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amphibolite facies 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 andesine (plagioclase)-hornblende 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 mineral P-T stability fields indicate that the facies represents a range of moderate-pressure, moderate- to high-temperature conditions.