New U-Pb zircon ages from Precambrian exposures in the White River Uplift of western 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 granite exposed along Mitchell Canyon northwest of Glenwood Springs, crystallized at 1765±9 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. Cross-cutting relationships with gneissic country rocks and other igneous units exposed in adjacent drainages were not observed. However, deformation and metamorphism are inferred to be between the age of intrusion (ca. 1765 Ma) and the age of younger, unfoliated, coarse-grained to K-feldspar megacrystic granite exposed near the mouth of No Name Canyon, which crystallized at 1743±8 Ma. This younger granite is cut by the Grizzly Creek shear zone to the north and only contains a locally developed magmatic foliation south of the shear zone. Foliated to mylonitic, fine-grained biotite granite exposed in the hanging wall of the shear zone along No Name Canyon crystallized at 1745±10 Ma, suggesting that it might be related to coarse-grained granite exposed in the shear zone footwall. These new ages define two granitic magmatic events in this area at ca. 1765 and 1745 Ma and provide a maximum age of deformation in the Grizzly Creek shear zone of 1743 Ma. Similarities in the orientation, structural style, and kinematics between the Grizzly Creek shear zone and other well-documented structures in the region raise the possibility that the shear zone records multiple episodes of both Paleoproterozoic and Mesoproterozoic (ca. 1.4 Ga) deformation, in which case the younger events would have occurred in the absence of local magmatism. Thus, the Grizzly Creek shear zone might represent a kinematic link between major crustal shear zones to the north and south throughout crustal assembly and stabilization in southern Laurentia, but details of the timing and kinematic relationships between these structures remain uncertain.

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