusions, in favo rable cases allows the derivation of prograde PT paths during subducti on and of retrograde ones during exhumation. Commonly, the gneisses ad jacent to the UHP rocks do not exhibit signs of ultradeep metamorphism , apparently because the kinetics of their mineral reactions are suffi ciently fast to allow complete reequilibration to shallower PT conditi ons during the retrograde path. It is also possible, however, that UHP equilibria were not attained throughout the rock volumes subducted, b ut only along zones of shearing and fluid introduction. If it is true that not all UHP metamorphic rocks return to the crustal orogenic belt s, but some continue to be subducted to greater mantle depths, the cla ssical geochemical pattern of a one-way mass transfer from mantle to c rust throughout the Earth's history is at stake. The assumed gradual g rowth of continents may have had a counterpart of continent destructio n during collision events. Most recent experimental studies at high pr essures and relatively low temperatures show that at least three new h ydrous (Mg)Al-silicates exist that were not found in nature thus far, but may be characteristic minerals in the cold portions of old subduct ion zones, thus extending the water retentivity of subducting slabs to greater, and hitherto unexpected, depths.