Gabbro is a dense, coarse-grained, dark-colored mafic rock. It is mostly a plutonic rock but can also be extrusive/subsurface. Mafic tells you it is rich in iron or magnesium.
Plutonic or intrusive igneous rocks solidify inside the Earth’s crust where cooling is slow. The slow cooling allows for large crystals to form. Thus, these rocks have a coarse-grained or phaneritic texture.
On the other hand, extrusive or volcanic means they form on the Earth’s surface where cooling is fast. Fast cooling history will create an aphanitic or fine-grained texture.
However, some extrusive rocks formed in the interior of thick lava can have a coarse-grained texture. The thick lava layer offers insulation, slowing down cooling.
Bottom line: Gabbro is an intrusive and extrusive rock, depending on where it forms. The International Union of Geological Sciences (IUGS) discourages the exclusive use of the term plutonic when describing this rock. However, most of the gabbros are plutonic.
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Properties and textures
Gabbro is a dense, hard, dark-colored rock with a 2.7-3.3g/cm3 density and a Mohs hardness scale of 6-7. It can be massive rock or layered.
Its color is usually dark green, dark gray, and black. However, the weathered specimens may have a reddish-brown or tan color due to iron oxidization.
The texture of gabbro is coarse-grain with equigranular hypidiomorphic interlocking, mostly dark crystals with a few lighter ones. Equigranular means nearly equal crystals or grains.
On the other hand, hypidiomorphic has mostly subhedral crystal forms. Subhedral crystal form lies between euhedral with well-formed, recognizable sharp faces and anhedral without well-formed crystal faces.
The coarse-grained texture can occur with other textures like porphyritic, pegmatitic, and orbicular.
Porphyritic gabbro has large mineral crystals called phenocrysts in a finer but coarse-grained groundmass or matrix.
On the other hand, pegmatitic gabbro has enormous interlocking crystals, some several inches or exceeding a meter.
What about orbicular? Orbicular gabbro has nearly spherical, onion-like, or concentric layered structures.
Lastly, microgabbro, also known as diabase or dolerite, is a rock with a medium-grained texture and a composition like a gabbro. It is a plutonic rock formed at shallower depths of less than 2 km. Such rocks are called subvolcanic or hypabyssal.
Chemical composition
Gabbro is a basic rock. It is low in silica, higher in iron, magnesium, and calcium, and lower in sodium, potassium, and calcium.
The typical percentage weight chemical composition of gabbro is 45-52% silica, more than 14% silica, nearly 10% calcium oxide, about 5-15% iron oxides, 5-12% magnesium oxide, and less than 5% alkalis (sodium and potassium) oxides.
Gabbro mineral composition
Gabbro is a mafic rock. It has calcic-plagioclase and augite. Also, it may have smaller amounts of olivine, enstatite, diopside, hornblende, quartz, alkali feldspar, and less often biotite.
Common accessory minerals are magnetite, chromite, zircon, ilmenite, spinel, and sulfides.
The calcium-rich plagioclase has more than 50 mol % anorthite. This plagioclase labradorite to bytownite.
Hornblende may rim augite in this rock, while alkali feldspar and quartz are usually interstitial.
On the QAPF classification, gabbro is a coarse-grained plutonic rock in which quartz is less than 5% of the QAPF content by volume and plagioclase is over 90% of the total feldspars.
It is one of the gabbroic rocks or gabbroids whose definition is broader. On the QAPF classification, gabbroic rocks are coarse-grained igneous rocks that have up to 10% feldspathoids and up to 20% quartz of the QAPF content by volume, with plagioclase accounting for at least 65% of the total feldspars.
Depending on mafic mineral volume or color index M, you can classify these rocks into leucogabbros if M is less than 35, mesogabbros if between 35-65, and melagabbros for those with M between 65 and 90. Above 90, you will have ultramafic rocks.
That is not all. Gabbro rocks are further classified according to the relative percentage of clinopyroxene, orthopyroxene, and olivine. This yields other rocks like norite, troctolite, and gabbronorite. Others are hornblende, orthopyroxene and olivine gabbro.
Lastly, like basalts, gabbros can fall into alkali and tholeiitic subcategories.
How is gabbro formed?
Gabbro forms when magma cools slowly deep inside the Earth’s crust to form plutonic type or in thick lavas to form extrusive.
In both scenarios, the slow cooling allows large mineral crystals to grow, creating a coarse-grained texture.
The mafic magma that forms this rock originates from partially melting mantle peridotites.
How do they occur?
Gabbro rocks are layered, non-layered (massive), or both.
Layered gabbro occurs in layered igneous intrusion. These layers are centimeter to meters thick and may vary in mineral composition.
The layering happens due to fractional crystallization and nucleation habits.
You can have layers with different colors like melanocratic, mesocratic, and leucocractic, alternating dark and lighter or lighter towards the upper parts.
Massive, on the other hand, occurs from magma crystallization in situ. Such will have an isotropic fabric, i.e., the same in all directions. Massive gabbros are common in the upper part of oceanic crusts and some intrusions.
Lastly, both layered and non-layered can occur in the same intrusion. For instance, you can layer on the lower side and massive on the upper part. Such occur in deep oceanic crusts or large intrusions like the Duluth Complex.
Where is gabbro found?
Gabbro rocks are common in the oceanic crust, with only a small amount occurring on continental crust.
In the oceanic crust, they occur below sheeted dikes that feed pillow lavas that cover the oceanic crust.
In continental crusts, they occur primarily in layered igneous intrusion and ophiolites. Ophiolites are exposed to ancient oceanic crust, and its upper mantle emplaced onto the continent.
In layered intrusions, they are a minor constituent. Most of these intrusions are lopoliths with a few dikes. Examples include the Dulux Complex in Minnesota, US, the Bushveld complex in South Africa, Skærgaard in Greenland, and the Kiglapait and Muskox Mountains in Canada. Others are the Great Dyke in Zimbabwe and the Kûngnât ring dike in Greenland.
On the other hand, ophiolites with gabbro include Semail in UAE and Orman, Coast Range in California USA, and Troödos.
Furthermore, some island arcs may have a small amount of gabbros. However, most are deeply buried, and much erosion is necessary to expose them.
Lastly, only a tiny amount occurs in orogenic belts like the Appalachian Mountains, the Alps in Europe, or the Andes. Some are ophiolite remnants.
Uses of gabbro
Gabbro is a tough, dense, and durable rock with many uses in the construction and dimensional stone industry. Also, it is associated with some valuable ores and has many other uses.
In the construction industry, crushed gabbro makes railroad ballast, roadstone, asphalt, mortar, and concrete for roads, bridges, buildings, and other construction projects.
Besides construction, gabbro is a popular dimensional stone sold as black granite. Cut and polished will make floor tiles, countertops, pavers, ashlars, windowsills, etc.
That is not all. Gabbroic complexes and other ultramafic and mafic rocks are associated with platinum, nickel, chromium, gold, silver, cobalt, titanium, and other valuable minerals. A good example is platinum deposits in Merensky Reef in the Bushveld complex.
More uses include making monuments, statues, curbing stones, ripraps, and headstones. However, being so tough, it is not the most preferred rock.