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Diorite

Any of various dark, granite-textured, crystalline rocks rich in plagioclase and having little quartz.

The texture of diorites is notably variable. Most often diorites are equigranular, with coarse, partly or mostly anhedral plagioclase and hornblende crystals, subordinate biotite, and interstitial quartz and orthoclase.

Diorite is found as isolated small bodies such as dikes, sills, and stocks, but it is also found in association with other plutonic rocks in batholithic bodies. It is closely associated with convergent plate boundaries where calc-alkalic magmatism and mountain building are taking place.

Diorite is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas.

Diorite has about the same structural properties as granite but, perhaps because of its darker colour and more limited supply, is rarely used as an ornamental and building material. It is one of the dark gray rocks that is sold commercially as black granite.

Most diorites are truly igneous; they have crystallized from molten material (magma). Occassionally, we find others that are products of reactions between magma and included fragments of foreign rock (xenoliths). Many have been chemically transformed (metasomatized) in the solid state from some pre-existing rock, such as gabbro, by the loss of certain constituent atoms and the gain of others.

Diorite occurs in small bodies such as sills (tabular bodies inserted while molten between other rocks), dikes (tabular bodies injected in fissures), stocks (bodies intruded upward), or as more irregular masses associated with gabbro. More commonly it occurs in batholiths (huge bodies) with granodiorite and granite. The igneous rock of the San Gabriel Mountains of southern California is predominantly diorite.

Material Notes:
Igneous rock, with a grain size 1- 3 mm (hypabyssal). It is intermediate in the acidic-basic range, with a total silica content of 55-65%. Predominent minerals in this generally medium-dark colored rock are horneblende and plagioclase feldspar.

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