amphibolite

—amphibolitic /am fib'euh lit"ik/, adj.

/am fib"euh luyt'/, n. Petrog.

a metamorphic rock composed mainly of amphibole and plagioclase.

[1825-35; AMPHIBOLE + -ITE¹]

* * *

Igneous or metamorphic rock composed largely or dominantly of amphibole minerals.

For igneous rocks, the term hornblendite is usually used and is more restrictive; hornblende is the most common amphibole. Metamorphic amphibolites are more widespread and variable than igneous ones. Typically, they are medium-to coarse-grained and are composed of hornblende and plagioclase. Basic igneous rocks such as basalts and gabbros (see acid and basic rocks) can be the parent rocks of amphibolite.

* * *

▪ rock

      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). 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. Mafic igneous rocks (e.g., basalts and gabbros) and sedimentary dolomite can be the parent rocks of amphibolite.

* * *