Gabbro


Gabbro is a coarse-grained and usually dark-colored igneous rock. Gabbro is an intrusive rock. It means that it formed as magma cooled slowly in the crust. Igneous rocks with similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead).

The most important mineral groups that make up gabbro are plagioclase and pyroxene. Plagioclase usually predominates over pyroxene. Plagioclase is sodium-calcium feldspar. It contains more calcium than sodium in gabbro. If there is more sodium in the plagioclase, then the rock type is named diorite. Diorite is usually lighter in color and contains more amphiboles than pyroxenes.

“Gabbro” in the strict sense of the term is an intrusive rock that is chiefly composed of monoclinic pyroxene (clinopyroxene, abbreviated Cpx) and plagioclase. If more than 5% of Cpx is replaced with orthorhombic pyroxene (orthopyroxene, abbreviated Opx), the rock is named gabbronorite. If more than 95% of the pyroxene is Opx, then we have norite. These rocks are all collectively called gabbroic rocks. Sometimes the term “gabbro” is used loosely to include them all. Rocks that contain more than 90% pyroxene is pyroxenite. Gabbroic rocks may contain small amounts of quartz (up to 5%). If there is more quartz present, the rock must be named quartz gabbro. If quartz makes up more than 20%, then it isn’t gabbro anymore. Such rocks are called granitoids1.

The classification scheme for gabbroic rocks without olivine and feldspathoids.

There are even more varieties possible. Olivine can be present or feldspathoids just like in basalt. So, rock names like nepheline-bearing gabbro or olivine norite are possible. Anorthosite and troctolite are similar rocks also. Anorthosite is almost monomineralic rock — more than 90% of the plagioclase-pyroxene pair is plagioclase. Troctolite contains mostly olivine and plagioclase, pyroxene again forms no more than 10% of the composition.

It is often mentioned that the term “gabbro” was brought into geological terminology by a German geologist Leopold von Buch. It is probably not correct. The term was used by Italian geologist Tozzetti in 1768. The term “gabbro” itself comes from Italy. Von Buch redefined the term in 1810 as a rock composed of diallage and feldspar or saussurite. Saussurite is a mixture of epidote, albite, and other alteration minerals. Saussuritized gabbro can be named metagabbro nowadays because its original composition has clearly been altered by metamorphism. Diallage isn’t defined as a mineral now but it is monoclinic pyroxene anyway (either diopside or augite). So, we can say that von Buch used the term roughly as we use it today.

Gabbros form in the crust. This is the magma that didn’t break to the surface to cool as a basalt. Gabbros are not as widespread as granitoids but they are definitely not rare rocks. Gabbroic rocks are usually equigranular (composed of similarly sized grains) mixtures of black, brown or greenish pyroxene and white, gray, or greenish plagioclase. Pyroxenite and other ultramafic rocks are darker- and diorite is lighter-colored.

Hand sample of gabbro I collected in the Isle of Skye in Scotland. I know it is gabbro in the strict sense because pyroxenes are monoclinic (they have inclined extiction). One edge of the sample is straight because I took a slice of it with a rock saw to make a thin section. Width of sample is 55 mm.

Norite from Rustenburg, South Africa. Black is orthopyroxene, gray and greenish gray is plagioclase. The rock contains very small amount of quartz. Width of view is 65 mm.

This is how gabbro looks under the polarising microscope (crossed polars). These colors are not real, they are caused by the interference between lightwaves. Plagioclase is striped black-gray-white, pyroxene is colorful, there are some small olivine grains as well. Yellow stripes in blue in the upper part of the image is an intergrowth of two pyroxenes (Opx in Cpx).

Pegmatitic gabbro from Cyprus.

Gabbro from the Caldera de Taburiente on La Palma (The Canary Islands). La Palma is an oceanic island which is largely composed of volcanic rocks. Deep-seated intrusive rock gabbro is exposed there because of uplift and subsequent deep erosion.

Gabbro from La Palma. Width of sample is 5 cm.

Gabbro from the Troodos ophiolite in Cyprus. Width of sample is 7 cm.

References

1. 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.


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