Gabbro

Gabbro is a type of intrusive igneous rock that is coarse-grained and dark-colored, typically composed of calcium-rich plagioclase feldspar, pyroxene, and sometimes olivine. It is similar in composition to basalt, which is its extrusive equivalent. Gabbro is typically found in plutonic bodies, such as batholiths and dikes, and is often associated with other mafic and ultramafic rocks.

Group: Plutonic Igneous Rock
Colour: Dark grey to black.
Texture: Phaneritic (medium to coarse grained).
Mineral content: Predominantly plagioclaseand pyroxene ( augite) with lesser olivine. Silica (SiO ₂) content – 45%-52%.

Composition and Classification

The most important minerals in the gabbron are plagioclase and pyroxene. plagioclase often appear more. It is a small amount of olivine and amphibole. Plagioclase is sodium-calcium feldspar. Gabrobro contains more calcium than sodium. If there is more sodium in the plagioclase, then the rock type is called diorite. Gabbro is greenish and dark.

Diorite, gabbro, anorthosite — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anorthosite (M < 10%), diorite (M > 10%, plagioclase An0
– An50), gabbro (M > 10%, plagioclase An50 –An100). Gabbros may be further subdivided. Either of the two synonymous terms dolerite or diabase may be used for medium-grained gabbros rather than the term microgabbro, if required.

Gabbroic rocks— the gabbros (sensu lato) of QAPF field 10, may be further subdivided according to the relative abundances of their orthopyroxene, clinopyroxene, olivine and hornblende as shown in Fig. Some of the special terms used are:
Gabbro (sensu stricto) = plagioclase and clinopyroxene
Norite = plagioclase and orthopyroxene
Troctolite = plagioclase and olivine
Gabbronorite = plagioclase with almost equal amounts of clinopyroxene and orthopyroxene
Orthopyroxene gabbro = plagioclase and clinopyroxene with minor amounts of orthopyroxene

Clinopyroxene norite = plagioclase and orthopyroxene with minor amounts of clinopyroxene
Hornblende gabbro = plagioclase and hornblende with pyroxene < 5%.

Gabbro Formation

Gabbro forms through the process of solidification of mafic magma, which is rich in iron and magnesium, and typically occurs at relatively deep levels within the Earth’s crust or upper mantle. The formation of gabbro involves several key stages:

1. Magma generation: Gabbro forms from the partial melting of the mantle or lower crust, typically in association with tectonic processes such as mantle upwelling, subduction, or continental rifting. Mafic magma, which has a composition rich in iron and magnesium, is generated through the melting of mantle rocks or crustal rocks that have been previously enriched in mafic minerals.
2. Magma migration: Once formed, the mafic magma tends to be less dense than the surrounding rocks, and it tends to rise towards the Earth’s surface due to buoyancy. The magma may travel through fractures, faults, or other pathways in the crust, moving towards the surface or getting trapped at intermediate depths.
3. Magma emplacement: As the mafic magma rises and reaches its final destination, it can either intrude into existing rocks or erupt onto the Earth’s surface as lava. In the case of gabbro, the magma typically intrudes into the crust, forming intrusive bodies. These bodies can take various shapes, such as dikes, sills, laccoliths, or larger bodies called plutons.
4. Solidification: Once the mafic magma is emplaced into the crust, it begins to cool and solidify. Gabbro forms as the magma cools slowly, allowing for the crystallization of minerals such as pyroxene, plagioclase feldspar, and sometimes olivine. The slow cooling of the magma results in the formation of coarse-grained crystals, which are characteristic of gabbro.
5. Post-emplacement processes: After solidification, gabbroic bodies may undergo various post-emplacement processes such as deformation, metamorphism, and weathering, depending on the geologic conditions and tectonic history of the region. These processes can further modify the composition and texture of gabbroic rocks.

Overall, gabbro forms through the solidification of mafic magma, which originates from partial melting of the mantle or lower crust, and it is typically emplaced into the crust as intrusive bodies that cool and crystallize slowly to form coarse-grained rocks with a characteristic mineral composition.

