What is a gabbro in geology ?

Gabbro : definition

Streckeisen diagrams (or QAPF diagrams) - Classification of plutonic and volcanic rocks
A gabbro is a basic grainy plutonic rock, blackish green in color, composed mainly of calcium plagioclase (labrador to anorthite) and interstitial pyroxene (diopside, augite or hypersthene), with brown hornblende, olivine and biotite as accessory minerals.

The varieties are essentially based on variations in mineralogical composition : troctolite (abundant olivine, rare pyroxene), norite (dominant hypersthene), quartz gabbro (abnormal presence of andesine quartz). Gabbros occur in clumps of reduced size, often associated with diorites or peridotites. Gabbro is the plutonic equivalent of basalt.

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