AGE AND DEFORMATIONAL CHARACTERISTICS OF THE PALEOPROTEROZOIC DENNY CREEK GRANODIORITE, COLLEGIATE PEAKS WILDERNESS AREA, COLORADO

New field mapping and geochronology help to constrain the age and tectonic significance of Paleoproterozoic (ca. 1.65 Ga) magmatism and deformation in exposures in the Collegiate Peaks Wilderness Area, central Colorado. This study focused on the Denny Creek pluton, a widely exposed igneous intrusive unit dominated by medium- to coarse-grained granodiorite with feldspar megacrysts up to 3 cm in size. It was emplaced into amphibolite-facies supracrustal rocks (mica schist, quartzite, and calc-silicate gneiss) and older granitic units. U-Pb zircon geochronology indicates that granodiorite crystallized ca. 1656 Ma, and the pluton is cut by granodiorite of the Paleoproterozoic Kroenke pluton and numerous Mesoproterozoic granitic plutons. Exposures of Denny Creek granodiorite were observed to contain a solid-state foliation defined by biotite and recrystallized and elongated quartz and feldspar phenocrysts. In general, deformation intensity and the foliation orientation vary throughout the range. However, exposures of Denny Creek granodiorite along Clear Creek preserve a km-thick high strain zone characterized by a penetrative, subvertical mylonitic foliation that strikes north–northwest (avg. 349/81SW). Asymmetric shear fabrics are locally present, but extensive boudinage of cm-thick mica-poor layers and tight to isoclinal folding of cross-cutting pegmatitic veins suggest that flattening strain was dominant during deformation. High-strain rocks are primarily found within a few kilometers of the contact between the Denny Creek and Kroenke plutons. This spatial association, combined with the observation that the orientation of mylonitic fabrics is subparallel to the mapped pluton contact, suggests that deformation occurred locally during emplacement of the Kroenke pluton.