A set of metamorphic mineral assemblages produced by the metamorphism of a wide range of starting rock types under the same metamorphic conditions, and typically characterized by the development of the mineral assemblage smectite–zeolites (in addition to relict igneous plagioclase and pyroxene) in rocks of basic igneous composition. Other rocks of contrasting composition, e.g. shales, would develop their own specific mineral assemblages, even though they are all 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 a range of conditions involving low pressures (1–4 kb) and low temperatures (300–500 °C), usually developed in (a) thick sedimentary sequences on continental margins, (b) rock sequences subjected to hot water-convecting systems, and (c) areas of tectonic burial at the leading edge of thrust sheets.