U-PB ZIRCON AGE CONSTRAINTS ON PROTEROZOIC GRANITIC MAGMATISM AND SHEAR ZONE DEVELOPMENT IN THE GLENWOOD SPRINGS AREA, COLORADO

New U-Pb zircon ages from Precambrian exposures in the Glenwood Springs area, Colorado, reveal at least two episodes of Proterozoic granitic magmatism and help to constrain the age of the 1-km-thick Grizzly Creek shear zone. Gneissic quartz monzonite exposed along Mitchell Canyon northwest of Glenwood Springs crystallized at ca. 1763 Ma and is the oldest igneous unit recognized in the area. The gneissic foliation is defined by alternating layers of biotite and elongated pink K-feldspar up to 5 cm in size, and the fabric strikes west-northwest and dips moderately to steeply north. The age of deformation is presently unknown, and cross-cutting relationships with other igneous units exposed in the area were not observed. Coarse-grained, K-feldspar megacrystic granite exposed north of Glenwood Canyon near the mouth of No Name Canyon crystallized at ca. 1741 Ma. This granite does not contain a tectonic foliation and is cut by the Grizzly Creek shear zone to the north. Foliated to mylonitic fine-grained biotite granite exposed in the hanging wall of the shear zone along No Name Canyon crystallized at ca. 1743 Ma, suggesting that it might be correlative with coarse-grained granite exposed in the shear zone footwall. These new ages indicate that granitic magmatism occurred in this area at ca. 1763 and 1740 Ma and require that shear zone development occurred after 1741 Ma. These ages also provide promising new correlations with other Proterozoic exposures to the northeast in Colorado and provide new opportunities to further understand the Proterozoic tectonic evolution of southern Laurentia.