granulite facies

granulite facies, one of the major divisions of the mineral facies classification of metamorphic rocks, the rocks of which formed under the most intense temperature-pressure conditions usually found in regional metamorphism. At the upper limit of the facies, migmatite formation may occur. Temperatures of 650–1,100 °C (1,200–2,000 °F) and pressures of 3 to 10 kilobars (1 kilobar equals about 15,000 pounds per square inch) may be reached. Under conditions of less intense metamorphism, rocks of the amphibolite facies (q.v.) are formed.

The minerals found in the rocks of the granulite facies include pyroxene, biotite, garnet, calcium plagioclase, and quartz or olivine. As in all metamorphic rocks, the composition of the parent rock exerts a strong control on the particular mineralogy that is observed. A characteristic of this facies is the low content of water, which has been forced out of the rock by the high pressure and temperature.