Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Xgd Granodiorite (Early Proterozoic) – Five intrusions of biotite granodiorite are
present in the southwestern part of the Maysville quadrangle. The two larger bodies, on
the west edge of the quadrangle, are apophyses connected to two of three irregular
intrusions that were mapped as Pikes Peak Granite in the southeastern part of the Garfield
15’ quadrangle by Dings and Robinson (1957). They were later interpreted to be Early
Proterozoic Denny Creek Granodiorite (Tweto, 1987, p. A26 and A27). The granodiorite
is light to medium gray to pinkish gray and medium to very coarse grained (fig. 28). It is
characterized by large 0.5 to 1.0 inch, tabular, subhedral to euhedral microcline
phenocrysts in a medium-grained matrix of plagioclase, quartz, and biotite, with
accessory magnetite and sphene. Microcline phenocrysts are randomly oriented to rarely,
very weakly flow aligned. The mineralogy of the granodiorite is variable and the
composition changes with proximity to contacts and to a lesser extent with variations in
the amount of microcline phenocrysts. Estimates of mineral modes indicate 15 to 30
percent quartz, 25 to 40 percent microcline, 25 to 35 percent plagioclase, and 5 to 10
percent biotite and magnetite. Most of the samples are granite b but are close to the
granite b-granodiorite join (IUGS classification). Rocks from the borders of intrusions are
typically more mafic and finer grained and are locally transitional with hybrid
granodiorite (Xgdh) locally developed in the contact zone.
The largest body is a 2,500-ft by 3,000-ft by 4,000-ft triangular area across the
mouth of Fooses Creek in the west edge of the Maysville quadrangle. The second
apophysis is a 2,000 ft long, east-west oriented body that is about 6,000 ft north of the
southwest corner of the quadrangle. Two smaller, isolated bodies of granodiorite, about
1,500 ft long are present in the area between and east of the two apophyses. The fifth
intrusion of granodiorite (Xgd) is present in a 4,000-ft long east-west zone on the north
side of the South Arkansas River between the North Fork and Lost Creek. Numerous
small, dike-like bodies of granodiorite, too small to map, are present throughout the
Proterozoic terrane in the southwest part of the quadrangle. The contacts of the
granodiorite intrusions with the older Proterozoic gneisses are rarely exposed. However,
float trends and most of the dikes are parallel, or subparallel and concordant to the
gneissic layering.
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