quartz diorite . (a) In the IUGS classification, a plutonic rock with Q between 5 and 20, P/(A+P) greater than 90, and plagioclase more sodic than An₅₀. (b) A group of plutonic rocks having the composition of diorite but with an appreciable amount of quartz, i.e. between 5 and 20 percent of the light-colored constituents, according to Streckeisen (1967, p.157); also, any rock in that group; the approximate intrusive equivalent of dacite. Quartz diorite grades into granodiorite as the alkali feldspar content increases. Syn: quartz-bearing diorite.