Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

in descriptions of the geologic units of the Maysville quadrangle may wish to skip to the
Description of Map Units section.
The rocks in the Maysville quadrangle range in age from Early Proterozoic to
recent and include high-grade metamorphic rocks, a wide variety of intrusive rocks,
sedimentary rocks, and unconsolidated surficial deposits. The central Colorado area was
subjected to at least three major, separate orogenic events: (1) Early Proterozoic orogeny
at about 1.7 Ga (Tweto, 1980a and 1980b; Reed and others, 1987); (2) Mississippian-
Pennsylvanian (Ancestral Rocky Mountain) orogeny (DeVoto, 1972; Tweto, 1980a;
Kluth and Coney, 1981); and (3) Laramide orogeny (Tweto and Sims, 1963; Tweto,
1975, 1980a and 1980c). During Late Cenozoic time the region experienced complex
crustal extension and broad regional epirogenic uplift associated with development of the
Rio Grande rift (Knepper, 1976; Eaton, 1979 and Tweto, 1979a and 1980a).
Precambrian rocks in the Maysville quadrangle include a complex sequence of
regionally metamorphosed volcanic, sedimentary, and igneous rocks that belong to the
1,800 Ma Proterozoic Layered Gneiss Complex (Tweto, 1980b and 1987). The
metamorphism of the gneisses peaked at about 1,740 m.y. ago. The late Early Proterozoic
gneisses in the Maysville quadrangle are dominated by felsic and hornblendic gneisses,
including abundant amphibolites that were metamorphosed to the upper amphibolite
facies. They are part of an east-west trending belt of similar age supracrustal rocks that
extends from southwest of Gunnison to east of Salida (Bickford and Boardman, 1984).
U-Pb zircon age determinations and petrologic character support that these supracrustal
and associated intrusive rocks are part of a 310 mi wide belt of rocks that were accreted
to the southern edge of the Archean craton during the Early Proterozoic between 1790
and 1660 Ma (Reed and others, 1987).
Largely on the basis of observations in the Front Range, Tweto (1980b) described
at least three periods of Precambrian folding that affected the gneisses. The Proterozoic
gneisses were metamorphosed and folded during an early period of deformation followed
by a second period of folding (Early Proterozoic orogeny) that was associated with
regional intrusions associated with the 1,700 Ma Routt Plutonic Suite (Tweto, 1987).
Tweto (1987) summarized the predominantly granodiorite to quartz monzonite intrusions
of the late Early Proterozoic Routt Plutonic Suite as generally concordant and foliated,
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