granulaatit

Granulaatit, known in English as granulites, are high-grade metamorphic rocks that form under granulite facies conditions. They typically develop at high temperatures, around 700–1000°C, and at relatively low to moderate pressures within the lower to middle crust. The name reflects their grainy, interlocking texture. A defining feature is their general lack of hydrous minerals, which distinguishes them from many other metamorphic rocks.

Mineral assemblage and texture commonly include garnet, orthopyroxene, clinopyroxene, and plagioclase, with quartz present in felsic varieties. Granulaatit display a granuloblastic texture, where mineral grains are intergrown in a mosaic due to high-temperature deformation and reaction histories. Dehydration melting can occur locally, leading to minor melts that further drive recrystallization and texture development.

Formation and varieties: Granulaatit can form during regional metamorphism at granulite facies or through partial melting of crustal rocks. They are often categorized into mafic granulites (rich in pyroxenes and garnet) and felsic granulites (more feldspathic). They may occur as xenoliths within igneous rocks or as large crustal blocks within metamorphic terrains.

Occurrence and significance: Granulaatit are widespread in ancient continental crust, especially in Precambrian shield regions and granulite terrains. They provide important clues about deep-crust conditions, crustal thickening, and the thermal evolution of the crust.

Etymology: The term granulite derives from Latin granulum, meaning a small grain, reflecting their characteristic coarse-grained yet granular texture.