greenschist facies


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Related to greenschist facies: Metabasites

greenschist facies

[′grēn‚shist ′fā·shēz]
(petrology)
Any schistose rock containing an abundance of green minerals and produced under conditions of low to intermediate temperatures (300-500°C) and low to moderate hydrostatic pressures (3000-8000 bars).
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This is based on the following evidences: Field relations between the different lithotypes of metabasites reflect the occurrence of a prograde metamorphism, which is manifested by an increase of metamorphism from the greenschist facies (chlorite and actinolite schists with or without garnet, and metabasalts) to the amphibolite facies (amphibolites with or without garnet).
These are generally deformed and exhibit greenschist facies metamorphism with the development of epidotes- amphibolite facies metamorphism at places.
Epidotes sericite chlorite and tremolite/actinolite are the alteration products of phenocrysts and ground mass phases during greenschist facies metamorphism.
The second one corresponds to greenschist facies (Figure 30, represented by chlorite + muscovite + epidotesr-plagioclase (albite) + quartz + actinolite.
Most samples are metabasalts that belong to the chlorite zone of the greenschist facies, but other rock types include metagrey wackes and a metabasalt of the prehnite/ pumpellyite facies, and a glaucophanic metachert and glaucophanic metabasalt of the blueschist facies.
1996) noted that the grade of metamorphism is very low to low (lower greenschist facies at most), with localized higher grade areas related to contact metamorphism by late Neoproterozoic and Devonian plutons.
These rocks comprise mylonitic peridotite and sub-ophiolitic metamorphic rocks that range from garnetiferous amphibolites facies to amphibolite facies to greenschist facies and un- metamorphosed sediments.
Then a second reactivation of deformation and metamorphism under greenschist facies conditions occurred between 47 to 27 Ma.
It was initially metamorphosed to lower amphibolite facies and was later retrograded to lower greenschist facies.
The stratigraphic sequence is a biotite-rich turbidite metamorphosed to upper greenschist facies.
Like their host rocks, the sills show evidence for greenschist facies metamorphism, as indicated by the mineral assemblage chlorite, actinolite, epidote, and albitic plagioclase.