Granulites

Granulites are high-grade metamorphic rocks that form under elevated temperatures and relatively low to intermediate pressures within the granulite facies. They are characterized by a coarse, granular or granuloblastic texture in which mineral grains are typically equant and interlock, indicating intense recrystallization at high temperature and low water activity.

Mineral assemblages in granulites commonly include garnet, orthopyroxene and clinopyroxene with plagioclase and quartz; in felsic granulites, feldspar-rich matrices are typical, whereas mafic granulites may be dominated by pyroxenes and garnet. Hydrous minerals such as biotite, muscovite and cordierite are generally absent or rare because dehydration accompanies granulolite-facies metamorphism. Sillimanite or kyanite may occur locally in high-temperature granulites.

Granulites form in deep crustal conditions during tectonically hot, dry metamorphism, and many represent remnants of former lower crust. They often arise from partial melting of dry rocks, producing granitic melts that segregate into felsic granulite bodies. Exsolution of early melts can leave behind migmatitic or leucosome and melanosome textures.

Common rock types within the granulite category include charnockites (alkaline orthopyroxene-bearing granulites) and enderbites, as well as various mafic granulites. The granulite facies is contrasted with higher-pressure eclogite facies and with lower-temperature, hydrous greenschist and amphibolite facies.

Granulites occur in ancient continental crust worldwide, especially in Precambrian metamorphic terrains and orogenic belts. Their study helps constrain crustal temperatures, pressures, and the thermal history of deep crustal sections.