Gabbro: A Coarse-grained, Dark-Colored Plutonic Rock

Gabbro is a dense, coarse-grained, dark-colored, mostly plutonic rock. Its essential minerals are calcic plagioclase and pyroxene, but it may have minor amounts of amphibole, olivine, and other minerals.

This basic, mafic rock forms from the slow cooling of silica-poor (45-52 wt. %), magnesium- and iron-rich magma primarily deep in the Earth’s crust. However, it may also form within thick basalt lavas. Therefore, IUGS doesn’t recommend using plutonic exclusively when describing this rock.

The term gabbro was introduced by Giovanni Targioni Tozzetti, an Italian geologist, in 1768 to describe ophiolites found in the Italian mountain range Apennines. This rock was named after Gabbro, a small settlement village near Rosignano Marittimo in Tuscany, Italy.

Later in 1809, Christian Leopold von Buch, a German geologist, restricted the use of gabbro to only the Apennines ophiolites metamorphosed gabbro (metagabbro). Today, it describes a mafic plutonic igneous rock with mainly calcium-rich plagioclase and pyroxene.

Gabbro is a relatively common rock with many uses. It occurs worldwide, mainly in deep ocean crust, continental arcs, intraplate hotspots, and rifts, and less often in island arcs. Also, rocks with gabbroic character occur in lighter lunar highlands, i.e., on the moon.  

This article will focus on gabbro and gabbroic rocks. We will start by telling you what it looks like or how to identify it, i.e., its appearance (texture and color). Afterward, we will discuss its chemical and mineral composition and related gabbroic rocks.

Also, there are parts on its classification, how it forms, where it occurs, some of its uses, and more.

Quick overview and properties

• Name: Gabbro
• Synonym: Finlandite
• Rock type: Igneous
• Origin: Mostly intrusive or plutonic with few subsurface
• Texture: Coarse-grained or phaneritic (1/16mm to 3cm grain size)
• Cooling rate or history: Slow
• Color: Dark colored – dark gray, dark green to black
• Mohs hardness scale: 6-7
• Density: 2.7-3.3g/cm³
• Magnetism: Magnetite-rich varieties are weakly magnetic
• Chemical composition: Mafic
• Silica content: 45-52 wt. %
• Mineral composition: Pyroxene and calcium-rich plagioclase with lesser amounts of hornblende and olivine.
• Extrusive equivalent: Basalt
• Medium-grained equivalent (microgabbro): Dolerite or diabase
• Tectonic environment: Deep inside the oceanic crust, divergent boundaries (volcanic arcs), convergent boundaries (continental collision), and continental intraplate hotspots and rifts.

What does gabbro look like?

In the field, you will identify gabbro by its color, texture, and the fact that it is a little bit denser and darker than the likes of granite, diorite, or other look-alikes. It differs from peridotites, which tend more towards greenish color since they are rich in olivine.  

Usually, gabbro is a tough, massive, or layered dark gray, dark green, or black rock with a coarse-grained texture. It has equigranular hypidiomorphic grains with a somewhat speckled appearance characterized by mostly darker minerals (mafic) and fewer lighter minerals (felsic).

However, altering olivine to iddingsite and other minerals may result in reddish splotches on the rock. Also, it may show different textures like medium-grains (microgabbro), porphyritic, orbicular, pegmatitic textures, and rarely ophitic.

Let us look at these textures a little more.

1. Porphyritic gabbro

Porphyritic gabbro has larger crystals (phenocrysts) embedded in a finer but coarse-grained groundmass. Phenocrysts are usually plagioclase, pyroxene, and other mafic minerals in a coarse-grained matrix of the same minerals.

It shows a two-stage cooling. An Earlier stage forms phenocrysts and a later stage forms the coarse-grained matrix. This texture also occurs in granite, rhyolite, andesite, basalt, lamprophyre, and pitchstone.

2. Gabbro pegmatite  

Gabbro pegmatites will have enormous crystals (3-4 inches to meters) of calcic plagioclase, pyroxene (augite), or sometimes hornblende set in a coarse-grained matrix or groundmass. The groundmass can be pyroxene, plagioclase, ilmenite, etc.

It forms during the crystallization of the superheated and fluidized volatile-rich last magma melt portion. The high volatiles in this portion depolymerize the silica present and prevent nucleation until it is supersaturated. Once supersaturated, any nucleation will result in rapid crystal growth, forming pegmatites.

These pegmatites may have valuable minerals. They also occur in rocks like rhyolite, granite, syenite, diorite, and granodiorite.

