U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF PALEOPROTEROZOIC ORTHOQUARTZITES IN THE SOUTHWESTERN UNITED STATES: AN ESSENTIAL TOOL FOR REGIONAL CORRELATION, TECTONIC STUDIES, AND MORE

Thick (>2 km) metasedimentary successions including abundant compositionally pure quartzite occur among Paleoproterozoic (1.8–1.6 Ga) exposures throughout North America and elsewhere around the world. These distinctive successions have the potential to elucidate numerous aspects of Proterozoic (2.5–0.5 Ga) Earth systems. New field studies combined with U-Pb detrital zircon geochronology constrain the ages of deposition and sedimentary provenance of Paleoproterozoic quartzite successions exposed throughout the southwestern United States. This approach is critical for regional correlation of these highly discontinuous units and provides new baselines for provenance studies of younger sedimentary rocks and modern sediment in the region. Our results indicate that two major episodes of widespread sedimentation occurred during the Paleoproterozoic that, in both cases, closely followed a major regional orogenic event. Quartzites throughout the region have relatively simple age spectra dominated by Paleoproterozoic-aged detritus. Archean-aged grains are minor or absent in almost all samples. Peak detrital zircon ages vary slightly from location to location and mimic the age of underlying basement. Unimodal detrital populations suggest local sources and a first-cycle origin for the orthoquartzites within a short time interval during unroofing of local underlying basement. Our model for syntectonic deposition involves extensional basin development followed by thrust closure, possibly due to opening and closing of slab rollback basins. The first-cycle origin of orthoquartzites during prolonged accretionary orogenesis 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.