Granulite

Granulite is a class of high-grade metamorphic rocks formed under granulite facies conditions, typically at high temperatures and relatively high pressures in the lower to middle crust. The term refers to both the rocks themselves and the metamorphic environment that produces them. Granulites are characterized by coarse to very coarse grains and a granuloblastic texture, in which mineral grains have a tightly interlocking, mosaic-like fabric.

Mineral assemblages in granulites vary with their protolith. Mafic granulites commonly contain orthopyroxene, clinopyroxene, plagioclase, and garnet, with accessory rutile or magnetite. Pelitic (aluminous) granulites typically feature garnet, sillimanite or cordierite, orthopyroxene, plagioclase, and quartz, and may also include spinel or hercynite. Many granulites preserve evidence of partial melting, producing leucosome bands or pods that indicate dehydration melting during high-temperature metamorphism.

Texture and origin. Granulites are usually medium to coarse-grained and may display a granoblastic texture with interlocking minerals. Dehydration melting during granulite facies metamorphism can generate granitic melts, leaving a residuum that records the high-temperature, relatively dry conditions of formation. The presence of melts is a key feature in some granulite belts and helps distinguish granulite gneisses from non-melting high-grade rocks.

Formation conditions and occurrence. Granulite facies forms at temperatures roughly 700–1000 °C and pressures around roughly 6–12 kilobars, though estimates vary by geographic setting. It commonly develops in thickened crust during regional metamorphism or in crustal segments subjected to high-temperature, low-water environments. Granulite rocks are widespread in ancient continental crust and are a major component of many Precambrian terrains.

Relation to other metamorphic facies. Granulite is distinct from amphibolite, which forms at lower temperatures, and from eclogite, which forms under high-pressure conditions often associated with subduction. Granulites provide records of high-temperature crustal evolution and deep-crustal processes.