Ultrahigh-temperature metamorphism (UHT) represents extreme crustal metamorphism with metamorphic temperatures exceeding 900 °C. Granulite-facies rocks metamorphosed at very high temperatures were identified in the early 1980s, although it took another decade for the geoscience community to recognize UHT metamorphism as a common regional phenomenon. Petrological evidence based on characteristic mineral assemblages backed by experimental and thermodynamic relations demonstrated that Earth’s crust can attain and withstand very high temperatures (900-1000 °C) with or without partial melting.
Metamorphism of crustal rocks in which peak temperature exceeds 900 °C, recognized either by robust thermobarometry or by the presence of a diagnostic mineral assemblage in an appropriate bulk composition and oxidation state, such as assemblages with orthopyroxene + sillimanite + quartz, sapphirine + quartz or spinel + quartz, generally at pressure conditions of sillimanite stability in metapelites [after Brown (2007) following proposal by Harley (1998)].
Petrological indicators of UHT metamorphism are usually preserved in extremely Mg-Al-rich rocks which are usually dry and restitic in nature. Mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartz, osumulite and spinel + quartz provide straight away evidence for such extreme conditions. Occasionally widespread assemblages like garnet + orthopyroxene, ternary feldspars, (F-Ti) pargasite or metamorphic inverted pigeonite are taken as typical indicators of UHT metamorphism.
UHT rocks are now identified in all major continents and span different geological ages ranging from c. 3178 to 35 million years associated with major geological events. More than 46 localities/terranes with diagnostic UHT indicators have been reported over the globe, related to both extensional and collisional tectonic environments; the two fundamental types of Earth orogenic systems. The major Archean UHT rocks are distributed in East-Antarctica, South Africa, Russia and Canada. Paleoproterozoic UHT granulites were reported from the North China Craton (during the accretion of the supercontinent Columbia), Taltson magmatic zone, northwestern Canada and South Harris, Lewisian complex, Scotland. UHT rocks from the Neoproterozoic Grenville orogeny are distributed in the Eastern Ghats Province, India. Neoproterozoic-Cambrian (Pan-African) UHT occurrences are mainly distributed in Lutzow-Holm Bay, East Antarctica, southern Madagascar, Sri Lanka and southern India  UHT rocks are also reported from younger terranes like the Triassic Kontum Massif, Vietnam, Cretaceous Higo belt, Japan and Paleogene Gruf Complex, central Alps. Three-million-year-old xenoliths erupted in Qiangtang imply that UHT metamorphism is ongoing beneath central Tibet.
A correlation has been proposed between the episodic formation of UHT granulites and the episodic assembly and disruption of supercontinents or the plume activity during various periods in Earth history. UHT granulites are generally characterized by dry mineral assemblages, the stability of which require low water activities. The direct evidence for the involvement of CO2-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO2 fluid inclusions in buffering the water activity and stabilizing the anhydrous mineralogy of UHT rocks have come from the finding of abundant pure CO2 fluid inclusions in these rocks. UHT metamorphism has been recently evaluated in the plate tectonic context using modern analogues and it has been suggested that both post-collisional extension and rifting play a crucial role. The abundant CO2 liberated by subsolidus decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect thereby triggering the deglaciation of snowball Earth. Based on the distributions of carbonated subcontinental mantle and crustal domains that have undergone CO2-aided dry metamorphism at extreme conditions, Santosh and Omori (2008b) speculated that the UHT rocks might represent windows for the transfer of CO2 from the mantle into the mid crust and ultimately to the atmosphere.
- S.L., Harley (1998). "On the occurrence and characterization of ultrahigh-temperature crustal metamorphism". Geological Society, London, Special Publications 138 (1): 81–107. Bibcode:1998GSLSP.138...81H. doi:10.1144/GSL.SP.1996.138.01.06.
- Brown, M., 2007, Metamorphic conditions in orogenic belts: a record of secular change. International Geology Review 49, 193-234
- Kelsey, D.E., 2008, On ultrahigh-temperature crustal metamorphism. Gondwana Research 13, 1-29
- Santosh, M., Omori, S., 2008a, CO2 flushing: a plate tectonic perspective. Gondwana Research 13, 86-102
- Santosh, M., Omori, S., 2008b, CO2 windows from mantle to atmosphere: Models on ultrahigh- temperature metamorphism and speculations on the link with melting of snowball Earth. Gondwana Research 14, in press, doi:10.1016/j.gr.2007.11.001
- Arima, M., and Barnett, R. L., 1984, Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain, Manitoba, Canada: Contributions to Mineralogy and Petrology, v. 88, p. 102-112.
