Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

y the North Fork leucogranite (Tnfg) about 400 ft west of the Maysville quadrangle west
boundary.
The Mount Princeton pluton is generally discordant to foliation and gneissic
layering in Proterozoic rocks and to bedding in Paleozoic sedimentary rocks. Shannon
(1988) reviewed the overall attitude of contacts along the margin of the pluton. Most of
the pluton contacts are steep with the exception of the northern and southern margins,
which plunge moderately to shallowly north and moderately south, respectively. The
southeastern contact, including the segment crossing the northwest corner of the
Maysville quadrangle, is nearly vertical to steeply inward (northwest) dipping. Within the
Mount Princeton pluton (largely preserved in the Mount Aetna cauldron collapse
structure in the St. Elmo quadrangle), low dips of broad, internal textural and
compositional units suggest that the pluton had a relatively broad, flat roof that was
gently tilted southward during collapse of the Mount Aetna cauldron.
Many previous workers have noted textural and compositional variations in the
Mount Princeton pluton (Crawford, 1913; Dings and Robinson, 1957; Limbach, 1975;
Sharp, 1976; R.P. Smith, 1979, unpublished reconnaissance map and 1981, personal
communication; Shannon, 1988; Toulmin and Hammarstrom, 1990; and McCalpin and
Shannon, 2005). Shannon (1988) described and delineated a number of systematic
textural and compositional variations in the pluton. The pluton was divided into a border
unit with three subunits and an interior unit with three subunits. The Mount Princeton
border unit is mostly preserved in the Mount Aetna cauldron collapse structure in the
southern part of the St. Elmo quadrangle.
The genetic relations of compositional-textural units along the roof of the Mount
Princeton pluton have been variably interpreted and are problematic. The Mount
Pomeroy quartz monzonite was interpreted as a separate earlier intrusion by Crawford
(1913) and Dings and Robinson (1957). It is mainly preserved in the Mount Aetna
cauldron collapse structure, in a 7 mile long and 0.5 to 1.5 mile wide, N60°E-trending
zone. Toulmin (1976), Shannon (1988) and Toulmin and Hammarstrom (1990) suggested
that the Mount Pomeroy quartz monzonite is a roof facies of the Mount Princeton pluton.
Shannon (1988) suggested that the Mount Princeton border unit be defined as the broad
zone of heterogeneous textures and compositions along the roof zone, and locally along
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