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.
Read more about this topic: Ultra-high-temperature Metamorphism