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Greenschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form the typical greenschist facies minerals chlorite, actinolite, and albite. Greenschist facies results from low temperature, moderate pressure metamorphism.
The greenschist facies is at low pressure and temperature. The facies is named for the typical schistose texture of the rocks and green colour of the minerals chlorite, epidote and actinolite. Characteristic mineral assemblages are: In metabasites: chlorite + albite + epidote ± actinolite, quartz; In metagreywackes:
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. [1]
It initially undergoes low-grade metamorphism to metabasalt of the zeolite and prehnite-pumpellyite facies, but as the basalt subducts to greater depths, it is metamorphosed to the blueschist facies and then the eclogite facies. Metamorphism to the eclogite facies releases a great deal of water vapor from the rock, which drives volcanism in the ...
In his pioneering work on metamorphic facies in the Scottish Highlands, G.M. Barrow identified the chlorite zone as the zone of mildest metamorphism. [12] In modern petrology, chlorite is the diagnostic mineral of the greenschist facies. [10] This facies is characterized by temperatures near 450 °C (840 °F) and pressures near 5 kbar. [13]
Most likely, fluids have been produced under prograde greenschist- to amphibolite-facies metamorphism (220–450 °C and 1–5 Kbar). [3] The generally low salinity of the hydrothermal fluids can be attributed to devolatilization of minerals associated with metamorphic phase reactions, involving dehydration and decarbonisation. [26]
The alteration occurs during either late-stages of magmatic crystallization at low temperatures (< 500 °C) or during low-grade metamorphic events (sub-greenschist facies metamorphism). The reaction is paramorphic, meaning that the structure of the mineral is modified, but its original chemistry is retained. [2]
Petrogenetic grid for metapelites (click to zoom). [1] [2] Each line represents a metamorphic reaction.Metamorphic facies included are: BS = Blueschist facies, EC = Eclogite facies, PP = Prehnite-Pumpellyite facies, GS = Greenschist facies, EA = Epidote-Amphibolite facies, AM = Amphibolite facies, GRA = Granulite facies, UHT = Ultra-High Temperature facies, HAE = Hornfels-Albite-Epidote facies ...