AUTOCHTHONEITY OF THE PROTEROZOIC UNCOMPAGHRE GROUP, NEEDLE MOUNTAINS, COLORADO: INSIGHT INTO REGIONAL ~1.7 GA QUARTZITE DEPOSITION

Rheologically weak horizons between basement and sedimentary cover sequences frequently serve as major detachments during orogenesis. In the Needle Mountains of southwestern Colorado, the Paleoproterozoic Uncompaghre quartzite is in tectonic contact with the underlying Twilight and Irving gneiss suites. This contact is marked by a highly deformed aluminous horizon and distinct angular unconformity. These relations have led to interpretation of this contact as a paleosol which accommodated shearing during Mazatzal tectonism (1.65 Ga) and/or emplacement of ~1.45 Ga plutons (Tewksbury 1989; Harris and others, 1987). The amount and sense of displacement along this contact is controversial and has implications on the age of the Uncompaghre Group relative to the underlying basement suite. Preliminary field studies suggest that the basal shear zone is discontinuous, requiring that the Uncompaghre Group was deposited directly on exhumed 1.78-1.69 Ga basement. The Uncompaghre Group is believed to correlate with a regional pulse of deposition of ultra mature quartzites at ~1.70-1.69 Ga that includes correlative quartzites throughout Colorado and New Mexico (Karlstrom and others, 2004). Frequent association of quartzite with metarhyolite and shallow level granites and a general distribution of these sequences within the accretionary orogens of southern Laurentia suggest that widespread silicic volcanism and quartzite deposition were a response to variations in tectonic style along the continental margin, possibly involving slab rollback, strike-slip tectonics and/or back arc spreading. Exposures in the Needle Mountains may constitute a unique sedimentary package that can provide vital insight into Proterozoic sedimentary processes and basin development. We are currently evaluating associations between the Uncompaghre Group, Vallecito Conglomerate, and shallow level ~1.69 Ga plutons through a combination of structural analysis and U-Pb geochronology in order to further our understanding of the quartzite-rhyolite phenomenon.