Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

sillimanite-quartz-muscovite that are characteristic of the muscovite-sillimanite gneiss
(Xmsg). It has some intergrown biotite, and localized textures suggest it was the original
phyllosilicate in the rock that has been variably replaced by muscovite.
The distribution of units suggests that the muscovite felsic schist occurs in a
stratigraphic position that is in between the muscovite-cordierite schist (Xmc) on the
south and a sequence of amphibolite gneiss (Xag) and biotite felsic gneiss (Xbfg) on the
north. The muscovite felsic schist forms a fairly large lithologically coherent area and it
is not interlayered with other Proterozoic units. Some float is mixed with amphibolite
gneiss and biotite felsic gneiss in contact zones, suggesting the possibility of transitional
or interlayered contacts.
The muscovite felsic schist is cut by abundant Middle Proterozoic granite and
pegmatite bodies (YXgp) and some bull-quartz veins related to the Berthoud Plutonic
Suite. It generally does not make good outcrop and in float areas the small pieces of
muscovite felsic schist are typically overwhelmed by the Berthoud-Type granite and
pegmatite float. The granite and pegmatite bodies appear to be predominantly concordant
with the foliation in the felsic gneiss. The best area of muscovite felsic schist outcrop is
about 6,000 ft along a strike-ridge that runs parallel to and north of Willow Creek. The
gneiss is generally oriented about N75°E with a shallow to moderate (27° to 60°) dip to
the south. This orientation is similar to the muscovite-cordierite schist (Xmc) on the ridge
south of Willow Creek and suggests that the two units are part of the same structural
domain.
The muscovite felsic schist has some characteristics similar to the muscovitesillimanite
gneiss (Xmsg) and the two units are juxtaposed for a short distance across one
of the lithologic-structural domain-bounding faults on the north side of Willow Creek.
However, they are treated as separate mappable units, although they could represent
transitional units of the same or similar stratigraphic horizon or rock type. The felsic
composition, lack of gneissic layering, and the relative homogeneity over large areas
suggest that the protolith of the muscovite felsic schist was probably a medium-grained
granitic intrusion.
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