Granodiorite (// or //) is a phaneritic texture intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. According to the QAPF diagram, granodiorite has a greater than 20% quartz by volume, and between 65% to 90% of the feldspar is plagioclase. A greater amount of plagioclase would designate the rock as tonalite.
Granodiorite is felsic to intermediate in composition. It's the intrusive igneous equivalent of the extrusive igneous dacite. It contains a large amount of sodium (Na) and calcium (Ca) rich plagioclase, potassium feldspar, quartz, and minor amounts of muscovite mica as the lighter colored mineral components. Biotite and amphiboles often in the form of hornblende are more abundant in granodiorite than in granite, giving it a more distinct two toned or overall a darker appearance. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals. minor amounts of oxide minerals such as magnetite, ilmenite, and ulvospinel, as well as some sulfide minerals may also be present.
On average the upper continental crust has the same composition as granodiorite.
Granodiorite is a plutonic igneous rock, formed by an intrusion of silica-rich magma, which cools in batholiths or stocks below the Earth's surface. It is usually only exposed at the surface after uplift and erosion have occurred.
The name comes from two related rocks: granite and diorite to which granodiorite is an intermediate. The grano- root comes from the Latin for 'grain', an English language cognate. Diorite is named after the contrasting colors of the rock.
Granodiorite is most often used as crushed stone for road building. It is also used as construction material, building facade, and paving, and as an ornamental stone. The Rosetta Stone is a stele made from granodiorite.
- This article incorporates public domain material from the United States Geological Survey document: "Granodiorite definition".
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