gabbro

Gabbro is a coarse-grained, mafic, plutonic igneous rock that forms at spreading centers in rift zones and mid-ocean ridges (so underlies oceanic crust). Gabbros can form as massive uniform intrusions or as layered ultramafic intrusions formed by settling of pyroxene and plagioclase feldspar (pyroxene-plagioclase cumulate). As an essential component of the oceanic crust, gabbros are found in many ophiolite complexes in the sheeted dike zone to massive gabbro zone (zones III and IV). Long belts of gabbroic intrusions are typical at proto-rift zones and around ancient rift zone margins, where they intrude into the rift flanks. Although gabbro is the plutonic equivalent of volcanic basalt, gabbros have much more varied mineral assemblages. Gabbros are denses rock that are greenish or dark-colored and comprises varied percentages of pyroxene, plagioclase feldspar, amphiboles, and olivine. Where olivine is present in large quantities, the rock is termed olivine gabbro. Gabbros are commercially important because they are often associated with Ni, Cr, and Pt ores, which occur almost exclusively in association with gabbroic or related ultramafic rocks. Gabbroic complexes are often associated with primary magnetite (Fe) and ilmenite (Ti) deposits. Banded, or layered, gabbroic complexes with well-developed monomineral or bimineral varieties are found in Montana, the Bushveld Complex in South Africa, and the Black Cuillin mountains of Skye (Ghreadaidh via Coire a Ghreadaidh) Inhomogeneous gabbro complexes that are not regularly layered are associated with the large, basinlike intrusion in Sudbury, Ontario, large diabase sills (tabular intrusions) in the Palisades, New Jersey, and many Karoo diabases (fine-grained gabbro) in South Africa. The Bushveld layered mafic intrusion has alternating mafic (chromite) and felsic (anorthosite) layers that are believed attributable to crystal fractionation in a large magma chamber; with alternating pulses of new magma, such as the vertical dike that cuts through the outcrop. A finer-grained rock with the same composition as gabbro is termed diabase.

images : formations: layered gabbro, North Cascades : Salem gabbro-diorite cut by a a composite dike with felsic margins and a central core of basaltic rock : White Mountain Magma Series; gabbro dike in Adirondacks, and chill zone of gabbro dike; Black Cuillin mtns., Skye: pegmatitic gabbro : oceanic crust exposed on Cyprus; Gabbro Dyke with some Visible Gold; dikes in gabbro, Lizard; hummocky terrain of gabbro-diabase, Hess Mine, Yuma; Bird's Eye Gabbro, with spotted bands of large hornblende crystals; hand-specimens: gabbro, 1, 2, 3, 4, 5, 6, l; gabbro that is roughly half pyroxene and half plagioclase, and close-up, and gabbro with lots of coarse, dark pyroxene crystals, and close-up, gabbro in which it is difficult to discern the minerals, and close-up (Pyroxene is typically duller and more blocky than hornblende, but most important are the cleavages intersecting at 90°); layered gabbro; mii; close-up: gabbro; gabbro, Lizard, 2; gabbro, Cuillin; polished surface; migmatitic gabbro, and garnet vein in gabbro, and veins crossing contacts; oceanic crust gabbro; thin sections: Oman Ophiolite gabbro; gabbro fsu, 2; thin section of olivine gabbro - pyroxene and olivine show bright colours, striped grey rectangular crystals are plagioclase feldspar : thin section of gabbro with plagioclase and hypersthene (orthopyroxene) : hypersthene gabbro : thin section with pyroxene and (striped) plagioclases : thin section orthopyroxenes crystals surrounded by alteration (uralite) : thin section : thin section with twinned plagioclases; gabbro, Mt. Megantic, Monteregian Hills; gabbro; gabbro, Hess Deep, plagioclase (gray tones) and clinopyroxene (brown to blue tones); pigeonite exsolution