Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

sillimanite gneiss (Xmsg) and the muscovite felsic schist (Xmfs). The foliations are
always parallel to the gneissic layering. Gneissic layering is fine-scale on the order of
fractions of an inch to tens of inches. Layering typically involves variations in biotite and
magnetite content. The mineralogy is variable with biotite as the predominant mafic
mineral, but some muscovite-rich layers or zones with both biotite and muscovite occur
locally. Minor sillimanite is locally visible in hand sample. Biotite felsic gneiss lacks
garnet porphyroblasts that are characteristic of the biotite felsic gneiss further north in the
Buena Vista West quadrangle (McCalpin and Shannon, 2005). Some samples have larger
crystals (to about 1 / 8 inch) of plagioclase and amphibole that may be remnant phenocrysts
or crystal fragments.
Two thin sections of biotite felsic gneiss were examined; these sections contain
variable quartz from about 30 to 60 percent, plagioclase from 20 to 25 percent, and
biotite from 10 to 25 percent, minor microcline from about 2 to 5 percent, and muscovite
from 0 to 8 percent. The suite of accessory minerals includes magnetite, apatite, sphene,
and zircon. Aside from rare remnant possible phenocrysts, the biotite felsic gneisses were
completely recrystallized during regional metamorphism. The quartz-plagioclase-rich
mineralogy and especially high quartz content suggests a siliceous protolith. The regular
fine-scale gneissic layering and fine-grained textures suggest probable sedimentary or
volcaniclastic protoliths.
The largest biotite felsic gneiss domain is in the southwestern part of the
quadrangle and forms a roughly 6,000 ft by 6,000 ft trapezoidal area that extends from
nearly the top of the ridge north of Willow Creek to the South Arkansas River. Overall,
the biotite felsic gneiss is poorly exposed and predominantly occurs as small fragment
float. The domain has abundant Berthoud-type granite and pegmatite (YXgp) bodies that
locally make up 50 percent of the area. The southern margin of the domain is transitional
with the northern part of the muscovite felsic schist domain to the south. The biotite felsic
gneiss (Xbfg) and muscovite felsic schist (Xmfs) domains contain the main locus of
Berthoud-type granite and pegmatite intrusions.
Biotite felsic gneiss is less abundant in the eastern part of the Proterozoic terrane
south of the South Arkansas River where there are small domains and thin horizons that
are interlayered with amphibolite gneiss (Xag). Similar narrow horizons of biotite felsic
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