Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

analysis by Crawford (1913), two analyses by Toulmin and Hammerstrom (1990), and
one new analysis show significant major element variations in SiO 2 (55.00 to 61.90
percent), Al 2 O 3 (16.27 to 18.26 percent), MgO (1.51 to 3.60 percent), CaO (5.10 6.52
percent), and K 2 O (2.70 to 4.59 percent). Crawford’s sample is monzodiorite, Toulmin
and Hammerstrom’s samples are tonalite and diorite, and the new sample is diorite based
on the chemical classification of De la Roche and others (1980). The relatively wide
variation in chemical composition corresponds with subtle but significant mineralogical
and textural variations (best observed in stained slabs and thin sections) and appears to be
a characteristic of the quartz monzodiorite intrusion.
The quartz monzodiorite is spatially associated with the Mount Princeton pluton
and has mineralogical and textural characteristics that are similar to the Mount Pomeroy
subunit. The quartz monzodiorite intrusion is roughly oriented about N35E and is about
9,000 ft southeast of and is subparallel to the N45E-trending Mount Princeton pluton
contact. Both rocks are characterized by predominantly coarser-grained, distinctly
medium-gray, tabular plagioclase grains in a finer-grained matrix enriched in quartz and
alkali feldspar (fig. 24). Both are dominated by biotite and hornblende with minor and
variable amounts of clinopyroxene. The quartz monzodiorite has more variation in
mineral modes than the Mount Pomeroy subunit mostly related to the proportions of
quartz, alkali feldspar and plagioclase, and the much higher abundance of plagioclase in
the quartz monzodiorite. A comparison of whole-rock chemistry (sample 05-821 with
sample 85-70 Tmpp from Shannon, 1988) shows significant chemical differences
between the quartz monzodiorite and the Mount Pomeroy subunit. All of the major
elements are different with SiO 2 varying the most, by about 8.0 percent and Fe 2 O 3 by
about 3.9 percent.
A sample (05-447A) of quartz monzodiorite from the Maysville quadrangle was
submitted to the Geochronology Laboratory at the University of Alaska, Fairbanks. A
40 Ar/ 39 Ar plateau age of 32.4 +/- 0.3 Ma was determined by Layer and Drake (2006) on
biotite from the monzodiorite. On the basis of the quality of the spectrum, Layer and
Drake (2006) interpreted the age to reflect the cooling of the intrusive rock. This 32.4 Ma
age is significantly younger than the average 36.6 Ma age of the Mount Princeton pluton
and is in between the average ages of 34.4 Ma for the Mount Aetna cauldron and 28.9 Ma
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