AGE AND TECTONIC SIGNIFICANCE OF THE CA. 1.4 GA BROWNS PASS PLUTON, COLLEGIATE PEAKS WILDERNESS AREA, COLORADO

New field mapping and geochronology constrain the age and extent of Mesoproterozoic granitic magmatism and deformation in the Collegiate Peaks Wilderness Area, central Colorado. The Browns Pass pluton, exposed between Mount Yale and Cottonwood Pass west of Buena Vista, Colorado, is composed of coarse-grained to K-feldspar megacrystic quartz monzonite that crystallized ca. 1438 Ma (U-Pb zircon). It intrudes amphibolite-facies metasedimentary schists and gneisses and strongly deformed granodiorite of the Paleoproterozoic Denny Creek batholith. Xenoliths of the gneisses, schists and granodiorite are found within the Browns Pass pluton constrained to the contacts. Exposures throughout the Browns Pass pluton are characterized by a foliation defined by aligned K-feldspar megacrysts 3–5 cm in size. This fabric strikes east–northeast and dips steeply south (avg. 083/70SE), and it is interpreted to be magmatic in origin. Quartz monzonite within a few hundred meters of the pluton margin is cut by a solid-state foliation defined by biotite and recrystallized quartz and feldspar. This fabric strikes east–southeast and dips steeply south (avg. 104/83S), roughly parallel to the pluton contact. It is also locally parallel to well developed tectonic fabrics in wall-rock gneiss and granodiorite, suggesting that deformation was localized along the intrusion margins after crystallization of the quartz monzonite. The age of Browns Pass quartz monzonite indicates that it was emplaced during a widely recognized regional pulse of granitic magmatism between ca. 1440–1430 Ma. The nature and extent of solid-state deformation within the Browns Pass pluton is consistent with similar-aged plutons in the surrounding region, but the local foliation orientation is not consistent with northeast-striking fabrics that are documented both regionally and in nearby ranges. Continued field mapping is expected to provide additional insights into the extent of tectonic foliations in exposures of Browns Pass quartz monzonite and the possible influence of pre-existing wall-rock fabrics on the orientation of fabrics locally at ca. 1.4 Ga.