Gabbro

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gabbro A coarse-grained, basic igneous rock, consisting of essential calcium-rich plagioclase feldspar (approximately 60%), clinopyroxene (augite or titanaugite), and orthopyroxene (hypersthene or bronzite), plus or minus olivine with accessory magnetite or ilmenite. Gabbros result from the slow crystallization of magmas of basaltic composition, and like the basalts they can be divided into tholeiitic and alkali types (compare ALKALI BASALT; and THOLEIITE). Tholeiitic gabbros are characterized by the presence of two pyroxene types (augite and hypersthene) and interstitial silica-rich glass, whereas alkali gabbros are characterized by one calcium/titanium-rich pyroxene (titanaugite) and scattered interstitial feldspathoid minerals. Many large gabbroic intrusions display mineral layering, testifying to the complex processes taking place within basic magma chambers. Gabbros are commonly found intruded as ring complexes (e.g. Ardnamurchan and Skye in Scotland), large lopoliths (Bushveld complex, S. Africa), or layered complexes (Skaergaard in eastern Greenland being the most famous).

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gabbro: see basalt.

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gabbro (min.) rock composed of feldspar and diallage. XIX. — It. (Tuscan) :- L. glaber, glabr- smooth.