gabbro

A usually coarse-grained igneous rock composed chiefly of calcic plagioclase and pyroxene. Also called norite.

[Italian, perhaps from Latin glaber, bald, beardless.]

The plutonic equivalent of its more abundant extrusive equivalent, basalt. Because it crystallized from a magma intruded deep within the crust, gabbro has a grain size visible to the naked eye with approximately equal amounts of calcic plagioclase (with 50% or more anorthite, the calcium aluminium feldspar) and pyroxene. Olivine is common as an early crystallized mineral, but either nepheline or quartz could be a late-stage crystallization product found in the matrix. Hornblende or biotite is commonly formed as an alteration product of pyroxene during the late stages of the magmatic crystallization, when water becomes enriched in the residual magma. See also Biotite; Hornblende.

Gabbro is found in diverse tectonic environments, ranging through oceanic ridges, convergent plate boundaries, stable continents, and rifts. The forms of the intrusive gabbro bodies include dikes, sills, pipes, laccoliths, stocks, batholiths, and large layered intrusive complexes.

The grain size of gabbroic rocks ranges from a millimeter to centimeters. The finer-gained gabbro is commonly referred to as a diabase (or dolerite in the United Kingdom) that usually has small granular pyroxene interstitially enclosed by randomly oriented laths of calcic plagioclase. This diabasic texture results from the faster cooling of the magma due to its injection as small dikes and sills in shallow crust. In coarser-grained gabbro found in large plutons injected in deeper crust, the pyroxenes are larger and enclose partially (a texture called subophitic) or fully (a texture called ophitic) randomly oriented labradorite. Gabbros with mineral gains larger than 2 cm (0.8 in.) are rare, but such rocks are referred to as gabbro pegmatites. At contacts with country rock, the gabbro is commonly very fine grained or glassy because of the fast chilling of the magma. In some plutons, especially near their margins, gabbroic minerals are aligned perpendicular to the walls, yielding comb layering. See also Dolerite; Magma; Pluton.

Unlike basalts, which are found in surface or near-surface environments, gabbros are found as shallow-to-deep intrusive bodies. Large blocks measuring more than 10 km (6 mi) thick, with a suite of rocks including serpentinite, pillow lavas, and chert, contain gabbro. This suite, called ophiolites, is commonly found on continents along convergent plate boundaries; it is thought to have been tectonically emplaced by thrusting onto the continental margins. It consists of a suite of rocks that are believed to represent the oceanic crust and upper mantle. See also Chert; Ophiolite; Serpentinite; Stratigraphy.

A coarse-grained, basic, igneous rock composed chiefly of calcium-rich plagioclase and pyroxene. Gabbro is formed through the crystallization of basaltic magma, usually as a large igneous intrusion deep within the earth's crust. The Black Cuillins of Skye furnish a British example.

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Igneous rock similar to diorite, predominantly composed of ferromagnetic minerals with crystals visible to the eye; has the same mineral composition as basalt.

A course-grained igneous rock rich in the minerals olivine, pyroxene, and plagioclase feldspar.

Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.

Close-up of gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.

Photomicrograph of a thin section of gabbro.

Gabbro (pronounced /ˈɡæbroʊ/) refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.

The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges.

Contents 1 Petrology 2 Distribution 3 Uses 4 Etymology 5 See also 6 External links

Petrology

A gabbro landscape on the main ridge of the Cuillin, Isle of Skye, Scotland.

Gabbro as a xenolith in a granite, eastern Sierra Nevada, Rock Creek Canyon, California.

Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in a large amount).

The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then a norite. Quartz gabbros are also known to occur and are probably derived from magma that was over-saturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid mineral nepheline. (Silica saturation of a rock can be evaluated by normative mineralogy). Gabbros contain minor amounts, typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.

Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called diabase, although the vernacular term microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse grained to pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro, although these may exhibit acicular crystal habits.

Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.

Distribution

Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.

Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes as parts of zones III and IV (sheeted dyke zone to massive gabbro zone). Long belts of gabbroic intrusions are typically formed at proto-rift zones and around ancient rift zone margins, intruding into the rift flanks. Mantle plume hypotheses may rely on identifying mafic and ultramafic intrusions and coeval basalt volcanism.

Uses

Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.

Ocellar varieties of gabbro can be used as ornamental facing stones, paving stones and it is also known by the trade name of 'black granite', which is a popular type of graveyard headstone used in funerary rites. It is also used in kitchens and their countertops, also under the misnomer of 'black granite'.

Etymology

Gabbro was named by the German geologist Christian Leopold von Buch after a town in the Italian Tuscany region. Essexite is named after the type locality in Essex County, MA, USA.

See also

• Peridotite
• Igneous differentiation
• Fractional crystallisation

External links

Wikimedia Commons has media related to: Gabbro

• Ocean drilling program gabbro petrology
• Scientists find the elusive gabbro

v • d • e Igneous rocks by composition Type Mafic < 52% SiO2 Intermediate 52-63% SiO2 Intermediate-Felsic 63-69% SiO2 Felsic >69 % SiO2 Volcanic rocks: Dikes & Sills: Plutonic rocks: Basalt Diabase Gabbro Andesite Diorite Dacite Granodiorite Rhyolite Aplite — Pegmatite Granite

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