Download Gabbro Igneous rock containing coarse, iron

Gabbro
Igneous rock containing coarse, iron-bearing hornblende and augite (dark), and scattered feldspar grains pink). Rock
crystallized from iron-rich magma at considerable depth beneath the surface. Gabbro is the coarse grained plutonic
(subsurface) equivalent of basalt lava.
Gabbro is a medium or coarsegrained 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.