Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

The field relations and characteristics of the Proterozoic quartzites in the Maysville
quadrangle are supportive of a chert protolith for these rocks.
Xmc Muscovite-cordierite schist (Early Proterozoic) – The muscovite-cordierite
schist is one of the most distinctive Proterozoic lithologic units recognized in the
Maysville quadrangle and is suggested to have the best potential for semi-regional,
mappable, key-stratigraphic horizons. The muscovite-cordierite schist is present as a
continuous, N70° W-trending layer for about 10,500 ft along the western part of the
southern margin of the quadrangle. It occurs in one of the largest lithologic-structural
domains that displays the most systematic and coherent stratigraphy. Three segments of
the schist are interpreted to be the same stratigraphic horizon, which has been offset by
three faults. The western segment is at least 2,400 ft long (about 500 ft in Maysville
quadrangle) and is right-laterally offset about 1,600 ft along a northwest-trending fault.
The middle segment is about 8,000 ft long and the eastern segment is about 1,700 ft long.
The middle and eastern segments are left-laterally offset about 1,750 ft along a northnorthwest-trending
fault. The eastern segment is offset to the north side of Willow Creek
and is truncated by a north-northwest-trending fault at its easternmost extent. In the
Maysville quadrangle, the schist unit does not occur on the east side of the fault,
suggesting that significant offset (greater than 5,000 ft if right-lateral offset or greater
than 9,000 ft if left-lateral offset) exists across this fault zone.
The muscovite-cordierite schist is light gray, fine to medium grained, and is
strongly foliated (figs. 31 and 32). It commonly contains cordierite porphyroblasts that
gradationally vary in size (from 1 / 16 to ¾ inches) along strike within the schist unit. The
western and eastern schist segments tend to have no cordierite or small (< 1 / 16 inch)
porphyroblasts. The western part of the central segment has small ( 1 / 16 to 1 / 8 inch)
cordierite porphyroblasts and the eastern part has abundant, large ( 1 / 4 to ¾ inch)
cordierite porphyroblasts. Most hand samples of muscovite schist are weakly magnetic
due to the presence of finely disseminated magnetite. A thin-section study of one sample
from the eastern part of the central segment has estimated mineral modes of 30 percent
quartz, 25 to 30 percent muscovite, 15 percent cordierite, 10 percent biotite, 10 percent
plagioclase, 1 to 2 percent sillimanite, 1 to 2 percent magnetite, and trace tourmaline. The
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