Blueschists

Blueschists are metamorphic rocks that form under high-pressure, low-temperature conditions, typically in subduction zone environments. They are named for their characteristic blue minerals, most notably the blue amphibole glaucophane, which gives the rock its distinctive color.

The mineral assemblage of blueschists commonly includes glaucophane and lawsonite, together with epidote, chlorite, and phengite. The exact composition varies with fluid availability and the starting material, but the presence of blue amphibole is a defining feature. Blueschists are often leaflet or schistose, exhibiting pronounced foliation that records strong shear during subduction.

Formation and conditions: Blueschist facies represent high-pressure, relatively cool metamorphism associated with subducting oceanic crust. P–T conditions are generally high pressure with low to moderate temperatures, roughly on the order of 0.5–1.5 GPa and about 200–500°C. Fluids released during subduction promote the growth of hydrous minerals such as lawsonite, stabilizing the blueschist assemblage in many cases. The rocks typically originate from mafic to intermediate shales or basalts.

Occurrence and significance: Blueschists occur in many subduction-related terranes around the world. Classic exposures include the blueschist belts of the Franciscan Complex in California and extensions in the Alps. They provide important records of subduction-zone pressures and temperatures and of tectonic processes that transport and then exhume oceanic crust.

Relation to other metamorphic facies: Blueschist metamorphism lies within a continuum of subduction-zone metamorphism. If rocks experience higher temperatures or deeper burial, they may progress to greenschist, amphibolite, or eclogite facies, and later be exhumed to the surface.