Gabbro
Gabbro (.
Etymology
The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy.[1] It was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany. Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his description of these Italian ophiolitic rocks.[2] He assigned the name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" (metamorphosed gabbro).[3]
Petrology
Gabbro is a coarse-grained (
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
The composition of the plagioclase cannot easily be determined
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.[12][7][8]
Coarse-grained gabbroids are produced by slow crystallization of
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.[8][13]
A more detailed classification is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are:[8][13]
- Normal gabbro (gabbro sensu strictoorthopyroxene.
- 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
Gabbros are also sometimes classified as alkali or tholleiitic gabbros, by analogy with alkali or tholeiitic basalts, of which they are considered the intrusive equivalents.[14] Alkali gabbro usually contains olivine, nepheline, or analcime, up to 10% of the mineral content,[15] while tholeiitic gabbro contains both clinopyroxene and orthopyroxene, making it a gabbronorite.[14]
Gabbroids
Gabbroids (also known as gabbroic-rocks[8]) are a family of coarse-grained igneous rocks similar to gabbro:[10]
- Quartz gabbro contains 5% to 20% quartz in its QAPF fraction. One example is the cizlakite at Pohorje in northeastern Slovenia,[16]
- 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.[8]
Gabbroids contain minor amounts, typically a few percent, of iron-titanium oxides such as
Gabbro is generally coarse-grained, with crystals in the size range of 1 mm or larger. Finer-grained equivalents of gabbro are called
Gabbro is usually
).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.[20][21] 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.[22] 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
Layered gabbros are also characteristic of
Uses
Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.[27][28][29] For example, the Merensky Reef is the world's most important source of platinum.[30]
Gabbro is known in the construction industry by the trade name of black granite.[31] However, gabbro is hard and difficult to work, which limits its use.[32]
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.[citation needed]
See also
- Peridotite – Coarse-grained ultramafic igneous rock type
- Igneous differentiation – Geologic process in formation of some igneous rocks
- Fractional crystallisation – Process of rock formation
References
- ISBN 978-90-481-4020-6
- ISBN 978-90-481-4020-6
- ^ Gabbro at SandAtlas geology blog. Retrieved on 2015-07-09.
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- ^ ISBN 0716724383.
- ^ S2CID 28548230.
- ^ a b c d e f g h i j "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
- ISBN 978-0-521-88006-0.
- ^ a b Jackson 1997, "gabbroid".
- ^ Blatt & Tracy 1996, p. 71.
- ^ Jackson 1997, "gabbro".
- ^ a b Philpotts & Ague 2009, p. 142.
- ^ a b Allaby 2013, "gabbro".
- ^ Jackson 1997, "alkali gabbro".
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- ^ Philpotts & Ague 2009, p. 102.
- ^ Philpotts & Ague 2009, pp. 370–374.
- ^ Philpotts & Ague 2009, pp. 95–99.
- ^ Philpotts & Ague 2009, p. 99.
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- ^ Philpotts & Ague 2009, pp. 384–390.
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