In geology ultrahigh-temperature metamorphism (UHT) is extreme crustal metamorphism with metamorphic temperatures exceeding 900 °C.Brown, M., 2007, Metamorphic conditions in orogenic belts: a record of secular change. International Geology Review 49, 193-234Santosh, M., Omori, S., 2008a, CO₂ flushing: a plate tectonic perspective. Gondwana Research 13, 86-102 Granulite-

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

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.

Definition

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 The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

+ sillimanite + quartz,

sapphirine Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula ( (with iron as a major impurity). Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed c ...

+ quartz or

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

+ quartz, generally at pressure conditions of sillimanite stability in meta pelites fter Brown (2007) following proposal by Harley (1998)

Identification

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, osumilite 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.

Global distribution

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.Kelsey, D.E., 2008, On ultrahigh-temperature crustal metamorphism. Gondwana Research 13, 1-29Santosh, 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, 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 The North China Craton is a continental crustal block with one of Earth's most complete and complex records of igneous, sedimentary and metamorphic processes. It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea. The ...

(during the

accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

of the supercontinent

Columbia Columbia may refer to: * Columbia (personification), the historical female national personification of the United States, and a poetic name for America Places North America Natural features * Columbia Plateau, a geologic and geographic region in ...

),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. 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 The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

Gruf Complex, central Alps. Three-million-year-old xenoliths erupted in Qiangtang imply that UHT metamorphism is ongoing beneath central Tibet.

Recent hypothesis

A correlation was proposed between the episodic formation of UHT metamorphic rocks and the episodic assembly of supercontinents in the Precambrian. However, inspection of extreme metamorphism at convergent plate margins indicates that supercontinental assembly is associated with regional HP to UHP eclogite-facies metamorphism at low thermal gradients of less than 10 °C/km, whereas continental rifting plays a crucial role in causing regional HT to UHT granulite-facies metamorphism at high thermal gradients of greater than 30 °C/km. In this regard, the episodic formation of HT to UHT granulite-facies metamorphic rocks is temporally and spatially coupled with the breakup or attempting rupture of supercontinents in the

plate tectonic 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 large te ...

s context. Because UHT rocks are generally characterized by low water contents, this led to an illusion for the involvement of CO₂-rich fluids in generating diagnostic UHT assemblages according to the finding of abundant pure CO₂ fluid inclusions in these rocks. However, the extraction of liquid phases such as aqueous solutions and hydrous melts from anatectic systems during UHT metamorphism is so efficient that the common occurrence of pure CO₂ fluid inclusions looks as if the incoming CO₂ could have buffered the water activity and stabilized the

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

mineralogy of UHT rocks. Anatectic melts were variably extracted from anatectic systems, leading to granulite-migmatite-granite associations in accretionary and collisional orogens.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. Metamorphic core complexes were emplaced due to the buoyant entrainment of granitic melts. The abundant water was liberated by heating dehydration of the lowest orogenic crust, contributing aqueous solutions to amphibolite-facies retrogression of the overlying crust.

Definition

Identification

Global distribution

Recent hypothesis

References

Further reading