Gabbro Occurrence and Distribution

Gabbro is a common rock type that occurs in various geologic settings around the world. Here are some details about its occurrence and distribution:

1. Oceanic crust: Gabbro is a major rock type in the oceanic crust, which makes up a significant portion of the Earth’s surface. It forms as part of the slow cooling and solidification of mafic magma that is erupted at mid-ocean ridges and intrudes into the surrounding oceanic crust. Gabbroic rocks are typically found in the lower part of the oceanic crust, underlying the basaltic rocks that make up the upper part.
2. Layered Intrusions: Gabbro is a common rock type in layered intrusions, which are large bodies of igneous rock that form when magma solidifies in horizontal layers over an extended period of time. Examples of layered intrusions with significant gabbroic components include the Bushveld Complex in South Africa, the Stillwater Complex in Montana, USA, and the Skaergaard Intrusion in Greenland.
3. Continental crust: Gabbro can also be found in the continental crust, usually associated with other mafic and ultramafic rocks. In some cases, gabbro bodies may be exposed at the surface due to erosion and weathering, or they may be buried beneath sedimentary rocks.
4. Mountain belts: Gabbroic rocks can be found in mountain belts and areas of tectonic activity where magmatic intrusions have occurred. For example, gabbroic rocks are found in the Appalachian Mountains in eastern North America, the Alps in Europe, and the Andes in South America.
5. Island arcs: Gabbroic rocks can also occur in island arcs, which are curved chains of volcanic islands formed at subduction zones. In these settings, gabbroic rocks can form as intrusive bodies beneath the volcanic arc.

Overall, gabbro is a widespread rock type that occurs in various geologic settings, including oceanic crust, layered intrusions, continental crust, mountain belts, and island arcs. Its distribution is influenced by the tectonic processes and geologic history of the region where it is found.

Gabbro and Basalt Difference 

The rocks are very common in the world. The reason why they are so widespread is that they are formed of magma or lava, which are cooled directly as they approach the surface of the earth. Although both are magmatic rocks with almost the same composition, the main difference between these two rocks is the formation process or the cooling rate of the liquid rock that they initiate.

Gabbro occurs when the liquid rock is cooled very slowly over a long period of time in the underground. This is an intrusive magmatic rock and is close to the very hot core of the world, which is why it takes longer to cool, and therefore the rock is visibly very different from the opposite of the basalt. The crystals are very large and clear to the naked eye and the texture is very coarse. This coarse grained texture can be labeled porphyric or a mixture of large and finer grained crystals, which are very large crystals. The time required for the liquid rock to cool will determine the texture.
On the other hand, the basalt is an extrusive magmatic rock. The surface is much closer to the surface of the earth and therefore the cooling process is much faster than the gabbone. The fact that the cooling rate is much higher gives the basal what it calls aphanitic texture, or it is so finely grained that it is not seen by the helpless human eye.

Uses of Gabbro

Gabbro can be polished to a bright black glow. Bright polished gabbro cemetery signs are used in kitchen stalls, floor tiles, facade stone and other size stone products. It is a highly desirable rock based on weather and wear. In the stone industry size gabbro is sold under the name “black granite”. Gabbro is also used to make a large number of rough cut products, such as borders, stones, paving stones and other products. The most common use of gabbro is like crushed stone or aggregate. The crushed gabbro is used as a basic material in construction projects, as a crushed stone for road construction, as a railway ballast and as a filler where a resistant crushed stone is needed.

Key points about Gabbro

• Composition: Gabbro is composed mainly of calcium-rich plagioclase feldspar, usually labradorite or bytownite, and pyroxene, typically clinopyroxene (such as augite) or orthopyroxene (such as hypersthene). It may also contain small amounts of olivine, amphibole, and other minerals.
• Texture: Gabbro has a coarse-grained texture, with individual mineral grains visible to the naked eye. The grains are typically interlocking, giving the rock its characteristic appearance.
• Color: Gabbro is typically dark-colored, ranging from dark gray to black, due to the presence of dark-colored minerals such as pyroxene and olivine.
• Occurrence: Gabbro is commonly found in large plutonic bodies, such as batholiths, which are large intrusive rock formations, and dikes, which are tabular intrusions that cut across other rocks. It can also be found in layered intrusions, such as in the Bushveld Complex in South Africa.
• Petrogenesis: Gabbro is typically formed through the slow cooling and crystallization of mafic magma beneath the Earth’s surface. As the magma cools and solidifies, mineral crystals begin to form, resulting in the coarse-grained texture of gabbro.
• Uses: Gabbro is used as a dimension stone for construction and architectural purposes, as it is durable and can be polished to a high sheen. It is also used as a crushed stone for road construction and aggregate in concrete. In some cases, gabbro can contain valuable minerals such as nickel, copper, and platinum group elements (PGEs), and may be mined for these resources.
• Geology: Gabbro is an important rock in the geology of the Earth’s crust, as it is a common component of the oceanic crust and is also found in continental crust. It plays a role in the formation of large igneous provinces, which are massive outpourings of igneous rock that can have significant geological and environmental impacts.
• Rock associations: Gabbro is often associated with other mafic and ultramafic rocks, such as basalt, peridotite, and norite, and can be part of layered intrusions where different rock types occur in distinct layers or bands.