- Harley, S. L., 1985, Garnet-orthopyroxene bearing granulites from Enderby Land, Antarctica: Metamorphic pressure-temperature-time evolution of the Archaean Napier Complex: Journal of Petrology, v. 26, p. 819-856.
- Harley, S. L., and Motoyoshi, Y., 2000, Al zoning in orthopyroxene in a sapphirine quartzite: Evidence for >1120°C UHT metamorphism in the Napier complex, Antarctica, and implications for the entropy of sapphirine: Contributions to Mineralogy and Petrology, v.138, p. 293–307.
- Fonarev, V. I., Pilugin, S. M., Savko, K. A., and Novikova, M. A., 2006, Exsolution textures of ortho-and clinopyroxene in high-grade BIF of the Voronezh Crystalline Massif: Evidence of ultrahigh-temperature metamorphism: Journal of Metamorphic Geology, v. 24, p. 135-151.
- Tsunogae et al., 2002
- Santosh, M. Sajeev K. and J. Li 2006, Extreme crustal metamorphism during Columbia supercontinent assembly: Evidence from North China Craton. Gondwana Research, v. 10, p. 256-266.
- Santosh, M., Tsunogae, T., Li, J.H., and Liu, S.J., 2007, Discovery of sapphirine- bearing Mg-Al granulites in the North China Craton: Implications for Paleoproterozoic ultrahigh- temperature metamorphism. Gondwana Research 11, 263-285.
- Santosh, M., Tsunogae, T., Ohyama, H. Sato, K., Li, J.H., and Liu, S.J., 2008, Carbonic metamorphism at ultrahigh-temperatures. Earth and Planetary Science Letters 266, 149-165.
- Farquhar; Chacko, Thomas; Ellis, David J. et al. (1996). "Preservation of oxygen isotope compositions in granulites from Northwestern Canada and Enderby Land, Antarctica: implications for high-temperature isotopic thermometry". Contributions to Mineralogy and Petrology 125 (2–3): 213–224. Bibcode:1996CoMP..125..213F. doi:10.1007/s004100050217. Retrieved 2010-10-06.
- Baba, S., 1998, Proterozoic anticlockwise P-T path of the Lewisian complex of South Harris, outer Hebrides, NW Scotland: Journal of Metamorphic of Geology, v. 16, p. 819–841.
- Baba, S., 1999, Sapphirine-bearing orthopyroxene-kyanite/sillimanite granulites from South Harris, NW Scotland: Evidence for Proterozoic UHT metamorphism in the Lewisian: Contributions to Mineralogy and Petrology, v. 136, p. 33–47.
- Baba, S., 2003, Two stages of sapphirine formation during prograde and retrograde metamorphism in the Paleoproterozoic Lewisian complex in South Harris, NW Scotland: Journal of Petrology, v. 44, p. 329–354.
- Hollis, J. A., Harley, S. L., White, R. W., and Clarke, G. L., 2006, Preservation of evidence for prograde metamorphism in UHT HP granulites, South Harris, Scotland: Journal of Metamorphic Geology, v. 24, p. 263–279.
- Dasgupta, S., Sanyal, S., Sengupta, P., and Fukuoka, M.,1994, Petrology of granulites from Anakapalle-evidence for Proterozoic decompression in the Eastern Ghats, India: Journal of Petrology, v. 35, p. 433–459.
- Motoyoshi, Y., and Ishikawa, M., 1997, Metamorphic and structural evolution of granulites from Rundvågshetta,Lützow-Holm Bay, east Antarctica, in Ricci, C. A., ed., The Antarctic region: Geological evolution and processes: Proceedings of the VII International Symposium on the Antarctic Earth Sciences, Siena, Terra Antarctica, p. 65–72.
- Jöns, N.; Schenk, Y. (2011). "The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context; implications for the amalgamation of the Gondwana supercontinentm". European Journal of Mineralogy 23: 127–156. doi:10.1127/0935-1221/2011/0023-2087.
- Sajeev, K. and Osanai, Y. 2004a, Ultrahigh-temperature Metamorphism (1150°C and 12 kbar) and Multi-stage Evolution of Mg-Al Granulites from Central Highland Complex, Sri Lanka, Journal of Petrology, v. 45, p. 1821-1844.
- Sajeev, K.; Osanai, Y. (2004b). "'Osumilite' and 'spinel+quartz' from Highland Complex, Sri Lanka: a case of cooling and decompression after ultrahigh-temperature metamorphism". Journal of Mineralogical and Petrological Sciences (JMPS) 99: 320–327. doi:10.2465/jmps.99.320.
- Sajeev, K.; Osanai, Y.; Connolly, J.A.D.; Suzuki, S. Ishioka; Kagami, H.; Rino, S. (2007). "Extreme Crustal Metamorphism during a Neoproterozoic Event in Sri Lanka: A Study of Dry Mafic Granulites". Journal of Geology v 115: 563–582. doi:10.1086/519778.
