Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Figure 35. Outcrop of Proterozoic hornblende intermediate gneiss (Xhig) with
localized mylonitic fabrics.
The largest area of hornblende intermediate gneiss is a 19,000 ft long semicircular
area that extends from the south shoulder of Mount Shavano to the South
Arkansas River. The eastern part of this area is an 18,000 ft long, crescent-shaped zone of
predominantly Routt-type () granite and pegmatite sills interlayered with narrow septa
or screens of hornblende intermediate gneiss (Xgp/Xhig). The presence of weak
foliations and gneissic layering in some of the granites associated with the pegmatites
provides the main support that they are Routt-type or Pre-Routt-type intrusions. The
western part of the large area of hornblende intermediate gneiss has less than 50 percent
granite and pegmatite intrusions. Some interlayered amphibolite gneisses generally
become more abundant toward the northeast. The large area of hornblende intermediate
gneiss is cut by the northeast-trending Tertiary quartz monzodiorite (Tqm) intrusion on
the south side of the North Fork and is cut by the Tertiary North Fork leucogranite (Tnfg)
on the north side of the North Fork.
The hornblende intermediate gneiss was completely recrystallized during
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