In summary, gabbro is a coarse-grained, dark-colored intrusive igneous rock composed mainly of calcium-rich plagioclase feldspar and pyroxene. It is commonly found in large plutonic bodies, has various uses in construction and industry, and plays an important role in geology and geophysics.

Gabbro FAQ

Here are some frequently asked questions (FAQs) about gabbro:

Q: What is gabbro?

A: Gabbro is a type of intrusive igneous rock that is coarse-grained and dark-colored, composed mainly of calcium-rich plagioclase feldspar, pyroxene, and sometimes olivine.

Q: How is gabbro formed?

A: Gabbro is formed through the slow cooling and crystallization of mafic magma beneath the Earth’s surface. As the magma cools, mineral crystals begin to form, resulting in the coarse-grained texture of gabbro.

Q: What minerals are typically found in gabbro?

A: Gabbro is typically composed of calcium-rich plagioclase feldspar (such as labradorite or bytownite), pyroxene (such as augite or hypersthene), and sometimes olivine. It may also contain small amounts of other minerals, such as amphibole.

Q: What is the color of gabbro?

A: Gabbro is typically dark-colored, ranging from dark gray to black, due to the presence of dark-colored minerals such as pyroxene and olivine.

Q: Where is gabbro commonly found?

A: Gabbro is commonly found in large plutonic bodies, such as batholiths and dikes, and can also be found in layered intrusions. It is a common component of the oceanic crust and can also occur in continental crust.

Q: What are the uses of gabbro?

A: Gabbro is used as a dimension stone for construction and architectural purposes, as it is durable and can be polished to a high sheen. It is also used as a crushed stone for road construction and aggregate in concrete. In some cases, gabbro can contain valuable minerals such as nickel, copper, and platinum group elements (PGEs), and may be mined for these resources.

Q: What is the texture of gabbro?

A: Gabbro has a coarse-grained texture, with individual mineral grains visible to the naked eye. The grains are typically interlocking, giving the rock its characteristic appearance.

Q: What other rocks are associated with gabbro?

A: Gabbro is often associated with other mafic and ultramafic rocks, such as basalt, peridotite, and norite, and can be part of layered intrusions where different rock types occur in distinct layers or bands.

Q: What is the geologic significance of gabbro?

A: Gabbro is an important rock in the geology of the Earth’s crust, as it is a common component of the oceanic crust and is also found in continental crust. It plays a role in the formation of large igneous provinces, which are massive outpourings of igneous rock that can have significant geological and environmental impacts.

Q: Are there any economic resources associated with gabbro?

A: Yes, gabbro can contain valuable minerals such as nickel, copper, and platinum group elements (PGEs), and may be mined for these resources. However, such occurrences are relatively rare and not all gabbro bodies contain economic concentrations of these minerals.

These are some common questions and answers about gabbro, a type of intrusive igneous rock with important geologic, economic, and industrial significance.

References

• Flexiblelearning.auckland.ac.nz. (2019). Geology – rocks and minerals. [online] Available at: https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/gabbro.html [Accessed 2 Mar. 2019].
• Sandatlas.org. (2019). Gabbro – Igneous rocks. [online] Available at: http://www.sandatlas.org/gabbro/ [Accessed 2 Mar. 2019].
• Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
• Ronald Louis Bonewitz, (2012) NATURE GUIDE AND MINERALS, Smithsonian NATURE GUIDE, LONDON, NEW YORK, MELBOURNE, MUNICH, AND DELHI