Gabbro

Gabbro

Gabbro (pronEng|ˈɡæbrəʊ) is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt. It is a plutonic rock, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.

The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges.

Petrology

Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in large quantities).

The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then a Norite. Quartz gabbros are also known to occur 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 mineral nepheline. (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, although these may exhibit acicular crystal habits.

Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.

Distribution

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 orthocumulate.

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.

Uses

Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.

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'.

Etymology

Gabbro was named by the German geologist Christian Leopold von Buch after a town in the Italian Tuscany region. Essexite is named after the type locality in Essex County, MA, USA.

See also

* Igneous rocks
* Cumulate rocks
* Ultramafic to mafic layered intrusions
* Essexite
* Norite
* Peridotite
* Olivine
* Pyroxene
* Igneous differentiation
* Fractional crystallisation
* Cumulate rocks

External links

* [http://www-odp.tamu.edu/publications/prelim/176_PREL/176IGNE.HTML Ocean drilling program gabbro petrology]
* [http://www.livescience.com/forcesofnature/060420_earth_drill.html Scientists find the elusive gabbro]


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Look at other dictionaries:

  • gabbro — [ gabro ] n. m. • 1778; mot florentin, p. ê. du nom d un village toscan ♦ Géol. Roche éruptive composée de plagioclases et de pyroxène. ● gabbro nom masculin (de Gabbro, nom propre) Roche plutonique basique constituée essentiellement de… …   Encyclopédie Universelle

  • Gabbro — Gabbro, in der Gesteinskunde ein granitisch körniges Gestein, das vorwiegend aus Feldspat und Diallag besteht, zu denen sich noch in einigen Arten der Olivin (Olivingabbro) als wesentlicher Gemengteil gestellt. Der Feldspat bildet meist größere,… …   Lexikon der gesamten Technik

  • gabbro — GABBRÓ s.n. v. gabro. Trimis de LauraGellner, 13.09.2007. Sursa: DN  gabró/gabróu s. n. rocă magmatică intruzivă cu structură holocristalină, din feldspaţi plagioclazi, piroxeni, amfiboli, olivinã şi biotit, folosit în construcţii şi în pavaje.… …   Dicționar Român

  • Gabbro — Gab bro, n. [It.] (Geol.) A name originally given by the Italians to a kind of serpentine, later to the rock called {euphotide}, and now generally used for a coarsely crystalline, igneous rock consisting of lamellar pyroxene (diallage) and… …   The Collaborative International Dictionary of English

  • gabbro — (n.) type of igneous rock, 1837, from It. (Tuscan) gabbro, from L. glaber bare, smooth, bald (see GLAD (Cf. glad)) …   Etymology dictionary

  • Gabbro — (Geogn.), ein körniges Gemenge von Diallag u. Labrador; gehört zu den plutonischen Gebirgsarten u. kommt häufig in Begleitung des Serpentins vor. Früher bezog man ihn von der Insel Corsica u. verarbeitet ihn zu Tischplatten u. anderen… …   Pierer's Universal-Lexikon

  • Gabbro — (Euphotid), massiges kristallinisches Gestein von meist grobkörniger, selten flaseriger (Flasergabbro) Struktur, besteht aus einem basischen Kalknatronfeldspat (Labrador bis Anorthit) und Diallag, zu denen als weiterer Gemengteil oft noch Olivin… …   Meyers Großes Konversations-Lexikon

  • Gabbro — Gabbro, körniges Eruptivgestein aus Diallag und Plagioklas, mit akzessorischem Titaneisen, Apatit, Enstatit etc., auch mit Olivin (Olivin G.), massiges, in mächtigen Stöcken zwischen Graniten, Gneisen und Glimmerschiefern auftretendes Gestein;… …   Kleines Konversations-Lexikon

  • gabbro — [gab′rō] n. [It < L glaber, bare: see GLABROUS] any of a group of usually dark, coarsegrained, igneous rocks, composed chiefly of pyroxene and plagioclase …   English World dictionary

  • Gabbro — Handstück eines Gabbro Star Galaxy: Ein Gabbro aus Indien. Muster ca. 25×15cm …   Deutsch Wikipedia

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