TECTONIC EVOLUTION OF THE BLACK CANYON OF THE GUNNISON, COLORADO: NEW U/PB GEOCHRONOLOGY AND EVIDENCE FOR BOTH PALEOPROTEROZOIC AND MESOPROTEROZOIC DEFORMATION AND METAMORPHISM

The Black Canyon region preserves a record of protracted (1.78-1.65 Ga) Paleoproterozoic growth and stabilization of juvenile crust and a similarly protracted (1.44-1.35 Ga) Mesoproterozoic episode of intracratonic tectonism accompanied by A-type plutonism. New U/Pb zircon and in situ monazite geochronology from the Black Canyon of the Gunnison, Colorado, provides timing constraints for both orogenic episodes. Zircon from amphibolite within migmatitic gneiss grew ca. 1.74 Ga, which we interpret to be the time of an early metamorphic event and therefore a minimum age for the basaltic protolith. Zircon and titanite dates indicate that crystallization of the Pitts Meadow granodiorite, development of the first generation of migmatite, and syntectonic development of L-S tectonite took place (ca. 1.71 Ga).

The Vernal Mesa monzogranite was emplaced ca. 1.43 Ga during predominantly dextral strike-slip reactivation of the northeast-striking Black Canyon shear zone, as recorded by pegmatite-filled tension gashes and shallowly southeast-plunging stretching lineations. 1.41 Ga titanite in the aureole suggest that the rocks surrounding the Vernal Mesa monzogranite either remained hot or were reheated millions of years after pluton emplacement. This is in agreement with in situ microprobe analysis of monazite inclusions in cordierite + plagioclase + garnet that, combined with thermobarometric data, indicate that metamorphism at pressures and temperatures of ~550-600°C and ~2.5-4 kbars took place ca. 1.4 Ga and extended at least 3 km southeast of the pluton.

The dominant regional NW-striking foliation in this part of Colorado is defined by surface foliations, aeromagnetic, and tomographic trends, and it is interpreted to record assembly of tectonostratigraphic terranes (1.72-1.70 Ga) in a complex, NW-striking subduction system south of the Cheyenne belt, possibly analogous to the modern Banda Sea. Our combined structural plus geochronologic studies suggest that the present complex fabric geometry involving northwest- and northeast- striking foliation domains can be deciphered in terms of early accretion across northwest-striking boundaries, then multiple episodes of NW-SW convergence.