gabbros

Gabbro is a coarse-grained, mafic intrusive igneous rock that is chemically equivalent to basalt but crystallizes at greater depth. The primary minerals are calcium-rich plagioclase feldspar and pyroxene, with smaller amounts of olivine in ultramafic varieties, and accessory minerals such as magnetite, ilmenite,apatite, and zircon. The plagioclase commonly ranges from labradorite to bytownite, and pyroxene is typically clinopyroxene (augite) or orthopyroxene.

Texture and appearance: gabbro is holocrystalline and coarse-grained, with grains large enough to be seen with the naked eye. It is generally equigranular, though some varieties are porphyritic with larger crystals of plagioclase.

Formation and occurrence: gabbro forms when basaltic magma crystallizes underground as dikes, sills, or within larger magma chambers, cooling slowly to produce a coarse texture. It constitutes the lower part of the oceanic and continental crust in many settings and is a major component of ophiolites, where mantle and lower crust materials are obducted onto continental margins.

Relation to basalt and variants: gabbro is the plutonic counterpart of basalt; the two share a similar chemical composition but differ in texture due to cooling rate. Ultramafic variants include norite (orthopyroxene + plagioclase) and troctolite (olivine + plagioclase), while anorthosite is dominated by plagioclase and is not considered a typical gabbro.

Uses and significance: gabbro is quarried as a dimension stone and decorative aggregate; some occurrences host magnetite and ilmenite, contributing to economic interest in certain localities.