Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

A small 400 by 400 foot body of North Fork leucogranite is present along the
western edge of the Maysville quadrangle about 3,500 ft north of the main body. It is the
end of an about 3,500 ft long, 500 to 700 ft thick, arc-shaped, dike-like extension on the
north side of the intrusion; the extension extends northward along the eastern edge of the
Garfield quadrangle and enters the western edge of the Maysville quadrangle in upper
McCoy Creek about 3,500 feet west-southwest of the summit of Mount Shavano. Most of
this extension consists of medium-grained leucogranite (Tnfg). The eastern part of the
North Fork leucogranite intrusion in the Maysville quadrangle intrudes Proterozoic rocks
(Xhig, Xag and YXp) and the western part of the intrusion in the Garfield quadrangle
intrudes the Mount Princeton pluton (Tmpp). On the basis of mapping talus block float,
the northern extension of the North Fork intrusion cuts the northeast-trending
Proterozoic-Mount Princeton pluton contact about 400 feet west of the western edge of
the Maysville quadrangle.
Two earlier studies attempted age determinations on the North Fork leucogranite
intrusion. Pulfrey (1971) reported a K-Ar biotite age of 30.8 +/- 1.1 Ma, which was
recalculated (Shannon, 1988) with revised decay constants from Dalrymple (1979) as
31.6 +/- 1.1 Ma. Shannon (1988) conducted fission-track dating analyses on a sample of
North Fork leucogranite from the western part of the intrusion in the Garfield quadrangle.
A fission-track zircon age of 20.0 +/- 2.7 Ma and a fission-track apatite age of 19.7 +/-
2.7 Ma were interpreted to record reset uplift-cooling ages for this part of the Sawatch
Range. Recent high-precision 40 Ar/ 39 Ar dating by McIntosh and Chapin (2004) included
eight samples of ‘Mount Antero Granite’. On the basis of rough sample location
information, one sample appears to be from the North Fork leucogranite intrusion. The
alkali-feldspar age of 28.65 +/- 0.53 Ma and altered biotite age of 29.12 +/- 0.18 Ma from
the same sample were rejected as inaccurate or imprecise.
Miscellaneous magmatism (late Eocene to early Oligocene ) – This grouping of
intrusive rocks includes minor quartz latite porphyry (Tqlp) that occurs as dikes in the
northwest quadrant of the Maysville quadrangle. The dikes occur in the same area as the
aphyric rhyolite (Tr) and rhyolite porphyry (Trp) dikes in a swarm that parallels the range
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