Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

locally tanish to pinkish gray. It contains two variable populations of larger crystals in a
fine grained to aphanitic groundmass (fig. 25). A population of black, subhedral to
euhedral crystals, mostly remnant hornblende grains, is interpreted to be phenocrysts.
Another minor population of larger, subhedral to rounded crystals may be resorbed
phenocrysts or xenocrysts. All hand samples of andesite are weakly to moderately
magnetic (due to abundant fine, disseminated accessory magnetite) and most samples
weakly effervescence with dilute HCl (due to the presence of carbonate alteration).
Three thin sections and seven slabs (stained for alkali feldspar) of different
andesite dikes were prepared and studied. The main purpose was to evaluate the intensity
of hydrothermal alteration and to see if any of the andesite dikes had suitable minerals for
40 Ar/ 39 Ar dating. The alkali-feldspar stained slabs show relatively abundant alkalifeldspar
(estimated 5 to 18 percent) in the groundmass (fig. 25). With a binocular
microscope, small amounts of quartz (estimated 1 to 4 percent) can barely be resolved.
Hand sample and thin section studies indicate the lack of suitable minerals for age
determinations.
Figure 25. Billets and slabs of Tertiary () andesite hybrid dikes (Ta) showing
variations in textures: chilled margins (left); dike interiors (middle) and
xenocryst bearing (right). Stained for alkali feldspar.
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