metamorphic facies

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metamorphic facies A set of metamorphic mineral assemblages derived from rocks of contrasting composition which have been subjected to the same grade or conditions of metamorphism. For example, a metamorphosed shale, basic lava, and limestone which are found adjacent to each other in a metamorphic terrain must have been subjected to the same grade of metamorphism, yet each of these contrasting rock types displays a different metamorphic mineral assemblage, the individual assemblages reflecting both the starting rock composition and the grade of metamorphism. Since all three rocks have been subject to the same grade of metamorphism, the set of contrasting mineral assemblages reflects only the contrasts in rock composition, and thus constitutes a metamorphic facies. Any change in the mineral assemblage observed in a particular rock composition represents a mineralogical response to changing metamorphic conditions and thus would define a new facies. The metamorphic conditions represented by a particular facies can be deduced from experimental studies of the overlapping pressure—temperature stability fields of mineral assemblages in that facies. However, the definition of a facies is purely descriptive and based entirely on the mineral assemblages observed. The concept of metamorphic facies was first proposed by the Finnish petrologist P. E. Eskola in 1920 after he had compared the mineral assemblages in similar rock compositions from Oslo, Norway, and Orijarvi, Finland, two areas which had been subjected to contrasting metamorphic conditions.