Editing Gabbro (section)

==Petrology==
[[File:Mineralogy igneous rocks EN.svg|thumb|upright=1.4|Mineral assemblage of igneous rocks]]

Gabbro is a coarse-grained ([[phaneritic]]) [[igneous rock]] that is relatively low in [[silica]] and rich in iron, magnesium, and calcium. Such rock is described as ''[[mafic]]''. Gabbro is composed of [[pyroxene]] (mostly clinopyroxene) and calcium-rich [[plagioclase]], with minor amounts of [[hornblende]], [[olivine]], orthopyroxene and [[accessory minerals]].<ref>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=gabbro}}</ref> With significant (>10%) olivine or orthopyroxene it is classified as olivine gabbro or gabbronorite respectively. Where present, hornblende is typically found as a rim around [[augite]] crystals or as large grains enclosing smaller grains of other minerals (''[[poikilitic]]'' grains).<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=gabbro}}</ref><ref name=BlattTracy1996>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=53}}</ref>

{{anchor|Gabbroid}}
[[File:Gabbroid qapf.png|thumb|QAPF diagram with the gabbroid/dioritoid fields highlighted in yellow. Gabbroids are distinguished from dioritoids by an anorthite content of greater than 50% of their plagioclase.]]
[[File:Gabbro qapf.png|thumb|QAPF diagram with the gabbro field highlighted in yellow. Gabbro is distinguished from diorite by an anorthite content of greater than 50% of its plagioclase and from anorthosite by a mafic mineral content greater than 10%.]]
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.<ref name="lebas-streckeisen-1991">{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen|first2=A. L.|title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–833|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|citeseerx=10.1.1.692.4446|s2cid=28548230}}</ref><ref name="bgs">{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref><ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |edition=2nd |pages=139–143}}</ref> 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%.{{sfn|Jackson|1997|loc="gabbroid"}}

The composition of the plagioclase cannot easily be determined [[Field work|in the field]], 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.{{sfn|Blatt|Tracy|1996|p=71}}

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.{{sfn|Jackson|1997|loc="gabbro"}}<ref name="lebas-streckeisen-1991"/><ref name="bgs"/>

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]].<ref name="lebas-streckeisen-1991"/><ref name="bgs"/>

=== 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 [[ultramafic rock]]. A gabbroic rock with less than 10% mafic mineral content will be classified as an anorthosite.<ref name="bgs"/>{{sfn|Philpotts|Ague|2009|p=142}}

A more detailed classification is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are:<ref name="bgs"/>{{sfn|Philpotts|Ague|2009|p=142}}
* Normal gabbro (gabbro ''sensu stricto''<ref name="bgs"/>) is composed almost entirely of plagioclase and [[clinopyroxene]] (typically augite), with less than 5% each of hornblende, olivine, or [[orthopyroxene]]. 
* [[Norite]] is composed almost entirely of plagioclase and [[orthopyroxene]], with less than 5% each of hornblende, clinopyroxene, or olivine.
* [[Troctolite]] is composed almost entirely of plagioclase and olivine, with less than 5% each of pyroxene or hornblende.
* [[Hornblende gabbro]] 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 [[gabbronorite]] (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.<ref name="bgs"/>

[[File:Main ridge of the cuillin in skye arp.jpg|thumb|right|A gabbro landscape – the main ridge of the [[Cuillin]], [[Isle of Skye]], [[Scotland]]]]
[[File:Čizlakit.jpg|230px|thumb|right|Cizlakite sample]]

Gabbros are also sometimes classified as alkali or tholleiitic gabbros, by analogy with [[alkali basalt|alkali]] or [[Tholeiitic magma series|tholeiitic]] basalts, of which they are considered the intrusive equivalents.{{sfn|Allaby|2013|loc="gabbro"}} Alkali gabbro usually contains olivine, nepheline, or [[analcime]], up to 10% of the mineral content,{{sfn|Jackson|1997|loc="alkali gabbro"}} while tholeiitic gabbro contains both clinopyroxene and orthopyroxene, making it a gabbronorite.{{sfn|Allaby|2013|loc="gabbro"}}

=== Gabbroids ===
Gabbroids (also known as gabbroic-rocks<ref name="bgs"/>) are a family of coarse-grained igneous rocks similar to gabbro:{{sfn|Jackson|1997|loc="gabbroid"}}
* Quartz gabbro contains 5% to 20% quartz in its QAPF fraction. One example is the ''cizlakite'' at [[Pohorje]] in northeastern Slovenia,<ref>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}}</ref>
* Monzogabbro contains 65% to 90% plagioclase out of its total feldspar content.
* [[Quartz monzogabbro]] 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 [[nepheline]]-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 [[leucite]]-bearing monzogabbro.<ref name="bgs"/>

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.<ref name=BlattTracy1996/>

Gabbro is generally coarse-grained, with crystals in the size range of 1&nbsp;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]].<ref name="bgs"/> Some pyroxene-plagioclase [[Cumulate rock|cumulates]] are essentially coarse-grained gabbro,<ref>{{cite journal |last1=Beard |first1=James S. |title=Characteristic mineralogy of arc-related cumulate gabbros: Implications for the tectonic setting of gabbroic plutons and for andesite genesis |journal=Geology |date=1 October 1986 |volume=14 |issue=10 |pages=848–851 |doi=10.1130/0091-7613(1986)14<848:CMOACG>2.0.CO;2|bibcode=1986Geo....14..848B }}</ref> and may exhibit acicular crystal habits.<ref>{{cite journal |last1=Nicolas |first1=Adolphe |last2=Boudier |first2=Françoise |last3=Mainprice |first3=David |title=Paragenesis of magma chamber internal wall discovered in Oman ophiolite gabbros |journal=Terra Nova |date=April 2016 |volume=28 |issue=2 |pages=91–100 |doi=10.1111/ter.12194|bibcode=2016TeNov..28...91N |s2cid=130338632 }}</ref>

Gabbro is usually [[equigranular]] in texture, although it may also show [[ophitic texture]]<ref name=BlattTracy1996/> (with laths of plagioclase enclosed in pyroxene<ref>{{cite journal |last1=Wager |first1=L. R. |title=A Note on the Origin of Ophitic Texture in the Chilled Olivine Gabbro of the Skaergaard Intrusion |journal=Geological Magazine |date=October 1961 |volume=98 |issue=5 |pages=353–366 |doi=10.1017/S0016756800060829|bibcode=1961GeoM...98..353W |s2cid=129950597 }}</ref>).