Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Xcs Calc-silicate gneiss (Early Proterozoic) – Almost all of the calc-silicate gneiss
zones are associated with amphibolite gneiss (Xag) and hornblende intermediate gneiss
(Xhig). The main areas of calc-silicate gneiss are present in a 17,000-ft long and about
3,000-ft wide, N30°W-trending zone that extends from Green Creek, on the southern
boundary of the quadrangle, to the South Arkansas River, opposite Lost Creek. Four of
the larger mappable areas of calc-silicate gneiss (Xcs) are shown within this larger belt of
amphibolite gneiss (fractional Xag/Xcs unit). Smaller calc-silicate gneiss zones, too small
or discontinuous to map separately, are associated with the amphibolite gneiss (Xag) belt
and with smaller amphibolite gneiss and amphibolite agglomerate (Xaa) zones in the
southwest corner of the quadrangle and a few areas extending northward to upper Como
Creek.
In most areas the cal-silicate rocks occur in narrow horizons (tens of feet thick)
that are apparently controlled by stratigraphy. The amount of calc-silicates present in the
horizons is variable and ranges from weak calc-silicates with epidote-dominant
assemblages (fig. 29), through moderate calc-silicates with more classic epidote-garnet
assemblages, to strong calc-silicates with clinopyroxene-amphibole-mica-garnet-epidote
assemblages. Calc-silicate rocks are interlayered with amphibolite and hornblende
gneisses in a few areas and individual calc-silicate horizons are broader, up to 2,000 ft
long and a few hundred feet thick, in some zones. Some areas of calc-silicate rocks are
associated with very fine-grained, hornfels-like hornblende felsic gneiss (Xhfg) with
weak calc-silicate overprint. Thus, the calc-silicate gneisses (Xcs) exhibit a transitional
character from weak epidote calc-silicates to moderate epidote-garnet calc-silicates to
strong calc-silicates with complex assemblages. The distribution of these various zones
does not show systematic patterns that support zonation about a center or to specific
intrusions. Consequently, the calc-silicate zones are considered to be regional, rather than
local features (related to contact metamorphism) and are generally interpreted to be
related to high-grade regional metamorphism (see Economic Geology section).
The calc-silicate gneisses are varicolored including light gray to black, green, tan
and pink with common light-orangeish to reddish oxide staining. They range from very
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