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Detrital zircon uranium-lead geochronology and hafnium-isotope analyses of passive margin and Roberts Mountains allochthon strata: Interpreting the Early Paleozoic tectonic evolution of western Laurentia
AdvisorTrexler, James H.
Geological Sciences and Engineering
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This dissertation investigated Neoproterozoic–Devonian units of the western Laurentian passive margin and Roberts Mountains allochthon (RMA) and determined U-Pb detrital ages and Hf isotope zircon analyses that provide new insights into the early Paleozoic tectonics of western Laurentia. The three chapters investigate several difficult questions and contradictions in the understanding of early Paleozoic tectonism in western Laurentia through analysis of sedimentary units. The provenance, depositional histories, and tectonic evolution of the lower Paleozoic sedimentary strata of north-central Nevada have long been subjects of speculation and debate. Detrital zircon U-Pb geochronology and Hf-isotope analyses indicate the provenance, sedimentary distribution patterns, and tectonic evolution of Upper Neoproterozoic–Cambrian passive margin strata and Ordovician – Devonian strata of the RMA, with a special emphasis on the enigmatic Harmony Formation.The study reported in Chapter 1 uses detrital zircon U-Pb geochronology to determine whether or not the Upper Neoproterozoic–Lower Cambrian Osgood Mountain Quartzite and the Upper Cambrian–Lower Ordovician Preble Formation in the Osgood Mountains of northern Nevada were units of the western Laurentian passive margin. Within the Osgood Mountain Quartzite, U-Pb age populations of the detrital zircons shift with stratal age. This shift indicates that the zircons were shed in different proportions from the source terranes, which suggests a change in provenance within the Osgood Mountain Quartzite. These changes are consistent across a Great Basin transect of coeval passive margin strata. The change in provenance is due to a shift in sedimentary transport patterns, which was caused by the Late Neoproterozoic-Early Cambrian uplift of theiiTranscontinental Arch. This study provided independent corroboration of the existence of the Transcontinental Arch and better precision for the timing at which the Arch uplifted. The study also recorded the impact of the uplifted Arch on continent-wide sediment dispersal patterns—the change in predominant source terranes—and confirmed the Arch as a sediment source for passive-margin strata. Regional coeval changes in detrital zircon U-Pb age patterns provide a correlative tool in unfossiliferous sediments and could be useful in future studies.Chapter 2 describes how detrital zircon U-Pb geochronology and Hf-isotope analyses were used to determine the provenance, sedimentary transport, and tectonic evolution of RMA strata. Workers have speculated for decades, with little agreement, on the origin, depositional basin(s), and subsequent tectonic transport of the RMA. Zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; approximately one-quarter of these grains were further analyzed for Hf isotope ratios. Five of the studied units have similar U-Pb age poulations and Hf-isotope ratios, while the U-Pb ages and Hf-ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of these data with known analyses of igneous basement rocks and other sedimentary units of Laurentia reveals that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. All of the RMA strata were deposited near the Peace River Arch region and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny. This study determined the origin, location of the depositional basin, andiiiproposed a subsequent tectonic evolution that accounts for origin, deposition, and current location of the RMA strata.Chapter 3 describes the origin, age, and tectonic development of the Harmony Formation. The Harmony Formation has always been difficult to explain—it is mostly an immature feldspathic arenite, which would argue for minimal transport from origin to deposition. However, its general position as the top thrust plate in the RMA stack argues for deposition oceanward of other more texturally mature RMA strata. The age of the Harmony Formation is equally contentious—published age determinations range from Cambrian to Mississippian. Zircon grains from ten arenite samples were analyzed for U-Pb ages; grains from eight of these samples were further analyzed for Hf-isotope ratios. Seven of the arenite units have similar U-Pb age peaks and Hf isotope ratios, whereas three differ significantly. The data confirmed the subdivision of the Harmony Formation into two petrofacies, quartzose (Harmony A) and feldspathic (Harmony B). Harmony A originated in the central Laurentian craton. Harmony B had a common source in eastern Alberta–western Saskatchewan, north of the source of the Harmony A. All of the Harmony Formation strata were deposited near eastern Alberta in Late Neoproterozoic through Cambrian time and subsequently tectonically interleaved with the Roberts Mountains allochthon strata. The entire package was tectonically transported south along the Laurentian margin. Subsequently, it was emplaced eastward onto the craton during the Late Devonian to Early Mississippian Antler orogeny. This study proposed a reasonable solution to one of the longest enduring and most puzzling conundrums of the western Cordillera—the origin, age, and transport of the Harmony Formation.ivThese three studies demonstrated the utility of detrital zircon U-Pb geochronology and Hf-isotope analyses in better understanding difficult sedimentary and tectonic problems. The studies also provided new insights into the Early Paleozoic tectonic evolution of western Laurentian.