Ultra-high-pressure metamorphism refers to metamorphic 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, the composition and evolution of Earth's crust. The discovery of UHP metamorphic rocks in 1984Chopin, C., 1984, Coesite and pure pyrope in high-grade blueschists of the western Alps: a first record and some consequences: Contributions to Mineralogy and Petrology, v. 86, p. 107–118.Smith, D. C., 1984, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics: Nature, v. 310, p. 641–644. 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 of microcontinents 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 diamondMassonne, H. J., and Nasdala, L., 2000, Microdiamonds from the Saxonian Erzgebirge, Germany: in situ micro-Raman characterisation: European Journal of Mineralogy, v. 12, p. 495-498.), 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.Ravna, E. J. K., and Terry, M. P., 2004, Geothermobarometry of phengite-kyanite-quartz/coesite eclogites: Journal of Metamorphic Geology, v. 22, p. 579-592. Most UHP rocks were metamorphosed at peak conditions of 800 °C and 3 GPa.Hacker, B. R., 2006, Pressures and temperatures of ultrahigh-pressure metamorphism: Implications for UHP tectonics and H2O in subducting slabs.: International Geology Review, v. 48, p. 1053-1066. At least two UHP localities record higher temperatures: the Bohemian and Kokchetav Massifs reached 1000–1200 °C at pressures of at least 4 GPa.Massonne, H.-J., 2003, A comparison of the evolution of diamondiferous quartz-rich rocks from the Saxonian Erzgebirge and the Kokchetav Massif: are so-called diamondiferous gneisses magmatic rocks?: Earth and Planetary Science Letters, v. 216, p. 347–364.Manning, C. E., and Bohlen, S. R., 1991, The reaction titanite + kyanite = anorthite + rutile and titanite-rutile barometry in eclogites: Contributions to Mineralogy and Petrology, v. 109, p. 1-9.Masago, H., 2000, Metamorphic petrology of the Barchi-Kol metabasites, western Kokchetav ultrahigh-pressure–high-pressure massif, northern Kazakhstan: The Island Arc, v. 9, p. 358–378.
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 obliteratedHacker, B. R., Kelemen, P. B., and Behn, M. D., 2011, Differentiation of the continental crust by relamination: Earth and Planetary Science Letters, v. 307, p. 501-516.Walsh, 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 continental orogenic belts; most occur in Eurasia.Liou, J. G., Tsujimori, T., Zhang, R. Y., Katayama, I., and Maruyama, S., 2004, Global UHP metamorphism and continental subduction/collision: The Himalayan model: International Geology Review, v. 46, p. 1-27. 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;Jahn, B. M., Caby, R., and Monie, P., 2001, The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications: Chemical Geology, v. 178, p. 143-158. the youngest is 8 Ma and exposed in the D'Entrecasteaux Islands of Papua New Guinea.Baldwin, S. L., Webb, L. E., and Monteleone, B. D., 2008, Late Miocene coesite-eclogite exhumed in the Woodlark Rift: Geology, v. 36, p. 735-738. A modest number of continental orogens have undergone multiple UHP episodes.Brueckner, H. K., and van Roermund, H. L. M., 2004, Dunk tectonics: a multiple subduction/eduction model for the evolution of the Scandinavian Caledonides: Tectonics, v. doi: 10.1029/2003TC001502.
UHP terrains vary greatly in size, from the >30,000 km2 giant UHP terrains in Norway and China, to small kilometer-scale bodies.Ernst, W. G., Hacker, B. R., and Liou, J. G., 2007, Petrotectonics of ultrahigh-pressure crustal and upper-mantle rocks: Implications for Phanerozoic collisional orogens: Geological Society of America Special Paper, v. 433, p. 27-49. 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 with a few percent mafic rock (eclogite) or ultramafic rock (garnet-bearing peridotite). Some include sedimentary or rift-volcanic sequences that have been interpreted as passive margins prior to metamorphism.Oberhänsli, R., Martinotti, G., Schmid, R., and Liu, X., 2002, Preservation of primary volcanic textures in the ultrahigh-pressure terrain of Dabie Shan: Geology, v. 30, p. 609–702.Hollocher, K., Robinson, P., Walsh, E., and Terry, M., 2007, The Neoproterozoic Ottfjället dike swarm of the Middle Allochthon, traced geochemically into the Scandian hinterland, Western Gneiss region, Norway: American Journal of Science, v. 307, p. 901-953.
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. UHP rocks of a wide variety of compositions have been identified as both regional metamorphic terrains and xenoliths.
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.Zheng, Y.-F., Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences, v. 145, p. 46-73. 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. 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,Schmid, R., Romer, R. L., Franz, L., Oberhänsli, R., and Martinotti, G., 2003, Basement-Cover Sequences within the UHP unit of the Dabie Shan: Journal of Metamorphic Geology, v. 21, p. 531-538. in the Arabian continental margin beneath the Samail ophiolite (in the Al Hajar Mountains, Oman),Searle, M. P., Waters, D. J., Martin, H. N., and Rex, D. C., 1994, Structure and metamorphism of blueschist-eclogite facies rocks from the northeastern Oman Mountains: Journal of the Geological Society of London, v. 151, p. 555-576. and in the Australian margin presently subducting beneath the Banda Arc.Hamilton, W., 1979, Tectonics of the Indonesian Region: U.S. Geological Survey Professional Paper, v. 1078, p. 1-345. 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.Plank, T., and Langmuir, C. H., 1993, Tracing trace elements from sediment input to volcanic output at subduction zones: Nature, v. 362, p. 739-742. 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.Hacker, B. R., Luffi, P., Lutkov, V., Minaev, V., Ratschbacher, L., Plank, T., Ducea, M., Patiño-Douce, A., McWilliams, M., and Metcalf, J., 2005, Near-ultrahigh pressure processing of continental crust: Miocene crustal xenoliths from the Pamir: Journal of Petrology, v. 46, p. 1661-1687.
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.