Three Proterozoic Orogenic Episodes and an Intervening Exhumation Event in the Black Canyon of the Gunnison Region, Colorado
Abstract
U/Pb zircon/titanite geochronology, in situ monazite geochronology, and 40Ar/39Ar thermochronology provide an unusually complete data set for reconstructing the tectonic history of Proterozoic rocks exposed in the Black Canyon, Gunnison, Colorado. These new geochronologic data record three protracted orogenic episodes and an exhumation event between orogenic pulses: (1) Yavapai orogeny (1741–1689 Ma), (2) exhumation marked by an angular unconformity beneath post‐Yavapai, pre‐Mazatzal quartzites, (3) Mazatzal orogeny (postquartzite deposition), and (4) 1434–1403 Ma intracratonic tectonism. Supracrustal rocks of the Black Canyon succession were deposited or crystallized at or prior to $$1741\pm 4$$ Ma and were intruded by the $$1713\pm 2$$ Ma Pitts Meadow granodiorite. Paleoproterozoic high‐temperature metamorphism (>650°C) associated with the Yavapai orogeny occurred between 1741 and 1690 Ma. Deposition of interorogenic quartzites took place after ∼1700 Ma. The Vernal Mesa monzogranite was emplaced at $$1434\pm 2$$ Ma followed by metamorphism ($$\sim 600^{\circ }\pm 50^{\circ }$$ C and $$\sim 3\pm 1$$ kbar) at $$\leq 1403\pm 23$$ Ma. 40Ar/39Ar thermochronology records Mesoproterozoic middle crustal temperatures of 350°–500°C, with the highest temperatures occurring near the Vernal Mesa monzogranite and the NE‐striking Black Canyon shear zone. The 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 pressure/temperature/time/deformation paths (PTtD), a new template for the evolution of Proterozoic rocks of southwestern Colorado and the southwestern United states emerges.
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References (29)
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