Quartz monzonites have the same general characteristics and compositions as true granites (= coarsely crystalline/phaneritic-textured, intrusive igneous rocks composed of quartz, K-feldspar, plagioclase feldspar, and usually one or more mafic minerals). Quartz monzonites, however, have less quartz than true granites. The quartz component of quartz monzonites is 5% to 20%, whereas true granites have 20% to 60% quartz.
Quartz monzonite (6.7 cm across) dominated by quartz + orthoclase feldspar + plagioclase feldspar + biotite from the Butte Quartz Monzonite (Boulder Batholith, Campanian Stage to Maastrichtian Stage, late Late Cretaceous, 68-78 m.y.) at an outcrop along the southern side of I-90, just east of Homestake, southeast of Butte, southwestern Jefferson County, southwestern Montana, USA (~45° 54’ 53” North, ~112° 22’ 34” West).