CROSS SECTION OF PROTEROZOIC ROCKS OF THE BLACK CANYON, COLORADO AND USE OF DATED PLUTONS AS "SNAP SHOTS" OF EVOLVING DEFORMATIONAL CONDITIONS

The Black Canyon (BC) of the Gunnison River, provides a spectacularly exposed ~50-km cross section of Proterozoic structures in SW Colorado. The BC cross section is characterized by NW-trending, moderately dipping S1, overprinted by steeply dipping NE-striking S2. The supracrustal package of the BC succession includes quartzitic gneiss, migmatite, mica schist and amphibolite. The Pitts Meadow, Vernal Mesa and Curecanti plutons of BC provide “snap shots” of lower- to upper middle-crustal tectonic conditions as the area evolved during crustal assembly (~1.76-1.68Ga) and subsequent intracratonic deformation (~1.4Ga and 1.2Ga). The Pitts Meadow granodiorite (~1.68Ga, Rb/Sr) was emplaced in the middle crust during upper amphibolite facies metamorphism and migmatization synchronous with as shown by diffuse S1-parallel contacts. The Vernal Mesa monzogranite (1.43Ga U-Pb) is little deformed but has a magmatic fabric defined by K-spar megacrysts aligned sub-parallel to regional NE S2 and was emplaced with a more restricted, but still ~100m wide, ductile deformational and thermal aureole. Regional vertical stretching lineation progressively shallows towards the pluton contact over a distance of 100’s of meters, and S-C fabrics near the contact show sinistral shear. The Curecanti monzogranite (~1.25Ga, Rb-Sr) was emplaced as a series of fine-grained undeformed sills with screens of the supracrustal rocks. The sharp, well exposed roof and floor contacts support a mechanical mode of emplacement at upper mid-crustal levels, where s3 was subvertical. A NW striking sheared boundary separates the BC succession and Dubois greenstone complex (~1.76Ga) to the east. A hypothesized suture between the BC and Dubois (S1 or earlier) is likely folded, NW-trending, initially shallowly dipping, and representative of lower P/T conditions. In contrast NE-trending, subvertical boundaries likely formed during the pluton enhanced tectonism at ~1.7-1.68Ga and/or ~1.4-1.3Ga. Based on pluton dates and correlation to other areas, the hypothesis presented here is that the region evolved through three main tectonic/ plutonic episodes: assembly of crust ~1.74-1.68Ga, intracratonic transpressive deformation ~1.42-1.35Ga, and intracratonic upper crustal pluton emplacement ca. ~1.2-1.1Ga.