On ultrahigh-temperature crustal metamorphism

Abstract

Ultrahigh-temperature (UHT) metamorphism is the most thermally extreme type of crustal metamorphism, with the crust capable of withstanding temperatures ≥ 900 °C. Mineral assemblages diagnostic of UHT metamorphism commonly occur in Mg–Al-rich rock compositions that are unfortunately relatively rare in nature. These include sapphirine + quartz, orthopyroxene + sillimanite ± quartz and osumilite. However, UHT metamorphism has been diagnosed using more common garnet + aluminous orthopyroxene assemblages, as well as ternary feldspars and metamorphic pyroxenes. The worldwide number of UHT localities exceeds 40, and may continue to increase as petrologists apply new retrieval methods for extracting information from mineral assemblages in conjunction with mineral chemistry, e.g. the aluminium content of orthopyroxene, and calculated phase equilibria, based on thermodynamic datasets that continue to be refined and improved. This contribution presents a review of UHT metamorphism, including: 1) the history of experiments that have ultimately lead to the precise P–T constraints we can now place on the generation and evolution of UHT mineral assemblages; 2) the diagnostic assemblages; 3) the age distribution of UHT metamorphism; 4) the use of calculated phase equilibria to constrain the evolution of UHT rocks; 5) the duration of UHT metamorphic episodes, which is a very active field of research at present; and, 6) the tectonic scenarios that have been proposed for the generation of UHT conditions in the deep crust. The two fundamental types of orogenic systems, namely accretionary and collisional, have been proposed to be potential sites for UHT metamorphism. In contrast to current geodynamic models that are typically unable to account for UHT metamorphic conditions in the deep crust, it may be possible that UHT metamorphism can occur during ‘normal’ tectonic events. If UHT metamorphism can occur on a regional scale during ‘normal’ tectonism, it is important to understand all aspects of UHT metamorphism and the implications it has for lithospheric rheology, crust–mantle interactions and the geodynamics of granulite facies metamorphism.