Metagabbro

Metagabbro is gabbro, a coarse-grained mafic igneous rock, that has undergone metamorphism. The original pyroxene and calcium-rich plagioclase partly or fully recrystallize into metamorphic minerals such as amphibole, epidote, garnet, and sodic plagioclase, while often preserving the coarse igneous texture.

Because gabbro shares basalt's chemistry, metagabbro develops similar mineral assemblages and is closely related to amphibolite and, under high pressure, to eclogite. It commonly appears dark grey-green to greenish-black with a mottled or patchy fabric.

Metagabbro is widespread in metamorphosed igneous complexes, ophiolites, and deep crustal sections, recording the alteration of intrusive mafic rocks.

Metagabbro forms when gabbroic intrusions are subjected to heat, pressure, and fluids during regional metamorphism, seafloor alteration, or subduction.

During metamorphism, pyroxene reacts to form amphibole (commonly hornblende or actinolite), and calcic plagioclase breaks down into sodic plagioclase plus epidote or other minerals. Distinctive 'corona' textures, where reaction rims grow around original crystals, are common in partially metamorphosed metagabbro.

At high pressure in subduction zones, metagabbro can transform into eclogite with garnet and omphacite. Metagabbro is found in ophiolite complexes, Precambrian shields, and deep-crustal terranes worldwide, including the Alps, Norway, and the Canadian Shield.

Identify metagabbro by its coarse, dark, mafic character combined with metamorphic minerals such as amphibole and epidote, often with a greenish tint and mottled texture.

Relict igneous texture (interlocking coarse crystals) frequently survives, and corona reaction rims around former pyroxene or olivine grains are diagnostic when present. The rock is hard and dense.

Look-alikes include fresh gabbro (which shows unaltered pyroxene and lacks amphibole-epidote assemblages), amphibolite (more thoroughly recrystallized and often foliated), and metabasalt (finer grained, from a volcanic protolith).

Metagabbro is used much like gabbro as crushed stone, aggregate, and durable dimension stone, sometimes marketed under trade names for countertops and facing stone because of its dark, attractive appearance.

Its study is important for understanding the metamorphism of the lower oceanic crust, ophiolites, and subduction processes, since metagabbro records how deep mafic rocks respond to changing pressure and temperature.

Metagabbro is not a gem material, though it can host minor ore minerals and is of interest to petrologists and collectors of metamorphic rocks.