A coarse-grained mafic plutonic rock composed of
calcic plagioclase (labradorite or bytwonite) and clinopyroxene
(augite), with or without olivine and orthopyroxene.
Common accessory minerals include apatite, magnetite, and
ilmenite. It is the intrusive equivalent of basalt, named after
the town of Gabbro in Tuscany, Italy.
Gabbro is a common constituent of oceanic crust, where
it crystallizes from magma chambers that form beneath
oceanic spreading centers. Oceanic gabbros have been
dredged from the seafloor and studied extensively in ophiolite
complexes that represent fragments of oceanic crust thrust
onto continents during tectonic collisions. Gabbro from these
complexes is quite variable in texture and composition. Highlevel
ophiolitic gabbros are typically homogeneous or
isotropic but grade downward into compositionally layered
varieties where variations in the amounts of plagioclase and
pyroxene result in a pronounced layering to the rock. These
layers are thought to represent layers that accumulated along
the sides and base of the magma chamber.
Many continental plutons also have gabbroic components
or are made predominantly of gabbro. Some continental
intrusions form large layered mafic-ultramafic layered
complexes that have large quantities of gabbro. Examples of
these complexes include the Skaergaard intrusion, Greenland;
the Bushveld, South Africa; and the Muskox, Canada.
See also IGNEOUS ROCKS; OPHIOLITES.
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