epidote-amphibolite facies

epidote-amphibolite facies, one of the major divisions of the mineral-facies classification of metamorphic rocks, the rocks of which form under moderate temperature and pressure conditions (250°–400° C [500°–750° F] and up to 4 kilobars [1 kilobar equals about 15,000 pounds per square inch]). This facies grades into the greenschist facies under less intense metamorphic conditions and into the amphibolite facies with greater temperature and pressure. Minerals typical of the rocks of the epidote-amphibolite facies include biotite, almandite garnet, plagioclase, epidote, and amphibole. Chlorite, muscovite, staurolite, and chloritoid may also occur. Biotite, which can be identified easily, is used to identify the lower boundary of this facies; amphibole may also be used as an index mineral to identify the facies. Depending upon the composition of plagioclase, this facies may be subdivided into two subfacies, the albite-epidote-amphibolite subfacies and the oligoclase-epidote-amphibolite subfacies.