amphibolitefacies

Amphibolite facies is a metamorphic facies that describes rocks formed during regional metamorphism under medium to high temperature and pressure conditions. It is most readily recognized by mineral assemblages that include amphibole, typically hornblende, together with plagioclase and quartz; biotite is common, and accessory minerals such as epidote, garnet, magnetite, titanite, and rutile may occur. The presence of hornblende distinguishes amphibolite facies from the lower greenschist facies, which contains chlorite and actinolite.

The temperatures and pressures characteristic of amphibolite facies generally fall in roughly the range of 500 to 750 degrees Celsius and several kilobars of pressure (often about 2 to 10 kbar), though exact boundaries vary with rock composition and tectonic setting. The rocks may record a range of P–T conditions within the facies, and they commonly preserve deformation features such as foliations formed during regional metamorphism.

Lithologies in amphibolite facies are diverse. Mafic rocks metamorphose into amphibolites with hornblende-bearing assemblages, while pelitic rocks can develop garnet- or hornblende-rich assemblages in which plagioclase, quartz, and biotite are common constituents. The facies is important in reconstructing tectonic histories because it marks a relatively high-temperature, mid- to high-pressure metamorphic stage, typically associated with mountain-building processes and crustal thickening.

Relation to other facies: amphibolite facies lies above greenschist and below granulite in the conventional metamorphic sequence. It is part of the broader framework of metamorphic facies used to interpret the pressure–temperature paths of rocks.