AMPHIBOLITE FACIES

one of the major divisions of the mineral-facies classification of metamorphic rocks, the rocks of which formed under conditions of moderate to high temperatures (500 C, or about 950 F, maximum) and pressures. Less intense temperatures and pressures form rocks of the epidote-amphibolite facies, and more intense temperatures and pressures form rocks of the granulite facies. Amphibole, diopside, epidote, plagioclase, almandite and grossularite garnet, and wollastonite are minerals typically found in rocks of the amphibolite facies. The disappearance of epidote and increase in calcium in plagioclase are characteristic chemical changes as metamorphic intensity increases through this facies. Water is usually lost from the parent rock as these changes take place. Amphibolite facies rocks are widely distributed in Precambrian gneisses (those 540 million to 3.8 billion years old); they are interpreted as having formed in the deeper parts of folded mountain belts.