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Ultra-high-pressure metamorphism refers to
metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect
plate tectonics Plate tectonics (from the la, label= Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of larg ...
, the composition and evolution of Earth's crust. The discovery of UHP metamorphic rocks in 1984 revolutionized our understanding of plate tectonics. Prior to 1984 there was little suspicion that continental rocks could reach such high pressures. The formation of many UHP terrains has been attributed to the
subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
of
microcontinent Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Caus ...
s or continental margins and the exhumation of all UHP terrains has been ascribed principally to buoyancy caused by the low density of continental crust—even at UHP—relative to Earth's mantle. While the subduction proceeds at low thermal gradients of less than 10°C/km, the exhumation proceeds at elevated thermal gradients of 10-30°C/km.


Definition

Metamorphism of rocks at pressures ≥27kbar (2.7GPa) to stabilize coesite, the high-pressure polymorph of SiO2, recognized by either the presence of a diagnostic mineral (e.g., coesite or diamond), mineral assemblage (e.g., magnesite + aragoniteKlemd, R., Lifei, Z., Ellis, D., Williams, S., and Wenbo, J., 2003, Ultrahigh-pressure metamorphism in eclogites from the western Tianshan high-pressure belt (Xinjiang, western China); discussion and reply: American Mineralogist, v. 88, p. 1153-1160), or mineral compositions.


Identification

Petrological indicators of UHP metamorphism are usually preserved in eclogite. The presence of metamorphic coesite, diamond, or majoritic garnet are diagnostic; other potential mineralogical indicators of UHP metamorphism, such as alpha-PbO2 structured TiO2, are not widely accepted. Mineral assemblages, rather than single minerals, can also be used to identify UHP rocks; these assemblages include magnesite + aragonite. Because minerals change composition in response to changes in pressure and temperature, mineral compositions can be used to calculate pressure and temperature; for UHP eclogite the best geobarometers involve garnet + clinopyroxene + K-white mica and garnet + clinopyroxene + kyanite + coesite/quartz. Most UHP rocks were metamorphosed at peak conditions of 800 °C and 3 GPa. At least two UHP localities record higher temperatures: the Bohemian and Kokchetav Massifs reached 1000–1200 °C at pressures of at least 4 GPa. Most felsic UHP rocks have undergone extensive retrograde metamorphism and preserve little or no UHP record. Commonly, only a few eclogite enclaves or UHP minerals reveal that the entire terrain was subducted to mantle depths. Many granulite terrains and even batholithic rocks may have undergone UHP metamorphism that was subsequently obliteratedWalsh, E. O., and Hacker, B. R., 2004, The fate of subducted continental margins: Two-stage exhumation of the high-pressure to ultrahigh-pressure Western Gneiss complex, Norway: Journal of Metamorphic Geology, v. 22, p. 671-689.


Global distribution

Geologists have identified UHP terrains at more than twenty localities around the globe in most well-studied
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anim ...
continental
orogenic Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
belts; most occur in Eurasia. Coesite is relatively widespread, diamond less so, and majoritic garnet is known from only rare localities. The oldest UHP terrain is 620 Ma and is exposed in Mali; the youngest is 8 Ma and exposed in the
D'Entrecasteaux Islands D'Entrecasteaux Islands () are situated near the eastern tip of New Guinea in the Solomon Sea in Milne Bay Province of Papua New Guinea. The group spans a distance of , has a total land area of approximately and is separated from the Papua New G ...
of Papua New Guinea. A modest number of continental orogens have undergone multiple UHP episodes. UHP terrains vary greatly in size, from the >30,000 km2 giant UHP terrains in Norway and China, to small kilometer-scale bodies. The giant UHP terrains have a metamorphic history spanning tens of millions of years, whereas the small UHP terrains have a metamorphic history spanning millions of years.Kylander-Clark, A., Hacker, B., and Mattinson, C., 2012, Size and exhumation rate of ultrahigh-pressure terrains linked to orogenic stage: Earth and Planetary Science Letters, v. 321-322, p. 115-120. All are dominated by quartzofeldspathic
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...
with a few percent mafic rock (eclogite) or ultramafic rock (garnet-bearing
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...
). Some include sedimentary or rift-volcanic sequences that have been interpreted as passive margins prior to metamorphism.


Implications and importance

UHP rocks record pressures greater than those that prevail within Earth's crust. Earth's crust is a maximum of 70–80 km thickness, and pressures at the base are <2.7 GPa for typical crustal densities. UHP rocks therefore come from depths within Earth's
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
. UHP rocks of a wide variety of compositions have been identified as both regional metamorphic terrains and
xenolith A xenolith ("foreign rock") is a rock fragment ( country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ig ...
s. UHP ultramafic xenoliths of mantle affinity provide information (e.g., mineralogy or deformation mechanisms) about processes active deep in Earth. UHP xenoliths of crustal affinity provide information about processes active deep in Earth, but also information about what kinds of crustal rocks reach great depth in Earth and how profound those depths are. Regional metamorphic UHP terrains exposed on Earth's surface provide considerable information that is not available from xenoliths. Integrated study by structural geologists, petrologists, and geochronologists has provided considerable data on how the rocks deformed, the pressures and temperatures of metamorphism, and how the deformation and metamorphism varied as a function of space and time. It has been postulated that small UHP terrains that underwent short periods of metamorphism formed early during continent subduction, whereas giant UHP terrains that underwent long periods of metamorphism formed late during continent collision.


