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" N, ~112 22' 34" W).



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