More properly greenschist refers to a grade or facies of Metamorphism. Greenschist facies results from low temperature, moderate pressure metamorphism. Temperatures of approximately 400 to 500 °C and depths of about 8 to 50 kilometers. Lower temperatures give rise to the Prehnite-pumpellyite facies and higher temperatures lead to to epidote-Amphibolite facies. Lower pressure, normally contact metamorphism produces albite-epidote Hornfels while higher pressures at great depth produces Eclogite.
The greenschist mineral assemblage depends on primary rock composition. A Mafic rock such as Basalt is altered to an assemblage of chlorite, Actinolite, epidote and albite. An Ultramafic rock such as Peridotite would be altered to an assemblage of Serpentine, Talc, Tremolite, Diopside and Brucite along with the persistant chlorite. Mudstones or siltstones develope an assemblage of Quartz, Plagioclase, chlorite, mica, Garnet, Pyrophyllite and some Graphite. In carbonate terrains the sequence consists of Calcite, Dolomite, quartz, and micas. In greater detail the greenschist facies is subdivided into subgreenschist, lower and upper greenschist.
Oceanic basalts in the vicinity of mid-ocean ridges typically exhibit greenschist alteration. The greenstone belts of the various Archean cratons are commonly altered to the greenschist facies. These ancient rocks are noted as host rocks for a variety of ore deposits in South Africa and Canada.