DIORITE

Diorite is a medium to coarse-grained intrusive igneous rock that commonly is composed of about two-thirds plagioclase feldspar and one-third dark-colored minerals, such as amphibole and/or biotite. The presence of sodium-rich feldspar, oligoclase or andesine, in contrast to calcium-rich plagioclase, labradorite or bytownite, is the main distinction between diorite and gabbro. The extrusive (volcanic) equivalent of diorite is andesite.

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.

Diorite, and its volcanic equivalent andesite, are thought to be the be the initial products of plate subduction at convergent margins. Indeed, huge bodies of diorite and granodiorite form the core of the Sierra Nevadas. Radiometric age dating of batholiths has shown that diorite is consistently older than adjacent granites supporting a model which has granitic plutons being emplaced after diorite. This model suggests that subducted basaltic crust is partially melted and may be combined with some subducted oceanic sediments to form andesites/diorites. Diorite is then partially melted to generate the younger granitic magmas.

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