- Brown, M., and Raith, M., 1996, First evidence of ultrahigh-temperature decompression from the granulite province of Southern India: Journal of the Geological Society, London, v. 153, p. 819–822.
- Morimoto, T., Santosh, M., Tsunogae, T., and Yoshimura, Y., 2004, Spinel + quartz association from the Kerala khondalites, southern India: Evidence for ultrahightemperature metamorphism: Journal of Mineralogical and Petrological Sciences, v. 99, p. 257–278.
- Tateishi, K., Tsunogae, T., Santosh, M. and Janardhan, A.S., 2004, First report of sapphirine+ quartz assemblage from southern India: Implications for ultrahigh- temperature metamorphism. Gondwana Research 7, 899-912.
- Sajeev, K., Osanai, Y. and Santosh, M. 2004, Ultrahigh-temperature metamorphism followed by two-stage decompression of garnet-orthopyroxene-sillimanite granulites from Ganguvarpatti, Madurai block, southern India. Contributions to Mineralogy and Petrology, v. 148, p. 29-46.
- Sajeev, K., Santosh, M. and Kim, H.S. 2006, Partial melting and P-T evolution of the Kodaikanal Metapelite Belt, southern India. Lithos v. 92, p. 465-483.
- Santosh, M., Sajeev, K., 2006. Anticlockwise evolution of ultrahigh-temperature granulites within continental collision zone in southern India. Lithos 92, 447–464.
- Shimpo, M., Tsunogae, T., Santosh, M., 2006. First report of garnet–corundum rocks from southern India: implications for prograde high-pressure (eclogite-facies?)metamorphism. Earth and Planetary Science Letters 242, 111–129.
- Prakash,D., Arima, M.and Mohan, A.2006, UHT metamorphism in the Palni Hills, South India: Insights from feldspar thermometry and phase equilibria. International Geology Review, v. 48, pp. 619-638.
- Prakash, D.; Arima, M.; Mohan, A. (2007). "Ultrahigh-temperature mafic granulites from Panrimalai, South India: Constraints from phase equilibria and thermobarometery". Journal of Asian Earth Sciences 29: 41–61. Bibcode:2007JAESc..29...41P. doi:10.1016/j.jseaes.2006.01.002.
- Osanai, Y., Nakano, N., Owada, M., Nam, T. N., Toyoshima, T., Tsunogae, T., and Binh, P., 2004, Permo-Triassic ultrahigh-temperature metamorphism in the Kontum Massif, central Viet Nam: Journal of Mineralogical and Petrological Sciences, v. 99, p. 225–241.
- Osanai, Y., Owada, M., Kamei, A., Hamamoto, T., Kagami, H., Toyoshima, T., Nakano N. and Nam T.N. 2006, The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo–Triassic metamorphic complexes in East Asia. Gondwana Research, v. 9, p. 152-166.
- Dunkley, D.J., Suzuki, K., Hokada, T., Kusiak, M.A., 2008, Contrasting ages between isotopic chronometers in granulites: Monazite dating and metamorphism in the Higo Complex, Japan, Gondwana Research, doi:10.1016/j.gr.2008.02.003.
- Droop, G. T. R., and Bucher-Nurminen, K., 1984, Reaction textures and metamorphic evolution of sapphirine-bearing granulites from the Gruf Complex, Italian Central Alps: Journal of Petrology, v. 25, p. 766–803.
- Hacker, B.R.; Gnos, L.; Grove, M.; McWilliams, M.; Sobolev, S.; Jiang, W.; Hu, Z. (2000). "Hot and dry xenoliths from the lower crust of Tibet". Science 287: 2463–2466. Bibcode:2000Sci...287.2463H. doi:10.1126/science.287.5462.2463.
- Clark, C., I.C.W. Fitzsimons, D. Healy, and S.L. Harley, 2011, How does the continental crust get really hot?, Elements, 7 (4), 235-240.
- Brown, M. and White, R.W. 2008, Processes in granulite metamorphism Journal of Metamorphic Geology, v. 26, p. 125-299.
- Sajeev, K. and Santosh, M. 2006, Extreme crustal metamorphism and related crust-mantle processes. Lithos v. 92 n. 3-4, p. 321-624.
- Santosh, M., Osanai, Y. and Tsunogae, T. 2004, Ultrahigh temperature metamorphism and deep crustal processes Journal of Mineralogical and Petrological Sciences v. 99 (part 1 & 2), n. 4-5, 137-365.
- Harley, S.L., 2008, Refining the P–T records of UHT crustal metamorphism. Geological Society, London, Special Publications, v. 138, p. 81-107.