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.
Plutonic igneous rock
• sodic plagioclase
• alkali feldspar
Q = 20 – 60%
Plagioclase/feldspars = 65 – 90%
• leucogranodiorite (M < 5%)
• melagranodiorite (M > 25%)
Extrusive equivalent: dacite
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