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Granodiorite

A phaneritic (visibly crystalline) plutonic rock composed chiefly of sodic plagioclase (oligoclase or andesine), alkali feldspar (microcline or orthoclase, usually perthitic), quartz, and subordinate dark-colored (mafic) minerals (biotite, amphibole, or pyroxene). Granodiorite is intermediate between granite and quartz diorite (tonalite). For convenience granite and granodiorite are commonly grouped and referred to as granite.

Granodiorite is an intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.

Medium- to coarse-grained rock that is one of the most abundant intrusive rocks. It contains quartz and is distinguished from granite by having more plagioclase feldspar than orthoclase feldspar; its other mineral constituents include hornblende, biotite, and augite. Granodiorite is similar to granite in appearance but darker. On average the upper continental crust has the same composition as granodiorite. The Rosetta Stone was carved out of grandiorite.

Granodiorite is typically intermediate colored with a subequal mixture of light colored sodium plagioclase/quartz, and dark colored amphibole and biotite. Appearance, like diorite, is often described as "salt and pepper" because of the mix.

The average composition of the upper continental crust is equivalent to the composition of granodiorite. Often forming large masses in the roots of mountain ranges, granodiorites occur worldwide, but are particularly abundant in North America, Scandinavia, and Brazil.

Tonalite–trondhjemite–granodiorite (TTG) series are rock aggregates formed by melting of wet mafic crust at high pressures. Evidence indicates that "wet melting" of garnet-amphibolite and/or eclogite was an important element of early continent formation (cratonization). It is widely accepted that most Archaean granite–greenstones are dominated by TTG, although Late Archaean terranes, such as in the Yilgarn Craton, are dominated by K-rich granitoid rocks derived from remelting of older felsic TTG-dominated crust. According to this model, a much greater degree of crustal reworking has occurred in the Pilbara craton than is required by TTG-dominated crust.

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