U-Pb geochronology of Proterozoic granites in the Sawatch Range, central Colorado, U.S.A.

We report new U-Pb zircon ages for four previously undated Proterozoic granitoid intrusions exposed in the southern Sawatch Range, central Colorado. Coarse-grained to K-feldspar megacrystic granite (Henry Mountain granite) exposed along Taylor Canyon 20 km north of Gunnison, Colorado, crystallized at 1697 ± 7 Ma. It cuts across high-temperature deformational fabrics in metavolcanic and metasedimentary country rocks, thus bracketing at least one Paleoproterozoic tectonic event locally. This granite also contains a well-developed northeaststriking, 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 1447 ± 9 Ma. It cuts across well-developed fabrics in metavolcanic host rocks and contains a widespread biotite foliation. Coarse-grained to K-feldspar megacrystic granite (Horsethief granite) exposed 5–10 km northwest of Taylor Park Reservoir was emplaced at 1437 ± 5 Ma, and it is locally deformed. Fine-grained, muscovite-biotite granite (Taylor River granite) that cuts across the southwestern part of the Henry Mountain pluton crystallized at 1428 ± 23 Ma. A subvertical, northeast-striking biotite foliation cuts across the contact between these two intrusions, suggesting that northwest-directed subhorizontal shortening occurred locally during the Mesoproterozoic. These ages provide new opportunities to constrain the age of tectonism in central Colorado and to further understand the Proterozoic tectonic evolution of southern Laurentia.