Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Tr Rhyolite dikes (early to late Oligocene) – Two aphyric rhyolite (Tr) dikes are
present in the northwest quadrant of the Maysville quadrangle. The dikes are part of a
northeast-trending set of dikes including rhyolite porphyry (Trp), rhyolite (Tr), finegrained
granite (Tcf), and quartz latite porphyry (Tqlp) that cut Proterozoic rocks in the
Maysville quadrangle and extend into the southern part of the Mount Antero quadrangle
where they cut the Mount Princeton pluton.
The rhyolite dikes are light gray, aphyric, and aphanitic, and consist of an
extremely fine-grained quartz-feldspar mosaic intergrowth. They are generally massive
and have minor irregular flow layering developed at some contacts. The dikes are not
well exposed but occur as concentrated float zones. The small angular float rock is
typically lost or overwhelmed in the coarse block talus characteristic of the Proterozoic
granodiorite (Xgdf) host.
Two segments of a probable 4,500 ft long continuous aphyric rhyolite dike cut
Early Proterozoic granodiorite (Xgdf) about 4,000 ft east-southeast of Shavano Lake. The
dike does not crop out and occurs as narrow concentrated float zones that suggest
thicknesses of less than ten feet and a northeast strike with a moderately steep dip to the
northwest. A second aphyric rhyolite dike is present along the main northeast-trending
range-front structure (Sawatch fault zone) north of Squaw Creek. Concentrated rhyolite
porphyry dike (Trp) float is almost continuous along this 6,000-foot-long fault zone and
aphyric rhyolite (Tr) is present at two localities at the center and north end of the
structure. The aphyric rhyolite occurs as minor subcrop and as concentrated float
suggesting dike thicknesses up to about 10 ft.
The field relations suggest that the fault zone has a rhyolite porphyry dike and
locally a composite rhyolite porphyry-aphyric rhyolite dike along it. The relative ages of
the rhyolite porphyry and aphyric rhyolite dikes was not established, but their presence as
subparallel dikes and presence in the same conduits suggests a similar age. Much of the
aphyric rhyolite along the Sawatch fault zone is brecciated. Two types of breccia are
present including (1) an annealed, partly open, crackle breccia that may represent a latestage
hydrothermal breccia and (2) a tectonized-silicified breccia suggesting continued
fault movement along the structure after the rhyolite dike was emplaced.
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