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Gabbro () is a (coarse-grained), formed from the slow cooling of magnesium-rich and iron-rich into a mass deep beneath the 's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained . Much of the Earth's is made of gabbro, formed at s. Gabbro is also found as s associated with continental . 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 is to .


Etymology

The term "gabbro" was used in the 1760s to name a set of rock types that were found in the s of the in Italy. It was named after , a hamlet near in . Then, in 1809, the German geologist used the term more restrictively in his description of these Italian ophiolitic rocks. He assigned the name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" ( gabbro).


Petrology

Gabbro is a coarse-grained () that is relatively low in and rich in iron, magnesium, and calcium. Such rock is described as '. Gabbro is composed of and calcium-rich , with minor amounts of , , and . When present, hornblende is typically found as a rim around crystals or as large grains enclosing smaller grains of other minerals (' grains). 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 , , or minerals, classification begins with the . The relative abundances of quartz (Q), (A), plagioclase (P), and feldspathoid (F), are used to plot the position of the rock on the diagram. The rock will be classified as either a gabbroid or a 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 (calcium plagioclase) fraction ot their total plagioclase of greater than 50%. The composition of the plagioclase cannot easily be determined , 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 , , or . 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 , which contains less than 10% mafic minerals. Coarse-grained gabbroids are produced by slow crystallization of having the same composition as the that solidifies rapidly to form fine-grained () .


Subtypes

There are a number of subtypes of gabbro recognized by geologists. Gabbros can be broadly divided into leucogabbros, with less than 35% mafic mineral content; mesogabbros, with 35% to 65% mafic mineral content; and melagabbros with more than 65% mafic mineral content. A rock with over 90% mafic mineral content will be classified instead as an . A gabbroic rock with less than 10% mafic mineral content will be classified as an anorthosite. A more detailed classification is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are: * Normal gabbro (gabbro ''sensu stricto'') is composed almost entirely of plagioclase and (typically augite), with less than 5% each of hornblende, olivine, or . * is composed almost entirely of plagioclase and , with less than 5% each of hornblende, clinopyroxene, or olivine. * is composed almost entirely of plagioclase and olivine, with less than 5% each of pyroxene or hornblende. * is composed almost entirely of plagioclase and hornblende, with less than 5% each of pyroxene or olivine. Gabbros intermediate between these compositions are given names such as (for a gabbro intermediate between normal gabbro and norite, with almost equal amounts of clinopyroxene and orthopyroxene) or olivine gabbro (for a gabbro containing significant olivine, but almost no clinopyroxene or hornblende). A rock similar to normal gabbro but containing more orthopyroxene is called an orthopyroxene gabbro, while a rock similar to norite but containing more clinopyroxene is called a clinopyroxene norite. Gabbros are also sometimes classified as alkali or tholleiitic gabbros, by analogy with or basalts, of which they are considered the intrusive equivalents. Alkali gabbro usually contains olivine, nepheline, or , up to 10% of the mineral content, while tholeiitic gabbro contains both clinopyroxene and orthopyroxene, making it a gabbronorite.


Gabbroids

Gabbroids (also known as gabbroic-rocks) are a family of coarse-grained igneous rocks similar to gabbro: * Quartz gabbro contains 5% to 20% quartz in its QAPF fraction. One example is the ''cizlakite'' at in northeastern Slovenia, * Monzogabbro contains 65% to 90% plagioclase out of its total feldspar content. * combines the features of quartz gabbro and monzogabbro. It contains 5% to 20% quartz in its QAPF fraction, and 65% to 90% of its feldspar is plagioclase. * Foid-bearing gabbro contains up to 10% feldspathoids rather than quartz. "Foid" in the name is usually replaced by the specific feldspathoid that is most abundant in the rock. For example, a -bearing gabbro is a foid-bearing gabbro in which the most abundant feldspathoid is nepheline. * Foid-bearing monzogabbro resembles monzogabbro, but containing up to 10% feldspathoids in place of quartz. The same naming conventions apply as for foid-bearing gabbro, so that a gabbroid might be classified as a -bearing monzogabbro. Gabbroids contain minor amounts, typically a few percent, of iron-titanium oxides such as , , and . , , and 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 (also known as ), although the term ''microgabbro'' is often used when extra descriptiveness is desired. Gabbro may be extremely coarse-grained to . Some pyroxene-plagioclase are essentially coarse-grained gabbro, and may exhibit acicular crystal habits. Gabbro is usually in texture, although it may also show (with laths of plagioclase enclosed in pyroxene).


Distribution

Nearly all gabbros are found in plutonic bodies, and the term (as the recommends) is normally restricted just to plutonic rocks, although gabbro may be found as a coarse-grained interior of certain thick lavas. Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of and , or as part of a as a formed by settling of pyroxene and plagioclase. An alternative name for gabbros formed by crystal settling is ''pyroxene-plagioclase adcumulate''. Gabbro is much less common than more silica-rich intrusive rocks in the of the Earth. Gabbro and gabbroids occur in some s 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 complexes as layered gabbro underling es and overlying rock derived from the . These layered gabbros may have formed from relatively small but long-lived s underlying s. Layered gabbros are also characteristic of s, which are large, saucer-shaped intrusions that are primarily in age. Prominent examples of lopoliths include the of South Africa, the of the of Canada, the of Scotland, the of Montana, and the layered gabbros near , Norway. Gabbros are also present in s associated with of .


Uses

Gabbro often contains valuable amounts of , , , , , , and s. For example, the is the world's most important source of platinum. Gabbro is known in the construction industry by the trade name of ''black granite''. However, gabbro is hard and difficult to work, which limits its use.


See also

* * *


References


External links


Ocean drilling program gabbro petrology
{{Authority control Plutonic rocks