Abstract

New field studies combined with U-Pb zircon geochronology constrain the ages of deposition and sedimentary provenance of Paleoproterozoic quartzite successions exposed in the southwestern United States. Orthoquartzites were deposited in short-lived basins at two times (ca. 1.70 and 1.65 Ga) during crustal assembly of southern Laurentia. The more voluminous ca. 1.70 Ga successions occur in southern Colorado, northern New Mexico, and central Arizona and are interpreted here to be time correlative, though not necessarily deposited in the same basins. Detrital zircon from quartzites and metaconglomerates exposed in southern Colorado and northern New Mexico is characterized by a single population with a relatively narrow range of ages (1.80–1.70 Ga) and minimal Archean input (<5% of grains analyzed). Peak detrital zircon ages (1.76–1.70 Ga) vary slightly from location to location and mimic the age of underlying basement. Unimodal detrital populations suggest local sources and a first-cycle origin of the orthoquartzites within a short time interval (1.70–1.68 Ga) during unroofing of local underlying basement. The maximum age of quartzite exposed at Blue Ridge, Colorado, is constrained by the 1705–1698 Ma coarse-grained granitoid basement on which quartzite was deposited unconformably. The minimum age of Ortega Formation quartzite (New Mexico) is constrained by ca. 1680–1670 Ma metamorphic monazite overgrowths. These dates agree with direct ages on the lower Mazatzal Group, Arizona, and suggest that orthoquartzite deposition occurred over a wide region during and soon after the ca. 1.70 Ga Yavapai orogeny. Regional structural arguments and the thrust style of quartzite deformation suggest that the metasedimentary successions were deformed during the ca. 1.66–1.60 Ga Mazatzal orogeny, thus making them important time markers separating the Yavapai and Mazatzal orogenic events.

Our model for syntectonic deposition involves extensional basin development followed by thrust closure, possibly due to opening and closing of slab rollback basins related to outboard subduction. The first-cycle origin of orthoquartzites near the end of the arc collisions of the Yavapai orogeny seems to contrast sharply with their extreme compositional maturity. This can be explained in terms of protracted, extreme diagenesis and/or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the Proterozoic. Similarities among quartzites exposed throughout the southwestern United States and along the Laurentian margin suggest that they represent a widespread regional, and perhaps global, episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions, a combination that might make these units a signature lithology for Paleoproterozoic time.

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