Ultradeep metamorphic rocks: The retrospective viewpoint

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 • 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...

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust

“…A small number of sialic crustal complexes, chiefly located within Eurasia, exhibit rare, scattered effects of UHP recrystallization (20,39). Well-recognized tracts include the Qinling-Dabie-Sulu belt of east-central China, the Kokchetav Massif of northern Kazakhstan, the Maksyutov Complex of the southern Urals, the Dora Maira Massif of the western Alps, and the Western Gneiss Region of Norway.…”

Stability limits of hydrous minerals in sediment and mid‐ocean ridge basalt compositions: Implications for water transport in subduction zones

“…Therefore it is necessary to determine the stability fields of hydrous minerals in the natural materials which compose the slab. The stabilities of hydrous minerals in the system A120 3-SiO2-H20 play a important role of the water content of the subducted sediment [Schreyer, 1995]. Topaz-OH (A12SiO4(OH)2) appears at pressures higher than 4 GPa [Wunder et al, 1993], and phase egg (A1SiO3(OH)) was observed at pressures greater than 10 GPa and at about 1000øC in this system [Eggleton et al, 1978].…”

Multianvil techniques in conjunction with synchrotron radiation at Deutsches ElektronenSYnchrotron (DESY) – HAmburger SYnchrotron LABor (HASYLAB)

“…The high-pressure SiO 2 polymorph coesite is an important mineral in the subduction process, including in crustal material as has been observed in rocks of the Dora Maira Massif (western Alps) by Chopin (1984), Gillet et al (1984), and also in other subducted continental rocks (Schreyer, 1995). The quartz to coesite transition is thus of fundamental importance in the effort to understand the processes within subducting lithosphere.…”