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Geology is the Way

Granodiorite

Granodiorite is a type of plutonic igneous rock with felsic to intermediate composition, representing – together with tonalite – the plutonic counterpart of dacite. Its name comes from the union of the words ‘granite‘ (granum = grain) and ‘diorite’, since its composition is halfway between these rock types. Granodiorites contain quartz, plagioclase, alkali feldspars, and femic minerals (most commonly biotite and/or hornblende). In the QAPF diagram, a rock can be classified as a granodiorite if quartz represents 20 to 60% of the felsic minerals and plagioclase is 65 to 90% of all feldspars. Plagioclase in granodiorites normally shows sodium-rich compositions (oligoclase to andesine). If the plagioclase composition is richer in calcium (above An50) the rock can be classified as a granogabbro, a very rare igneous rock.

granodiorite

Biotite-bearing granodiorite outcropping at Graniteville, Vermont (USA). Barre Pluton, New Hampshire Plutonic Series (Devonian). Photo © James St. John.

granodiorite

Granodiorite with well-visible prismatic crystals of black amphibole. Tobacco Root Batholith (Late Cretaceous), Madison County, Montana, USA. Photo © James St. John.

Granodiorite
Plutonic igneous rock
Felsic minerals:
quarzo
• sodic plagioclasio
alkali feldspar
Mafic minerals:
biotite
hornblende

QAPF classification:
Q = 20 – 60%
Plagioclase/feldspars = 65 – 90%
Colored varieties:
• leucogranodiorite (M < 5%)
• melagranodiorite (M > 25%)
Other varieties:
granogabbro
Extrusive equivalent: dacite

granodiorite

Granodiorite with quartz, plagioclase, alkali feldspar, and hornblende amphibole. Giant Forest Granodiorite (middle Cretaceous). Moro Rock, Sequoia National Park, California, USA. Photo © James St. John.

Neves granodiorite

Biotite granodiorite from Neves, Sud Tirol, Italy.

Bibliografia
Frost, T. P., & Mahood, G. A. (1987). Field, chemical, and physical constraints on mafic-felsic magma interaction in the Lamarck Granodiorite, Sierra Nevada, California. Geological Society of America Bulletin99(2), 272-291.
Glazner, A. F., Coleman, D. S., & Bartley, J. M. (2008). The tenuous connection between high-silica rhyolites and granodiorite plutons. Geology36(2), 183-186.
Martin, H., Smithies, R. H., Rapp, R., Moyen, J. F., & Champion, D. (2005). An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos79(1-2), 1-24.
Streckeisen, A. (1976). To each plutonic rock its proper name. Earth-science reviews12(1), 1-33.

        

Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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