1 Precambrian Geology

Wednesday, 17 May 2006
10:00 AM - 11:40 AM

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

Paper number: 1-1
Program: Technical Sessions
Time: 10:00 AM – 10:20 AM

Authors

Abstract

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.