Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

on the North Fork leucogranite intrusion. The border unit forms an arcuate outcrop
pattern along the eastern and northeastern part of the North Fork intrusion. The southern
contact of the intrusion is concealed in the floor of the North Fork valley; it is not known
if it is an intrusive or fault contact with the Proterozoic rocks. A zone of fine-grained
leucogranite forms a border rind around a large pendant of Proterozoic rocks (Xag, Xq,
Xgd, Xgdh) that occur along the west edge of the Maysville quadrangle. This pendant
and another zone of Proterozoic pendants on the ridge on the west side of McCoy Creek
in the Garfield quadrangle suggest that the roof zone of the North Fork intrusion is
dipping southward into the North Fork valley. Dikes of fine-grained leucogranite are
common in the contact zone of the North Fork intrusion in the Maysville quadrangle
(with Proterozoic rocks) and in the Garfield quadrangle (with Tmpp). The dikes are
generally 1 to 20 feet thick, but range up to 80 feet thick in McCoy Creek in the Garfield
quadrangle. The North Fork fine-grained leucogranite dikes are distinguished from the
California fine-grained leucogranite dikes by the lack of accessory garnet.
Figure 21. Slabs of North Fork border (Tnfb, left) and leucogranite (Tnfg, right)
stained for alkali feldspar.
83