QUARTZ MONZONITE
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).