3. Orbicular

Orbicular gabbro is a rare kind with rounded, spherical structures with concentric layering known as orbiculates. These orbiculates have radially arranged plagioclase and olivine crystals with regular shells of troctolitic composition.

The first occurrence of this texture was reported in the Baskil island-arc magmatic suite in SE Turkey. However, it is now known to occur in other places.

4. Microgabbro

Microgabbro is a medium-grained rock with gabbroic composition. Its crystal sizes are between aphanitic or fine-grained such as that seen in basalt, and coarse-grained, like in gabbro or granite.

Lastly, microgabbro is common on edges of gabbroic intrusions where cooling is slightly faster.

5. Ophitic

Ophitic texture is a poikilitic texture variation that occurs when olivine or augite surrounds plagioclase laths.

Peridotite composition

Peridotite chemical composition and mineralogy is as follows:

1. Chemical composition

Chemically, gabbro is a basic, mafic rock. It is low in silica (45% to 52 wt.%), relatively high in magnesium, iron, and calcium, and low in sodium or potassium.

Please note that mafic only implies the rock is high in darker ferromagnesian elements. However, it is not an IUGS-recognized classification.

Using data from Perkins (2020), the typical weight percentage chemical composition of gabbro is SiO₂: 50.78%, TiO₂: 1.13%, Al₂O₃: 15.68%, Fe₂O₃: 2.26%, FeO: 7.41%, MnO: 0.18 %, MgO: 8.35%, CaO: 10.85%, Na₂O: 2.14%, K₂O: 0.56%, H₂O: 0.48%, and P₂O₅: 0.18%.

This data confirms that gabbro rock is low in silica, relatively high in magnesium, iron, and calcium oxides, and poor in sodium and potassium.

2. Mineralogy

Gabbro has mainly calcium-rich plagioclase and clinopyroxene. It may also have a minor amount of olivine, alkali feldspar, quartz or feldspathoids, orthopyroxene, hornblende, and rarely biotite. Any alkali feldspar, quartz, or feldspathoids occur as interstitial minerals. However, quartz doesn’t coexist with feldspathoids.

Accessory minerals in this rock are chromite, magnetite, ilmenite, sulfide, zircon, apatite, and ulvospinel.

Usually, the calcium-rich plagioclase in gabbroic rocks has more than 50 mol % anorthite and is usually labradorite to bytownite. It appears lighter (light gray to whitish) and has a primarily stout prismatic subhedral crystal structure with normal zoning.

On the other hand, clinopyroxene is usually augite and less often aluminum-bearing diopside. Also, this rock may have minor amounts of enstatite (hypersthene or ferrosilite) orthopyroxene. You can tell these two apart by allowing the rock to weather slightly. Augite tends to turn green to black, and orthopyroxene turns brownish.

Hornblende is the common amphibole and often rim augite or as a poikilitic crystal enclosing other minerals.

What alteration does it undergo? Gabbro alteration includes olivine → serpentine and iddingsite; pyroxene → uralite and chlorite; and plagioclase or feldspathoids → sericite.

Also, plagioclase may weather clay minerals like kaolinite chemically in weak carbonic acid or via hydrolysis. However, it is resistant if it forms clasts.

Let us now look at gabbroids or gabbroic rocks classification and subtypes.

1. Gabbroids classification – QAPF

Gabbroids are coarse-grained igneous rocks that resemble gabbro. Sometimes, they are known as gabbroic.

On the Q (quartz), A (alkali), P (plagioclase), and F (feldspathoids) classification of coarse-grained rocks, gabbroids are defined as rocks with less than 20% quartz and less than 10% feldspathoids of the QAPF content by volume with plagioclase accounting for more than 65% of total feldspar by volume.

On the other hand, gabbro is defined as a rock with quartz less than 5% of the QAPF content by volume and plagioclase accounting for over 90% of the total feldspars. It has no feldspathoid and coincides with anorthosite and diorite.

However, anorthosite has less than 10% mafic content by volume compared to gabbros, over 35%. Also, anorthosite is not a gabbroic rock.

A decrease in plagioclase grades gabbro into monzogabbro, while an increase in quartz grades it into quartz gabbro. Those where feldspathoids replace quartz are foid bearing.

Here is more on gabbroids:

Note

Despite using the name gabbro, foid gabbro and monzogabbro are not gabbroids because in these coarse-grained plutonic rocks, foids are 20-60 % of the QAPF content by volume with no quartz. In contrast, foids are only up to 10% of QAPF content by volume in gabbroids, and some of these rocks may have quartz.  

