prehnite–pumpellyite facies

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prehnite–pumpellyite facies A set of metamorphic mineral assemblages produced by metamorphism of a wide range of starting rock types under the same metamorphic conditions and typically characterized by the development of the mineral assemblage prehnite (Ca2Al[Si3AlO10](OH)2), pumpellyite (Ca2Al2 (Mg,Fe2+,Fe3+,Al)[SiO4][Si2O7](OH)2(H2O.OH)) and quartz, with relict plagioclase and pyroxene, in rocks of basic igneous composition. Other rocks of contrasting composition (e.g. 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 pressure—temperature stability fields indicates that the facies represents a range of moderate pressure (2.5–5.0 kb), low temperature (150–300°C) conditions usually developed during burial metamorphism in thick sedimentary sequences on continental margins and in intracontinental basins.