Formation of UHP rocks

Eclogite-facies HP to UHP metamorphic rocks are produced by subduction of crustal rocks to the lower crust to mantle depths for extreme metamorphism at the low thermal gradients of less than 10°C/km. All of these rocks occur at convergent plate margins, and UHP rocks only occur in collisional orogens. There is general agreement that most well-exposed and well-studied UHP terrains were produced by the burial of crustal rocks to mantle depths of >80 km during
subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
. Continental margin subduction is well documented in a number of collisional orogens, such as the Dabie orogen where South China Block passive-margin sedimentary and volcanic sequences are preserved, in the Arabian continental margin beneath the Samail ophiolite (in the
Al Hajar Mountains The Hajar Mountains ( ar, جِبَال ٱلْحَجَر, Jibāl al-Ḥajar, ''The Rocky Mountains'' or ''The Stone Mountains'') in northeastern Oman and also the eastern United Arab Emirates are the highest mountain range in the eastern Arabi ...
, Oman), and in the Australian margin presently subducting beneath the
Banda Arc The Banda Arc (main arc, Inner, and Outer) is a set of island arcs in eastern Indonesia. It is the result of the collision of a continent and an intra-oceanic island arc. The presently active arc is located on what appears to be oceanic crust whe ...
. Sediment subduction occurs beneath volcanoplutonic arcs around the worldScholl, D. W., and von Huene, R., 2007, Crustal recycling at modern subduction zones applied to the past—Issues of growth and preservation of continental basement, mantle geochemistry, and supercontinent reconstruction, in Robert D. Hatcher, J., Carlson, M. P., McBride, J. H., and Catalán:, J. R. M., eds., Geological Society of America, Memoir Boulder, Geological Society of America, p. 9-32. and is recognized in the compositions of arc lavas. Continental subduction may be underway beneath the Pamir.Burtman, V. S., and Molnar, P., 1993, Geological and geophysical evidence for deep subduction of continental crust beneath the Pamir: Geological Society of America Special Paper, v. 281, p. 1-76. Subduction erosion also occurs beneath volcanoplutonic arcs around the world, carrying continental rocks to mantle depths at least locally.


