A set of metamorphic mineral assemblages produced by metamorphism of a wide range of starting rock types under the same high-pressure/low-temperature metamorphic conditions and typically characterized by the development of the mineral assemblage glaucophane–lawsonite–quartz in rocks of basic igneous composition. Other rocks of contrasting composition, for instance shales or limestones, would each develop their own specific mineral assemblage, even though they are all being metamorphosed under the same conditions. The variation of mineral assemblage with starting rock composition reflects a particular range of pressure, temperature and P(H2O) conditions. Experimental studies of mineral P–T stability fields indicate that the facies represents high-pressure/low-temperature conditions which can be met during subduction of oceanic crust under continental crust. Because rocks of basic igneous composition within this facies are characterized by the blue-coloured mineral glaucophane, the facies is sometimes referred to as the ‘blueschist facies’.
Subjects: Earth Sciences and Geography.