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Gabbro

Gabbro is a plutonic igneous rock and the intrusive counterpart of basalt. The name ‘gabbro’ derives from the Italian town of Gabbro (Rosignano Marittimo, Tuscany) and was first coined by the Italian geologist Tozzetti in 1768. Tozzetti used the term to refer to a suite of ophiolitic rocks. Only in 1809, the German geologist Christian Leopold von Buch, restricted the use of this term to plagioclase – clinopyroxene-bearing rocks (although he actually described metagabbros). Gabbros consist of calcic plagioclase (>An50), clinopyroxene (augite, diopside), and iron oxides (ilmenite, magnetite). Olivine, hornblende, orthopyroxene, and spinel may be present as accessory minerals and can constitute important, rock-forming phases in some varieties of gabbros. Plagioclase is generally comprised between 35 and 65% of the rock volume. Gabbros may also contain small amounts of quartz, indicating oversaturation in silica, or feldspathoids, present in silica-undersaturated gabbros. Additional details about the classification of gabbros are available at the page below.

Classification gabbroid rocks

  • Classification of gabbros Gabbroid rocks are a large family of plutonic rocks that contain almost exclusively plagioclase as the only felsic mineral, hence plotting close to the P vertex of the QAPF classification diagram (quartz < 5% and plagioclase > 90% feldspars). The relatively simple classification through the QAPF diagram hides, however, the true complexity of these rocks, which may contain variable proportion… Read More »Classification of gabbros

gabbro

Gabbro with large phenocrysts of augite pyroxene (black) surrounded by white plagioclase. Tangen, Norway. Width: 11 cm. © Siim Sepp.

Gabbro
Plutonic igneous rock
Felsic minerals:
• Ca-plagioclase
Mafic minerals:
pyroxene
olivine

QAPF classification:
Q = 0 – 5%
Plagioclase/feldspars > 90%
Colored varieties:
• leucogabbro (M < 35%)
• melagabbro (M > 65%)
Other varieties: quartz gabbro, foid-bearing gabbro, olivine gabbro, hornblende gabbro
Extrusive equivalent: basalt

Gabbros have also been classified based on their grain size, which is very variable in nature. Very coarse-grained varieties, showing crystals larger than several centimeters, are known as pegmatoid gabbro. On the other hand, fine-grained varieties have been called microgabbro, diabase, or dolerite – three terms that are considered synonyms in the present literature.

Gabbro is a fundamental constituents of ophiolite sequences and of the deeper part of the oceanic crust. It is also common in association with lower crustal rocks, as gabbros intrude at the base of the crust, and in rift areas, where mafic magma ascends within the crust. Mafic complexes, like layered intrusions, contain gabbro as a product of cumulus processes.

gabbro

Gabbro from Tangen, Norway, showing black clinopyroxene crystals surrounded by plagioclase, varyiing in color from white to grey. Width: 10 cm. © Siim Sepp.

gabbro

Gabbro tent to easily whether. This sample shows patches of orange iddingsite that formed due to olivine alteration. Flakstadøya, Lofoten Archipelago, Norway. Width: 12 cm. © Siim Sepp.

gabbro

Gabbro sample with black pyroxene and white plagioclase (bluish colors are an artifact from scanning). Unknown locality. © James St. John.

pegmatoid gabbro

Pegmatoid gabbro with very coarse-grained pyroxene (dark colors) surrounded by white plagioclase. Troodos ophiolite, Cyprus. © Siim Sepp.

outcrop of gabbro

Outcrop of gabbro on the shore of Castiglioncello, Italy. The grain size varies in the field from ‘regular’ to pegmatoid gabbro. © Samuele Papeschi/GW.

outcrop of gabbro

Detail of gabbro from Castiglioncello (Tuscany, Italy). Augitic pyroxene is dark green with metallic lustre. Plagioclase is white to light green. © Samuele Papeschi/GW.

pegmatoid gabbro

Pegmatoid gabbro from Castiglioncello, Italy. Large grains of clinopyroxene (here appearing light because of their strong metallic lustre) surrounded by altered, dark green calcic plagioclase. © Samuele Papeschi/GW.

References
Cox, K. G. (Ed.). (2013). The interpretation of igneous rocks. Springer Science & Business Media.
Le Bas, M. J., & Streckeisen, A. L. (1991). The IUGS systematics of igneous rocks. Journal of the Geological Society148(5), 825-833.
Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
Streckeisen, A. (1976). To each plutonic rock its proper name. Earth-science reviews12(1), 1-33.
Wilson, D. S., Teagle, D. A., Alt, J. C., Banerjee, N. R., Umino, S., Miyashita, S., … & Ziegler, C. (2006). Drilling to gabbro in intact ocean crust. science312(5776), 1016-1020.

        

See also
Gabbro – Sandatlas.org
Gabbro – Alexstrekeisen.it

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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