In foid monzogabbro, foids account for 10-60% of the QAPF content by volume and plagioclase 65-90% of the total feldspars.

On the other hand, a foid gabbro has 10-60% of the QAPF content by volume feldspathoids, and over 90% of the total feldspars are plagioclase.

Lastly, when naming, replace the word foid with the dominant feldspathoids. For instance, you may have nepheline, sodalite, cancrinite, or analcime monzogabbro or gabbro if they are the dominant ones.

2. Gabbro rock subclassification or subtypes

We can further subdivide this according to coloration/mafic content or on plagioclase, pyroxene, and hornblende content.

i. Coloration/mafic content

Depending on the mafic content or color index M, gabbroids fall into three categories. Leucogabbros with mafic minerals < 35% by volume, mesogabbros 35-65%, and melagabbros with more than 65% and up to 90%. Rocks with more than 90 vol.% mafic minerals are ultramafic or ultrabasic.

ii). classification based on plagioclase pyroxene, olivine, and hornblende

Gabbro rock may also be classified according to their percentage of pyroxene (Px), clinopyroxene (Clx), Orthopyroxene (Oxp), and olivine (Ol) by volume. It yields three ternary diagrams shown below.

a). Endmember of these ternary diagrams are:

b). Intermediate members of the ternary diagrams are:

iii). Calc-alkaline tholeiitic classification

We can also classify gabbros according to oxygen fugacity or magma’s redox state, i.e., into tholeiitic or calc-alkaline, just like basalts.

Alkali gabbro has olivine, analcime, and nepheline accounting for up to 10 vol% and sometimes with minor biotite and hornblende. In contrast, tholeiitic gabbro is essentially a gabbronorite with orthopyroxene and clinopyroxene.

How is gabbro formed?

Gabbro forms from the slow cooling of molten basaltic magma, mostly deep inside the Earth’s crust. The overlaying thick layer of rock provides insulation, slowing cooling rates. This favors mineral crystals to grow large, i.e., have a coarse-grained texture. Such rocks will be plutonic.

However, this rock can also form in interior parts of thick basaltic lava. These thick layers similarly offer insulation to allow crystal growth. However, such rocks are not plutonic but rather subsurface.

Lastly, the origin of basaltic magma that makes these rocks is the partial melting of mantle rocks.

Where is gabbro found or formed?

Gabbro is the most abundant rock in the deep oceanic crust beneath pillow lava basalts and sheeted dike complexes. It also occurs in continental crust, including continental arcs, intraplate hotspots, and rifts. However, it is less often seen on island arcs but does occur.

Going to specific locations, this rock is distributed worldwide. In the US, notable gabbroic rock occurrences are in Washington, Idaho, Oregon, California, Michigan, and Minnesota states.

Here is more on where this rock is found:

1. Ophiolites

Gabbros are common in the ophiolite complexes (obducted ancient oceanic crusts). These rocks occur beneath sheeted dikes complex as massive (on upper parts) and layered bodies (on lower portion) that overlay ultramafic rocks.

These ophiolites include Semail (Oman and UAE), Troödos in Cyprus, Coast Range in California, USA, Dun Mountain-Maitai Terrane in New Zealand, and Dongwanzi in China.

2. Igneous rock intrusions

These rocks may intrude on continents, forming batholiths, laccoliths, lopoliths, or large dikes. Such intrusions are a few to hundreds of kilometers long and have different geometries, e.g., funnel-shaped, saucer-shaped, etc.

However, they are less common than their silica-rich counterparts, like granodiorites or diorites. Why?  Because their high iron content makes them dense. Thus, they lack the upthrust force or buoyancy necessary to intrude.

In larger intrusions, it often occurs with anorthosites or ultramafic rocks. These rocks may be layered, massive, or both layered and massive.

Some of the notable intrusions with gabbro rock include the Duluth Complex in Minnesota, USA, Skærgaard in Greenland, Bushveld complex in South Africa (largest lopolith), Fongen-Hyllinden in Norway, Muskox and the Kiglapait Mountains in Canada.  

Also, it occurs on dikes like Great Dyke in Zimbabwe or Tugt û toq and Kûngnât ring dikes in Greenland.

3. Island arcs

Gabbro rocks also occur in the oceanic island arcs. However, you are unlikely to see them in situ since much erosion is necessary to expose these rocks. Thus, they are less exposed and common compared to continental arcs.

4. Subsurface deposits

Some gabbro-bearing thick basaltic lavas include Columbia River basalt floods in Eastern Washington and Oregon to western Idaho and Northern Nevada, USA, and Deccan Traps in India.

