FOLIATION OF THE CRIPPLE CREEK QUARTZ MONZONITE, COLORADO

The tectonic settings of mid-continent 1.4 Ga granitoids have been a source of debate over the past 15 years. Early hypotheses favored an anorogenic origin for these granitoids, while later models favor emplacement during compressional tectonic events (c.f. Anderson, 1989 and Nyman et al., 1994). Widespread magmatic foliations with consistent ~N30E strikes and steep NW dips have been recognized in many of these plutons, offering a key piece of evidence for synorogenic emplacement histories. The 1.4 Ga granitic body in the Front Range of Colorado known as the Cripple Creek Quartz Monzonite (Wobus et al., 1976) has been cited as an example of this style of magmatism. Magmatic foliations (interpreted to represent pre-full crystallization fabrics) have been recognized throughout this pluton, and have been interpreted as evidence for synorogenic origin. Detailed field mapping, however, suggests that the attitudes of foliations within the granitoid are broadly conformable to the pluton's contacts, and may simply represent the development of flow foliation during emplacment. This fabric was mapped by recording feldspar lineations in outcrops around Cripple Creek, Colorado. These data produced curvilinear trends on stereonet plots, which are most consistent with either flow foliations or post-emplacement folding of the pluton. If these foliations were related to far field compressional stresses, they would be expected to show a greater conformity to regional trends. Likewise, the pluton lacks structural indicators of post-emplacement ductile folding (e.g., myolinitic foliation, sigma-delta grain tails), and structural features indicating brittle folding are inconsistent with other observations. As the attitude of the foliation appears to be roughly parallel to the pluton's contacts (sweeping around the perimeter of the pluton), they are interpreted to represent flow foliations developed during emplacment. Although the foliation is well developed in parts of the pluton (especially along its margins), evidence for passive or dynamic emplacement histories are considered equivocal at present.