Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

The southeastern contact of the Mount Pomeroy subunit is in intrusive contact
with Early Proterozoic granodiorite (Xgdf) for about 10,000 ft across the Maysville
quadrangle (plate 1). The contact trends about N60°E and extends from about 1,500 ft
southeast of the summit of Mount Shavano to Lake Shavano along Squaw Creek. The
overall relation between the contact and topography suggest that the Mount Pomeroy
subunit contact is vertical to steeply inward (northwestward) dipping across the
Maysville quadrangle. The contact is mostly discordant and oblique to foliations in the
Early Proterozoic granodiorite (Xgd). No finer-grained, chilled textures are evident in
the Mount Pomeroy subunit along the contact. Evidence for this being an intrusive
contact is supported by dikes (2 to 100 ft thick) of Mount Pomeroy subunit that cut
Proterozoic rocks along the contact zone.
Whole-rock chemical analyses were compiled on the various subunits of the
Mount Princeton pluton (Shannon, 1988). Three analyses of the Mount Pomeroy subunit
(from St. Elmo and Whitepine quadrangles) show the most within-group variation. A new
whole-rock chemical analysis of the Mount Pomeroy subunit from the Maysville
quadrangle is given in table 1. This sample has the lowest SiO 2 and K 2 O contents and
highest Fe-total and MgO contents compared to all Mount Princeton pluton samples. The
sample is chemically classified (De la Roche and others, 1980) as tonalite. The tonalitic
composition of the Mount Pomeroy subunit contrasts with the composition of the interior
of the Mount Princeton pluton which is predominantly chemically classified as
granodiorite.
Tmpf Mount Princeton finer-grained quartz monzonite subunit (late Eocene) –
The Mount Princeton finer-grained quartz monzonite subunit is a light-gray to slightly
pinkish-gray, medium- to coarse-grained equigranular rock (fig. 23). Plagioclase is white
in contrast to the purplish-gray plagioclase in the Mount Pomeroy subunit. Biotite and
hornblende, and relatively coarse and abundant accessory sphene, are conspicuous in
hand sample. Thin section studies of this subunit showed minor clinopyroxene in one
sample and accessory allanite, apatite, and zircon (Shannon, 1988). The average of three
modal analyses showed 23.6 percent quartz, 29.3 percent alkali feldspar, 36.5 percent
plagioclase, 9.8 percent hornblende-biotite, and 0.8 percent magnetite (Shannon, 1988),
100