5. Continental arcs and orogenic belts

Gabbro also occurs in mountainous or orogenic belts. Examples are the Appalachian Mountains in eastern North America (Canada to the USA), the Alps in Europe, and the Andes in South America.

These orogenic belts occur due to both continent-continent collisions or subduction zones associated with orogenesis. However, in continental arcs associated with a subduction zone, tonalite, granodiorite, and granite are the dominant rocks.

How does gabbro occur?

Gabbro rocks can occur as layered, non-layered, or both layered and non-layered bodies.

Let us discuss each of these three a little more.

1. Layered gabbro

Gabbro rocks may form centimeters to meters layers with differences in mineral composition. i.e., modal layering.

Modal layering occurs due to fractional crystallization and consequent crystal settling as denser cumulates. For instance, pyroxene and plagioclase may form such cumulates. Also, nucleation habits may cause layering.

In layered gabbro, larger intrusions may preserve magma crystallization occurrence. Also, they indirectly show how the melt evolved chemically in composition via mineralogical and chemical layering.

Furthermore, you may have melanocratic, mesocratic, or leucocratic bands. Such occurs due to the internal grading of mafic minerals. For instance, some gabbroids may have rhythimic alternating, nearly uniform dark pyroxene and lighter plagioclase-rich layers. Sometimes, layering can occur in layers with darker mafic minerals that grade into lighter ones upwards.

Some layered gabbro rocks are the Bushveld complex in South Africa (associated with platinum), the Stillwater Complex in Montana, USA, and the Skaergaard intrusion in Greenland. Others are Muskox intrusion in North Territories, Canada, Rum layered Intrusion, and Island of Skye in Scotland. Also, they occur near Stavanger in Norway.

2. Massive gabbro rocks

Sometimes, gabbroic rocks may occur as massive bodies with isotropic fabric in intrusions or even upper parts of the oceanic crust. Such bodies don’t show preferred crystal orientation and happen due to in situ crystallization of minerals.

However, some may show local inhomogenity or streaking without regular layering patterns.

3. Both layered and non-layered

Lastly, some gabbro rock bodies may be both layered and non-layered. Such may have a layered lower part and a homogenous upper portion and are common in deep oceanic crust and some large intrusions. A good example is the Duluth Complex in Minnesota, USA, one of the largest gabbro intrusions.

What is gabbro used for?

Gabbro is a hard and durable rock that resists weathering with properties comparable to granite. Some uses are in making gravel as a dimensional stone, landscaping, building monuments, making grave markers, etc.

Here is more on gabbro rock uses:

• Crushed gabbro makes excellent quality aggregate for concrete, railroad ballast, road bases, fill, and other construction projects. You can also use it in unpaved walkways, driveways, or patios.
• Cut and polished to make dimensional stones for various uses, including flooring tiles, ashlars, facing stones, countertops, and paving. In the dimensional stone industry, gabbro and others like diorite, diabase, norite, basalt, and anorthosite are sold as black granite.  
• Some valuable minerals like chromium, nickel, and platinum are almost exclusively associated with gabbroids and other mafic or ultramafic rocks. Also, gabbroic complexes may have gold, silver, copper sulfide, cobalt, titanium, and palladium ores. For instance, the Bushveld Complex’s Merensky Reef is one of the most valuable platinum deposits.
• They make cemetery markers, monuments, curbing stones, landscaping stones, etc. However, they are hard, making them difficult to work with and limiting their use.

Indigo gabbro or mystic merlinite

Indigo gabbro is a semi-precious rock mined in Madagascar with a mottled black and lilac gray appearance, with some occurrences reported in Alaska. It is also known as mystic merlinite.

Some of the uses of indigo gabbro include making jewelry like beads, bracelets, pendants, necklaces, or as a decorative stone.

As a decorative stone, it may make cabochons, towers, or spheres, polished or sold as raw or tumbled stone.

Besides these uses, some people also associated it with various healing and metaphysical uses.

Lastly, from what we have seen online, it seems to have some mafic minerals and most likely a feldspar (the lighter-colored part). Unfortunately, we haven’t had a chance to confirm whether the rock is gabbroic.

Frequently asked questions

References

• Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
• Gruenewaldt, G. V. (1989). Gabbro and norite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 175-177). New York: Van Nostrand Reinhold.  (pp. 175-177
• Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.
• Frost, B. R. (2014). Essentials of igneous and metamorphic petrology. Cambridge University Press.
• Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
• Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company
• Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
• Perkins, Dexter, “Mineralogy” (2020). Open Educational Resources. 20. https://commons.und.edu/oers/20