hornblende—hornfels facies

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hornblende—hornfels facies A set of metamorphic mineral assemblages produced by contact metamorphism of a wide range of starting rock types under the same metamorphic conditions and typically characterized by the development of the mineral assemblage hornblendeplagioclase in rocks of basic igneous composition. Other rocks of contrasting composition, e.g. shales and 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 the lowpressure, moderate-temperature conditions met close to igneous intrusions.