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.
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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 + sillimanite + quartz, sapphirine + quartz or spinel + quartz, generally at pressure conditions of sillimanite stability in metapelites [after 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, 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.
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. 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.
Recent hypothesis
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.