Gabbro

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Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.
Gabbro classification on QAPF diagram
Close-up of gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.
Photomicrograph of a thin section of gabbro.

Gabbro /ˈɡæbr/ refers to a large group of dark, often phaneritic (coarse-grained), mafic intrusive igneous rocks chemically equivalent to plutonic basalt. It forms when molten magma is trapped beneath the Earth's surface and slowly cools into a holocrystalline mass.

The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basaltic magmatism at mid-ocean ridges that produce ophiolites. It is also present in association with continental volcanism. Due to its variant nature, the term "gabbro" may loosely be applied to a wide range of rocks, many of which are merely "gabbroic", technically speaking. In comparison, their extrusive counterparts, basalt and andesite cover most of the gabbroic designations.

Mineral assemblage of igneous rocks

Etymology[edit]

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, Massachusetts, U.S.A.

Petrology[edit]

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, and minor amounts of amphibole and olivine.

The pyroxene content is mostly clinopyroxene, generally augite, but small amounts of orthopyroxene may also be present. If the amount of orthopyroxene is more than 95% of the total pyroxene content (5% or less clinopyroxene content), then the rock is termed norite. On the other hand, Gabbro has more than 95% of its pyroxenes in the form of the monoclinic clinopyroxene/s. Intermediate rocks are termed gabbro-norite. The calcium rich plagioclase feldspar (labradorite-bytownite) and pyroxene content vary between 10% - 90% in gabbro. If more than 90% plagioclase is present, then the rock is an anorthosite. If on the other hand, the rock contains more than 90% pyroxenes (often both are present), it is termed pyroxenite. Gabbro may also contain small amounts of olivine ("olivine gabbro" if substantial amount of olivine is present), amphibole and biotite. The quartz content in gabbro is less than 5% of total volume. '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 minerals nepheline, cancrinite, and sodalite as accessory minerals rather than quartz. (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, some 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.

Zuma Rock, Nigeria, a massive uniform intrusion

Distribution[edit]

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 adcumulate.

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[edit]

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'.

See also[edit]

External links[edit]