GABBRO

Gabbro is a medium or coarse-grained rock that consists primarily of plagioclase feldspar and pyroxene. Essentially, gabbro is the intrusive (plutonic) equivalent of basalt, but whereas basalt is often remarkably homogeneous in mineralogy and composition, gabbros are exceedingly variable. Gabbros are found widely on the Earth and on the Moon as well. Gabbros are sometimes quarried for dimension stone (the black granite of commerce), and the San Marcos Gabbro of southern California is used for gauge blocks, but the true economic value of gabbro is minor. Far more important are the nickel, chromium, and platinum that occur almost exclusively in association with gabbroic or related ultramafic rocks. Primary magnetite (iron) and ilmenite (titanium) mineralizations are often intimately associated with gabbroic complexes.

Banded, or layered, gabbroic complexes in which monomineral or bimineral varieties are well developed have been described from Montana, the Bushveld in South Africa, and the island of Skye. There are also gabbro complexes that are inhomogeneous and not regularly layered, as the large, basinlike intrusion at Sudbury, Ontario, and some of the larger diabase sills (tabular intrusions), as the Palisades, New Jersey; and many of the Karoo diabases (fine-grained gabbro) in South Africa. A lopolith at Duluth, Minn., is a notable exception to the rather arbitrary division between layered and unlayered gabbro complexes. The lower part of this mass has the average composition of an olivine gabbro but is strongly banded. The upper portion is a comparatively homogeneous feldspathic gabbro, not sharply banded.

Although gabbro forms in diverse tectonic settings, much is thought to form at divergent plate margins. Here, the gabbro is a product of mantle-derived partial melts of peridotite. These partial melts rise bouyantly in the oceanic crust and solidify. The upper portion of the magma chamber crystallizes as the fine-grained, ubiquitous, pillow lavas characteristic of the ocean floor, while the middle and lower portions of the system soldify as diabasic dikes and cumulus textured gabbro.

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