blueschist

Blueschist is a metamorphic rock that forms under high-pressure, low-temperature conditions typical of subduction zones. It is named for the blue color imparted by the characteristic blueschist minerals, especially glaucophane, a blue amphibole.

Blueschists form when oceanic or continental crust is buried to substantial depths, reaching pressures of roughly 0.5–1.2 gigapascals and temperatures around 200–500°C. Under these conditions mineral assemblages stable in the blueschist facies crystallize, most notably glaucophane and often lawsonite, together with epidote, phengite, chlorite, albite, and quartz.

Texture and identification: blueschist rocks are typically schistose, with flattened, platy minerals aligned by deformation. The presence of glaucophane gives the rock its distinctive blue hue, making blueschist a recognizable indicator of high-pressure, low-temperature metamorphism. Lawsonite-bearing blueschists provide important constraints on pressure–temperature history.

Occurrence and significance: blueschist facies rocks occur in ancient subduction complexes and metamorphic belts around the world. Classic examples include the Franciscan Complex of California and various units in the Alps and other orogenic belts. The blueschist record helps reconstruct subduction zone dynamics, including burial depths, tectonic burial rates, and subsequent exhumation during or after orogenic collision.

Relationship to other metamorphic processes: blueschist represents an early, high-pressure stage in subduction zones, typically preceding progression to higher-temperature assemblages such as eclogite if deeper burial occurs, or returning to lower temperatures during exhumation.