Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

The Early Proterozoic foliated granodiorite (Xgdf) in the northwest part of the
quadrangle exhibits a moderate to strong, penetrative deformation fabric (foliation,
alignment of alkali-feldspar augen, and mylonitic fabrics) that is variable in orientation.
In the central part of the body the foliation is roughly east-west with moderate to steep
dips, mostly to the north. Along the southern contact with amphibolite and hornblende
gneisses the foliations are typically concordant with the contacts and the gneissic
layering. These moderate to strong penetrative deformation fabrics are not present in the
small intrusions of Proterozoic granodiorite (Xgd) in the southwestern part of the
quadrangle. Keller and others (2004) showed a roughly north-south oriented zone of
Proterozoic foliated granodiorite (Xgdf) that is about 32,000 ft long and 2,500 to 7,000 ft
wide within the large Denny Creek granodiorite (Xgd) body in the Southern Mosquito
Range on the east side of the rift in the Buena Vista East quadrangle. These observations
suggest that the Proterozoic foliated granodiorite (Xgdf) probably represents localized,
but broad zones of Early Proterozoic shearing that may have developed just after
emplacement of the Routt-type granodiorite intrusions and during the regional
metamorphic event. The zone of Proterozoic foliated granodiorite in the Maysville
quadrangle together with a zone preserved in the Mount Aetna cauldron in the Garfield
quadrangle (Shannon, 1988) suggest they may be remnants of a major Proterozoic shear
zone trending roughly N75°E. The shear zone is 42,000 ft long and at least 6,000 ft wide.
In the southwest quadrant of the quadrangle, the metamorphic foliations
developed in the various Proterozoic lithologic units always parallel the gneissic layering.
In some of the lithologic-structural domains the foliations and gneissic layering are
parallel to the orientation of the key lithologic units (Xmc, Xag, and Xq), suggesting that
most or all of the gneissic layering is a remnant of original bedding and stratigraphy. The
main trend of this stratigraphy and parallel metamorphic fabric is about N70°E. However,
a number of smaller domains suggest a subsidiary N35°W orientation. Only minor
evidence of small-scale folding of the gneisses was found during this study. The presence
of some small-scale folding, the local warping of the orientation of the gneisses and the
sharp change in foliations and gneiss orientations in some domains suggest that the
Proterozoic sequence may have been significantly folded prior to being disrupted by
younger faulting. Major changes in domain internal structural orientation may also be
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