Exhumation of UHP rocks

The specific processes by which UHP terrains were exhumed to Earth's surface appear to have been different in different locations. If continental lithosphere is subducted because of its attachment to downgoing oceanic lithosphere, the downward slab pull force may exceed the strength of the slab at some time and location, and necking of the slab initiates. The positive buoyancy of the continental slab—in opposition principally to ridge push—can then drive exhumation of the subducting crust at a rate and mode determined by plate geometry and the
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid ( liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an ap ...
of the crustal materials. The Norwegian Western Gneiss Region is the archetype for this exhumation mode, which has been termed 'eduction' or subduction inversion. If a plate undergoing subduction inversion begins to rotate in response to changing boundary conditions or body forces, the rotation may exhume UHP rocks toward crustal levels. This could occur if, for example, the plate is small enough that continental subduction markedly changes the orientation and magnitude of slab pull or if the plate is being consumed by more than one subduction zone pulling in different directions. Such a model has also been proposed for the UHP terrain in eastern Papua New Guinea, where rotation of the Woodlark microplate is causing a rift in the
Woodlark Basin The Woodlark Basin is a geologic structural basin located in the southwestern Pacific Ocean and can be found to the southwest of the island country of Papua New Guinea. The basin is an extensional basin that is actively spreading and has a seafloo ...
). If a subducting plate consists of a weak buoyant layer atop a stronger negatively buoyant layer, the former will detach at the depth where the buoyancy force exceeds slab pull, and extrude upward as a semi-coherent sheet. This type of delamination and stacking was proposed to explain exhumation of UHP rocks in the Dora Maira massif in
Piedmont it, Piemontese , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = , demographics1_footnotes = , demographics1_title1 = , demographics1_info1 = , demographics1_title2 ...
, Italy, in the Dabie orogen, and in the Himalaya. In addition it was demonstrated with analogue experiments. This mechanism is different from flow in a subduction channel in that the exhuming sheet is strong and remains undeformed. A variant of this mechanism, in which the exhuming material undergoes folding, but not wholescale disruption, was suggested for the Dabie orogen, where exhumation-related stretching lineations and gradients in metamorphic pressure indicate rotation of the exhuming block;Hacker, B. R., Ratschbacher, L., Webb, L. E., McWilliams, M., Ireland, T. R., Calvert, A., Dong, S., Wenk, H.-R., and Chateigner, D., 2000, Exhumation of ultrahigh-pressure continental crust in east–central China: Late Triassic–Early Jurassic tectonic unroofing: Journal of Geophysical Research, v. 105, p. 13339–13364. The buoyancy of a microcontinent locally slows the rollback of and steepens the dip of subducting mafic lithosphere. If the mafic lithosphere on either side of the microcontinent continues to roll back, a buoyant portion of the microcontinent may detach, allowing the retarded portion of the mafic slab to roll quickly back, making room for the UHP continental crust to exhume and driving back-arc extension. This model was developed to explain repeated cycles of subduction and exhumation documented in the Aegean and Calabria–Apennine orogens. UHP exhumation by slab rollback has not yet been extensively explored numerically, but it has been reproduced in numerical experiments of Apennine-style collisions. If continental material is subducted within a confined channel, the material tends to undergo circulation driven by tractions along the base of the channel and the relative buoyancy of rocks inside the channel; the flow can be complex, generating nappe-like or chaotically mixed bodies.Gerya, T. V., Perchuk, L. L., and Burg, J.-P., 2007, Transient hot channels: perpetrating and regurgitating ultrahigh-pressure, high temperature crust-mantle associations in collision belts: Lithos, v. 103, p. 236-256.Warren, C. J., Beaumont, C., and Jamieson, R. A., 2008, Modelling tectonic styles and ultrahigh pressure (UHP) rock exhumation during the transition from oceanic subduction to continental collision: Earth and Planetary Science Letters, v. 267, p. 129-145. The material within the channel can be exhumed if: # continuous introduction of new material into the channel driven by traction of the subducting plate pushes old channel material upward; # buoyancy in the channel exceeds subduction-related traction and the channel is pushed upward by the asthenospheric mantle intruding between the plates; or # a strong indenter squeezes the channel and extrudes the material within. Buoyancy alone is unlikely to drive exhumation of UHP rocks to Earth's surface, except in oceanic subduction zones. Arrest and spreading of UHP rocks at the Moho (if the overlying plate is continental) is likely unless other forces are available to force the UHP rocks upward. Some UHP terrains might be coalesced material derived from subduction erosion. This model was suggested to explain the North
Qaidam The Qaidam, Tsaidam, or Chaidamu Basin is a hyperarid basin that occupies a large part of Haixi Prefecture in Qinghai Province, China. The basin covers an area of approximately , one-fourth of which is covered by saline lakes and playas. Around ...
UHP terrain in western China. Even subducted sediment may rise as diapirs from the subducting plate and accumulate to form UHP terrains. Studies of numerical geodynamics suggest that both subducted sediment and crystalline rocks may rise through the mantle wedge
diapir A diapir (; , ) is a type of igneous intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–T ...
ically to form UHP terranes.Stöckhert, B., and Gerya, T. V., 2005, Pre-collisional high pressure metamorphism and nappe tectonics at active continental margins: a numerical simulation: Terra Nova, v. 17, p. 102-110.Yin, A., Manning, C. E., Lovera, O., Menold, C. A., Chen, X., and Gehrels, G. E., 2007, Early Paleozoic tectonic and thermomechanical evolution of ultrahigh-pressure (UHP) metamorphic rocks in the northern Tibetan Plateau, northwest China: International Geology Review, v. 49, p. 681-716.Behn, M. D., Kelemen, P. B., Hirth, G., Hacker, B. R., and Massonne, H. J., 2011, Diapirs as the source of the sediment signature in arc lavas: Nature Geoscience, v. DOI: 10.1038/NGEO1214. Diapiric rise of a much larger subducted continental body has been invoked to explain the exhumation of the Papua New Guinea UHP terrain. This mechanism was alo used to explain the exhumation of UHP rocks in Greenland. However, the mantle wedge above continental subduction zones is cold like cratons, which do not allow for diapirically ascending of the crustal materials. Foundering of the gravitationally unstable portions of continental lithosphere locally carries quartzofeldspathic rocks into the mantleGerya, T. V., and Meilick, F. I., 2011, Geodynamic regimes of subduction under an active margin: effects of rheological weakening by fluids and melts: Journal of Metamorphic Geology, v. 29, p. 7-31. and may be ongoing beneath the Pamir.


See also

* High pressure terranes along the Bangong-Nujiang Suture Zone * Eclogitization * Subduction zone metamorphism


References


Further reading

* Coleman, R.G., and Wang, X. (Editors), 1995. Ultrahigh Pressure Metamorphism. Cambridge University Press, 528 pp. * Hacker, B.R., and Liou, J.G. (Editors), 1998. When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks. Kluwer Academic Publishers, 323 pp. * Liou, J.G., and Ernst, W.G. (Editors), 2000. UltraHigh Pressure Metamorphism and Geodynamics in Collision-Type Orogenic Belts. Geological Society of America, International Book Series, volume 4, 293 pp. * Hacker, B.R., McClelland, W.C., and Liou, J.G. (Editors), 2006. Ultrahigh-Pressure Metamorphism: Deep Continental Subduction. Geological Society of America Special Paper 403, 206 pp. {{Metamorphic petrology Metamorphic petrology