AMPHIBOLITE

a rock composed largely or dominantly of minerals of the amphibole group. The term has been applied to rocks of either igneous or metamorphic origin. In igneous rocks, the term hornblendite is more common and restrictive; hornblende is the most common amphibole and is typical of such rocks. Hornblendite is an ultramafic rock (dominantly dark minerals) and is also ultrabasic (low in silica content). True hornblendites contain little other than amphibole and are probably derived from the alteration of pyroxene and olivine. Metamorphic amphibolites are a more widespread and variable group of rocks formed through metamorphism. Typically, they are medium- to coarse-grained and are composed of hornblende and plagioclase. These are the diagnostic rocks of the amphibolite facies of regional metamorphism and may be derived from premetamorphic rocks of various types. Basic igneous rocks (e.g., basalts and gabbros) and sedimentary dolomite can be the parent rocks of amphibolite.