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Gabbro ( ) is a phaneritic (coarse-grained), mafic (magnesium- and iron-rich), intrusion igneous rock formed from the slow cooling magma into a crystallinity mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro has the same chemical composition and mineralogy as rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at . Gabbro is also found as associated with continental volcanism. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.
Geologists use rigorous quantitative definitions to classify coarse-grained igneous rocks, based on the mineral content of the rock. For igneous rocks composed mostly of silicate minerals, and in which at least 10% of the mineral content consists of quartz, feldspar, or feldspathoid minerals, classification begins with the QAPF diagram. The relative abundances of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoid (F), are used to plot the position of the rock on the diagram. (2025). 9780521880060, Cambridge University Press. ISBN 9780521880060 The rock will be classified as either a gabbroid or a dioritoid if quartz makes up less than 20% of the QAPF content, feldspathoid makes up less than 10% of the QAPF content, and plagioclase makes up more than 65% of the total feldspar content. Gabbroids are distinguished from dioritoids by an anorthite (calcium plagioclase) fraction of their total plagioclase of greater than 50%.
The composition of the plagioclase cannot easily be determined Field work, and then a preliminary distinction is made between dioritoid and gabbroid based on the content of mafic minerals. A gabbroid typically has over 35% mafic minerals, mostly pyroxenes or olivine, while a dioritoid typically has less than 35% mafic minerals, which typically includes hornblende.
Gabbroids form a family of rock types similar to gabbro, such as monzogabbro, quartz gabbro, or nepheline-bearing gabbro. Gabbro itself is more narrowly defined, as a gabbroid in which quartz makes up less than 5% of the QAPF content, feldspathoids are not present, and plagioclase makes up more than 90% of the feldspar content. Gabbro is distinct from anorthosite, which contains less than 10% mafic minerals.
Coarse-grained gabbroids are produced by slow crystallization of magma having the same composition as the lava that solidifies rapidly to form fine-grained (aphanitic) basalt.
A more detailed classification is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are:
Gabbros are also sometimes classified as alkali or tholleiitic gabbros, by analogy with alkali basalt or tholeiitic basalts, of which they are considered the intrusive equivalents. Alkali gabbro usually contains olivine, nepheline, or analcime, up to 10% of the mineral content, while tholeiitic gabbro contains both clinopyroxene and orthopyroxene, making it a gabbronorite.
Gabbroids contain minor amounts, typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel. Apatite, zircon, and biotite may also be present as accessory minerals.
Gabbro is generally coarse-grained, with crystals in the size range of 1 mm or larger. Finer-grained equivalents of gabbro are called diabase (also known as dolerite), although the term microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse-grained to pegmatitic. Some pyroxene-plagioclase Cumulate rock are essentially coarse-grained gabbro, and may exhibit acicular crystal habits.
Gabbro is usually equigranular in texture, although it may also show ophitic texture (with laths of plagioclase enclosed in pyroxene).
Gabbro is much less common than more silica-rich intrusive rocks in the continental crust of the Earth. Gabbro and gabbroids occur in some but these rocks are relatively minor components of these very large intrusions because their iron and calcium content usually makes gabbro and gabbroid magmas too dense to have the necessary buoyancy. However, gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes as layered gabbro underling sheeted dike complexes and overlying ultramafic rock derived from the Earth's mantle. These layered gabbros may have formed from relatively small but long-lived underlying .
Layered gabbros are also characteristic of , which are large, saucer-shaped intrusions that are primarily Precambrian in age. Prominent examples of lopoliths include the Bushveld Complex of South Africa, the Muskox intrusion of the Northwest Territories of Canada, the Rum layered intrusion of Scotland, the Stillwater complex of Montana, and the layered gabbros near Stavanger, Norway. Gabbros are also present in stocks associated with alkaline volcanism of continental rifting.
Gabbro is known in the construction industry by the trade name of black granite.
The term "indigo gabbro" is used as a common name for a mineralogically complex rock type often found in mottled tones of black and lilac-grey. It is mined in central Madagascar for use as a semi-precious stone. Indigo Gabbro can contain numerous minerals, including quartz and feldspar. Reports state that the dark matrix of the rock is composed of a mafic igneous rock, but whether this is basalt or gabbro is unclear.
Distribution
Nearly all gabbros are found in plutonic bodies, and the term (as the International Union of Geological Sciences recommends) is normally restricted just to plutonic rocks, although gabbro may be found as a coarse-grained interior facies of certain thick lavas. (2025). 9781444330656, Wiley-Blackwell. ISBN 9781444330656 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 rock formed by settling of pyroxene and plagioclase.
An alternative name for gabbros formed by crystal settling is pyroxene-plagioclase adcumulate.
Uses
Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper . (1991). 9789401054843 ISBN 9789401054843 For example, the Merensky Reef is the world's most important source of platinum.
(1994). 9783540576266, Springer-Verlag. ISBN 9783540576266 However, gabbro is hard and difficult to work, which limits its use. (1982). 9780309032759 ISBN 9780309032759
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