PROTEROZOIC GRANITOID MAGMATISM IN THE SAWATCH RANGE, CENTRAL COLORADO: NEW AGE CONSTRAINTS FROM U-PB GEOCHRONOLOGY

New U-Pb zircon geochronology constrains the ages of four Proterozoic granitoid intrusions exposed in the Sawatch Range, central Colorado. Only one intrusion yielded a Paleoproterozoic age. Coarse-grained to K-feldspar megacrystic granite (Henry Mountain granite) exposed along Taylor Canyon 20 km north of Gunnison crystallized at ca. 1700 Ma. It cuts across high-temperature deformational fabrics in metavolcanic and metasedimentary host rocks, thus bracketing at least one Paleoproterozoic tectonic event locally. This granite also contains a well developed NE-striking, subvertical foliation that postdates emplacement. The three other intrusions all yielded Mesoproterozoic ages. The Monarch Pass pluton, comprising coarse- to medium-grained granodiorite exposed 50 km east of Gunnison, crystallized at ca. 1440 Ma. It cuts across well developed fabrics in metavolcanic host rocks and contains a widespread biotite foliation. Coarse-grained to K-feldspar megacrystic granite exposed 5-10 km NW of Taylor Park Reservoir was emplaced at ca. 1437 Ma, and it is locally deformed. Fine-grained muscovite-biotite granite (Taylor River granite) that cuts across the SW part of the Henry Mountain pluton in Taylor Canyon crystallized at ca. 1440 Ma. A subvertical, NE-striking biotite foliation cuts across the contact between these two intrusions, suggesting that NW-directed subhorizontal shortening occurred locally during the Mesoproterozoic. The overall tectonic significance of these Proterozoic intrusions has yet to be determined, but these ages provide new opportunities to constrain the age of tectonism locally and to further understand the Proterozoic tectonic evolution of southern Laurentia.