Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

One age determination on the rhyolite porphyry dikes in the Mount Antero region
was reported by Limbach (1975). A K-Ar whole rock age determination of 25.4 +/- 1 Ma
for a rhyolite porphyry dike in Chalk Creek was recalculated (Shannon, 1988) by using
revised constants from Dalrymple (1979) as 26.1 +/- 1 Ma.
Table 1 gives a whole rock chemical analysis (sample 05-542) of the main
northeast-trending rhyolite porphyry dike. The analysis indicates an alkali rhyolite
composition (De la Roche and others, 1980) with a very high SiO 2 content (75.82
percent), and high Na 2 O content (4.32 percent) and K 2 0 content (4.41 percent). This
analysis is very similar to a sample of rhyolite porphyry dike from the Buena Vista West
quadrangle (McCalpin and Shannon, 2005) except for slightly higher Al 2 O 3 and Na 2 O
contents. The chemistry of the rhyolite porphyry dike is also very similar to the North
Fork leucogranite (sample 05-575A, table 1).
Tcf California leucogranite-rhyolite dikes (early Oligocene) – These porphyritic,
fine-grained, biotite granite-rhyolite dikes (Tcf) are white to light gray and have variable
texture with generally moderate phenocryst content and aplitic groundmass with a 0.1 to
0.5 mm grain size. In general, the phenocrysts tend to be less euhedral and the
groundmass is coarser in the fine-grained granite-rhyolite dikes (Tcf) in comparison to
the rhyolite porphyry dikes (Trp). The distinction between phenocrysts and groundmass
is also less clear than in the rhyolite porphyry, because of the coarser and variably
textured groundmass. The overall phenocryst content varies from about 20 to 40 percent
with 8 to 15 percent quartz (1 to 3 mm), 7 to 12 percent alkali feldspar (1 to 3 mm), 5 to
12 percent plagioclase (1 to 2 mm), 2-4 percent biotite-muscovite (0.5 to 2.0 mm), 1 to 3
percent orangeish garnet (0.2 to 2.0 mm), and 0.5 to 2 percent magnetite.
Two dikes of porphyritic, fine-grained, biotite granite-rhyolite (Tcf) are mapped
in the northeast corner of the Maysville quadrangle (plate 1). In addition, numerous 1 to 5
ft thick dikes are present in the area around the summit of Mount Shavano and are
especially abundant around the California leucogranite intrusion (Tcm) in Squaw Creek.
One dike occurs as a 4,000 ft long, arcuate, northeast-trending dike crossing the ridge just
north of the summit of Mount Shavano; the dike extends for another 2,500 ft into the
southwest corner of the Mount Antero quadrangle. The dike is 8 to 20 ft thick and strikes
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