Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

(Xmsg) with minor amphibolite gneiss in the western part and lineated amphibolite (Xal)
in the eastern part. The two lithologies form a coherent structural block where the
contacts between the units appear concordant and the foliations and lineations are similar
and dip to the east and east-northeast. The muscovite-sillimanite gneiss forms a roughly
north-south-trending, crescent-shaped zone that is about 11,000 ft long and up to 3,000 ft
wide; this zone extends from Green Creek north-northwest across Willow Creek. It is
poorly to moderately well exposed and locally makes good, continuous ledge outcrops.
The muscovite-sillimanite gneiss varies from a white to a mostly light- to
medium-orangeish-brown color. The orangeish-brown color is related to a weak to
moderate, pervasive limonite staining that generally increases to the south. Most of the
muscovite-sillimanite gneiss is nonmagnetic and the limonite stain may be related to
pervasive oxidation of disseminated magnetite. It is fine to medium grained (locally
coarse grained) and moderately to strongly foliated. The muscovite-sillimanite gneiss
generally lacks gneissic layering but is characterized by small (0.25 to 1.0 inch) eyes,
lenses, or flattened nodules consisting of fine-grained sillimanite-quartz-muscovite (fig.
33). The amount, size, and composition of the eyes vary within the unit. Minor zones
within the muscovite-sillimanite gneiss lack the eyes and other areas have small (0.25 to
0.5 inch) muscovite porphyroblasts. The eyes tend to be the same size in any given area
and the size and abundance gradationally varies from place to place. Local gneissic
structures are produced by discontinuous layers of medium- to coarse-grained microcline
and quartz.
A study of two thin sections shows 35 to 40 percent quartz, 20 to 25 percent
microcline, 5 to 15 percent plagioclase, 10 to 15 percent muscovite, 10 to 12 percent
sillimanite, 3 to 10 percent biotite, with accessory zircon. On the basis of estimated
modes the rock has a granitic composition that would plot near the join between the
granite a and granite b fields (IUGS classification). The sillimanite tends to be
concentrated in the eyes and occurs as very fine-grained bundles of needles that are
locally intergrown with muscovite. The sillimanite and muscovite locally fringe each
other, but textures usually suggest that sillimanite replaces the muscovite. Both thin
sections show evidence of larger microcline grains with well-developed perthitic textures
that appear to be pegmatitic segregations. The microcline grains are locally extensively
136