MULTIPLE PROTEROZOIC OROGENIC EPISODES AND AN INTERVENING EXHUMATION EVENT IN THE BLACK CANYON OF THE GUNNISON REGION, COLORADO

New geochronologic data from Proterozoic rocks exposed in the Black Canyon of the Gunnison record a four-stage tectonic evolution that involved three orogenic episodes and an intervening exhumation event: 1) Yavapai orogeny (1741-1689 Ma) related to the assembly of arc-terranes, 2) exhumation marked by an angular unconformity beneath post-Yavapai, pre-Mazatzal quartzites, 3) Mazatzal orogeny (post-quartzite deposition), and 4) 1434-1403 Ma intracratonic tectonism. U/Pb zircon/titanite geochronology, in situ monazite geochronology, 40Ar/39Ar thermochronology, and geothermobarometry provide constraints for reconstructing this tectonic history. Supracrustal rocks of the Black Canyon succession were deposited or crystallized at or prior to 1741±4 Ma and subsequently intruded by the 1713±2 Ma Pitts Meadow granodiorite. Paleoproterozoic high temperature/moderate pressure metamorphism associated with the Yavapai orogeny occurred between 1741-1690 Ma at >650°C and 2-4 kbars. Deposition of inter-orogenic quartzites occurred after ~1700 Ma. The Vernal Mesa monzogranite was emplaced along the NE-striking Black Canyon shear zone at 1434±2 Ma. A younger set of pegmatite dikes cross-cut the Vernal Mesa monzogranite and are interpreted as tension gashes that record predominantly dextral, with a small component of northwest-side-up, reactivation of the Black Canyon shear zone at 1413±2 Ma. The maximum age of metamorphic conditions (~600±50°C and ~3±1 kbars) reached by schist ~3.5 km from the Vernal Mesa is constrained by 1403±8 Ma monazite inclusions in cordierite and garnet porphyroblasts. 40Ar/39Ar thermochronology records Mesoproterozoic middle crustal temperatures of 350-500°C, with the highest temperatures occurring near the Vernal Mesa monzogranite and the Black Canyon shear zone. The Black Canyon area cooled through 350°C by ~1385 Ma, but variably cooled through 300°C from 1370 to 1100 Ma, suggesting long-term residence of rocks above the 250°C isotherm at ~10 km crustal depths. When these results are combined with geologic data to construct generalized P-T-t-D (pressure/temperature/time/deformation) paths, a new model for the evolution of Proterozoic rocks of southwestern Colorado and the southwestern U.S. emerges.