Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

intrusion in the Trout Creek area and Bickford and Boardman (1984) reported a U-Pb
zircon age determination of 1,672 +/- 5 Ma from the southern part of the intrusion in the
Cameron Mountain quadrangle.
Two main areas of Proterozoic biotite granodiorite occur in the Maysville
quadrangle: a large area of deformed, foliated biotite granodiorite (Xgdf) in the northwest
quadrant and small, scattered intrusions of undeformed biotite granodiorite (Xgd) in the
southwest quadrant. The foliated granodiorite is equivalent to Proterozoic foliated
granodiorite (Xgdf) of Keller and others (2004) in the Buena Vista East quadrangle
Xgdf Foliated augen granodiorite (Early Proterozoic) – Foliated augen granodiorite
is present in an 18,000-ft long and 1,000- to 5,000-ft wide, east-northeast-trending zone
crossing the southeast slopes of Mount Shavano (plate 1). It is medium to dark gray and
medium to very coarse grained. It is composed of large (0.5 to 1.5 inch) microcline
phenocrysts and augen in a medium-grained matrix of plagioclase, quartz and biotite,
with accessory magnetite and sphene (fig. 27). The microcline crystals range from
tabular, subhedral to euhedral, moderately aligned phenocrysts to more commonly highly
stretched and flattened augen. Estimated modes include 20 to 25 percent quartz, 30 to 40
percent microcline, 30 to 35 percent plagioclase, and 8 to 12 percent biotite and
magnetite. The foliated granodiorite is mineralogically variable and plots in the granite b
field close to the quartz monzonite join (IUGS classification). Crawford (1913) provided
a whole-rock chemical analysis of foliated granodiorite from Jennings Creek (Garfield
quadrangle) that indicates a chemical classification (De la Roche and others, 1980) of
granodiorite.
The foliated granodiorite displays variable textural development of moderate to
strong penetrative mylonitic deformation fabrics. It displays a weak to strong foliation
with the preferential alignment of biotite and remnant alkali-feldspar phenocrysts that are
locally drawn out to a well-developed augen structure. Overall, the fabrics are
protomylonitic to orthomylonitic. Discrete mappable zones of stronger mylonitic fabrics
were not identified but they are preferentially developed along contacts with Early
Proterozoic gneiss.
The foliated granodiorite intrudes older Early Proterozoic gneisses (Xag and
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