Gabbro specimen; Rock Creek Canyon, eastern
Sierra Nevada, California; scale bar is 2.0 cm.
Close-up of gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California; scale bar is 5.0 mm.
() is a dark, coarse-grained, intrusive
igneous rock chemically equivalent to
basalt. It is a
plutonic rock, formed when molten
magma is trapped beneath the
Earth's surface and cools into a crystalline mass.
The vast majority of the Earth's surface is underlain by gabbro within the
oceanic crust, produced by basalt magmatism at
mid-ocean ridges.
Petrology
Gabbro is dense, greenish or dark-colored and contains
pyroxene,
plagioclase,
amphibole, and
olivine (olivine gabbro when olivine is present in large quantities).
The pyroxene is mostly
clinopyroxene; small amounts of
orthopyroxene may be present. If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then a
Norite.
Quartz gabbros are also known to occur and are probably derived from magma that was over-saturated with
silica.
Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the
feldspathoid mineral
nepheline. (Silica saturation of a rock can be evaluated by
normative mineralogy). Gabbros contain minor amounts, typically a few percent, of iron-titanium oxides such as
magnetite,
ilmenite, and
ulvospinel.
Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called
diabase, although the vernacular term
microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse grained to
pegmatitic, and some pyroxene-plagioclase
cumulates are essentially coarse grained gabbro, although these may exhibit acicular crystal habits.
Gabbro is usually
equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.
Distribution
Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase, or as part of a
layered intrusion as a
cumulate formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.
Gabbro is an essential part of the oceanic crust, and can be found in many
ophiolite complexes as parts of zones III and IV (
sheeted dyke zone to massive gabbro zone). Long belts of gabbroic intrusions are typically formed at proto-rift zones and around ancient
rift zone margins, intruding into the rift flanks.
Mantle plume hypotheses may rely on identifying mafic and ultramafic intrusions and coeval
basalt volcanism.
Uses
Gabbro often contains valuable amounts of
chromium,
nickel,
cobalt,
gold,
silver,
platinum, and
copper sulfides.
Ocellar varieties of gabbro can be used as ornamental facing stones, paving stones and it is also known by the trade name of 'black granite', which is a popular type of graveyard headstone used in funerary rites. It is also used in kitchens and their countertops, also under the misnomer of 'black granite'.
Etymology
Gabbro was named by the German geologist
Christian Leopold von Buch after a town in the
Italian Tuscany region. Essexite is named after the type locality in
Essex County, MA, USA.
See also
