Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

forming an arcuate layer about 6,500 ft long. The layer of amphibolite gneiss forms a
concentrated float zone about 600 to 1,000 ft east of, and subparallel to, the main
muscovite-sillimanite gneiss and amphibolite contact. On the basis of the orientation of
foliations and the contact patterns this lithologic-structural domain consists of a
structurally lower felsic gneiss sequence to the west and a structurally higher mafic
amphibolite sequence to the east.
The muscovite-sillimanite gneiss is cut by abundant, undeformed pegmatite dikes
(YXp) and some white bull quartz (+/- muscovite) bodies. Some of the pegmatite bodies
are moderately foliated, suggesting they are related to the Routt Plutonic Suite, and some
pegmatites appear to be segregations within the muscovite-sillimanite gneiss. The
muscovite-sillimanite gneiss is cut by a possibly Proterozoic microdiorite dike (Xmd) in
the north part of the lithologic-structural domain and by a possibly Tertiary andesite dike
on the south side of Willow Creek. The protolith of the muscovite-sillimanite gneiss is
problematic. Previous workers have interpreted it as a metarhyolite lapilli-crystal tuff
(Alers and Shallow, 1996). This interpretation is supported by the local interlayering and
the conformable relationship to the amphibolite gneiss. However, the overall lack of
remnant textures and layering and the relative uniformity and coarse-grain size of this
unit may suggest a metamorphosed and deformed granitic intrusion as the protolith.
Xmfs Muscovite felsic schist (Early Proterozoic) – The Early Proterozoic muscovite
felsic schist (Xmfs) is present in a large lithologic-structural domain in the southwest
quadrant of the Maysville quadrangle. It also occurs in a separate, smaller domain about
4,000 feet southwest of Maysville. The larger area of muscovite felsic schist is about
2,500 ft by 11,000 ft and forms an east-northeast-trending zone in the lithologic-structural
domain that contains the muscovite-cordiertite schist (Xmc).
The muscovite felsic schist is light tanish white to medium gray, fine to
predominantly medium grained, and moderately to strongly foliated (fig. 34). It is a
muscovite-rich, quartzo-feldspathic-rich rock that has the general appearance of a
uniform metagranitic rock. It does not exhibit gneissic layering or significant variations
in grain size. Muscovite is locally present as porphyroblasts to about ½ inch in size. It
lacks visible sillimanite in hand sample and does not contain the eyes or nodules of
138