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Gabbro // 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.
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' or monzogabbros are also known to occur, for example the cizlakite at Pohorje in northeastern Slovenia, 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 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.
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'.
- Le Maitre, R. W.; et al., eds., 2005, Igneous Rocks: A Classification and Glossary of Terms, Cambridge Univ. Press, 2nd ed., p. 69, ISBN 9780521619486
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