greenschist facies

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greenschist 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: chlorite—*actinolite—*albite—*epidote—*quartz 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 indicates that the facies represents a range of moderate-pressure (about 4–7 kb), moderate-temperature (about 400–500 °C) conditions, usually developed in orogenic belts associated with continent—continent or ocean—continent plate collision zones.

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