Gabbro
Gabbro is a dark colored plutonic rock composed
mainly of coarse grains of calcium rich plagioclase
feldspar and pyroxene. In addition, the rock may contain
olivine, hornblende, biotite, garnet, rutile, apatite,
zircon, magnetite, ilmenite, and chromite. Chemically and
mineralogically, gabbro is equivalent to basalt. Gabbros
may occur as border rocks around granitic and other
plutons, or as small individual plutons or dikes. Their
most common occurrence is in the lower parts of large,
layered complexes. Most gabbros appear to intrude the rocks
surrounding them and so are thought to be of igneous
origin, although some may also be produced by metamorphic
processes. It is lower in silica and darker in color than
diorite, and chemically the equivalent of diabase.