It forms during the crystallization of the superheated and fluidized volatile-rich last magma melt portion. The groundmass can be pyroxene, plagioclase, ilmenite, etc. 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. This texture also occurs in granite, rhyolite, andesite, basalt, lamprophyre, and pitchstone. An Earlier stage forms phenocrysts and a later stage forms the coarse-grained matrix. Phenocrysts are usually plagioclase, pyroxene, and other mafic minerals in a coarse-grained matrix of the same minerals. Porphyritic gabbro has larger crystals (phenocrysts) embedded in a finer but coarse-grained groundmass. Let us look at these textures a little more. Also, it may show different textures like medium-grains (microgabbro), porphyritic, orbicular, pegmatitic textures, and rarely ophitic. However, altering olivine to iddingsite and other minerals may result in reddish splotches on the rock. It has equigranular hypidiomorphic grains with a somewhat speckled appearance characterized by mostly darker minerals (mafic) and fewer lighter minerals (felsic). ![]() Usually, gabbro is a tough, massive, or layered dark gray, dark green, or black rock with a coarse-grained texture. It differs from peridotites, which tend more towards greenish color since they are rich in olivine. 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. Tectonic environment: Deep inside the oceanic crust, divergent boundaries (volcanic arcs), convergent boundaries (continental collision), and continental intraplate hotspots and rifts.Medium-grained equivalent (microgabbro): Dolerite or diabase.Mineral composition: Pyroxene and calcium-rich plagioclase with lesser amounts of hornblende and olivine.Magnetism: Magnetite-rich varieties are weakly magnetic.Color: Dark colored – dark gray, dark green to black.Texture: Coarse-grained or phaneritic (1/16mm to 3cm grain size).Origin: Mostly intrusive or plutonic with few subsurface.Afterward, we will discuss its chemical and mineral composition and related gabbroic rocks.Īlso, there are parts on its classification, how it forms, where it occurs, some of its uses, and more.ĭark-gray gabbro rock with calcic plagioclase (light-colored) and darker mafic minerals from Duluth, Minnesota, in the USA. We will start by telling you what it looks like or how to identify it, i.e., its appearance (texture and color). This article will focus on gabbro and gabbroic rocks. ![]() ![]() Also, rocks with gabbroic character occur in lighter lunar highlands, i.e., on the moon. It occurs worldwide, mainly in deep ocean crust, continental arcs, intraplate hotspots, and rifts, and less often in island arcs. Gabbro is a relatively common rock with many uses. Today, it describes a mafic plutonic igneous rock with mainly calcium-rich plagioclase and pyroxene. Later in 1809, Christian Leopold von Buch, a German geologist, restricted the use of gabbro to only the Apennines ophiolites metamorphosed gabbro ( metagabbro). This rock was named after Gabbro, a small settlement village near Rosignano Marittimo in Tuscany, Italy. The term gabbro was introduced by Giovanni Targioni Tozzetti, an Italian geologist, in 1768 to describe ophiolites found in the Italian mountain range Apennines. Therefore, IUGS doesn’t recommend using plutonic exclusively when describing this rock. However, it may also form within thick basalt lavas. %), magnesium- and iron-rich magma primarily deep in the Earth’s crust. This basic, mafic rock forms from the slow cooling of silica-poor (45-52 wt. Its essential minerals are calcic plagioclase and pyroxene, but it may have minor amounts of amphibole, olivine, and other minerals. ![]() Gabbro is a dense, coarse-grained, dark-colored, mostly plutonic rock.
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