Building the Precambrian Laurentian Core of the North American craton: Resolving the Age and Tectonic Setting of the Snowbird Tectonic Zone of Canada

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The Canadian Shield, the core of the North American continent, is one of the great Archean ( 2.5 billion year) cratons on Earth and one from which many inferences have been drawn about the early evolution of Earth and the nature of early tectonic processes. It is generally accepted that the core of the continent consists of a number of relatively small Archean microcontinents (or protocontinents) that were stitched together by Proterozoic (2.0-1.8 billion year) orogenic belts. Many workers interpret the boundaries between these crustal blocks in terms of modern plate tectonic processes. That is, it can be argued that by approximately 2 billion years ago, plate tectonic processes were beginning to resemble those that of the modern-day Earth. There is one major crustal boundary that has remained enigmatic and might challenge interpretations about the evolution of North America and of plate tectonics in general. The Snowbird Tectonic Zone of central Canada is a 2,800-kilometer-long linear discontinuity that roughly corresponds to the boundary between the Hearne and Rae geologic domains (microcontinents?) of a mass of ancient rocks that forms the Canadian Shield. This boundary has been interpreted by geologists and geophysicists in terms of very different tectonic processes that have occurred at widely different times in Earth history. This research is aimed at clarifying the origin and significance of the Snowbird Tectonic zone by first constraining the age of the feature itself, and then by comparing and contrasting basement and cover rocks on either side of the enigmatic boundary. In addition to the scientific goals of the research, the project involves a significant intellectual collaboration between Canadian and American researchers from both government and university organizations. The project will contribute to education in a STEM discipline by the training of graduate and undergraduate students and the development of research results into classroom curricula and the creation of an online course for high school Earth Science students. Results of the research will be widely disseminated through presentations at professional geoscience meetings and the peer-reviewed scientific literature. The Snowbird Tectonic zone is the 2800-kilometer-long boundary between the Rae and Hearn domains (proto-continents) in the western Canadian Shield. It is widely taken to be one of the fundamental growth structures, or sutures, within the core of Laurentia, but interpretations of the age and tectonic setting of the zone vary widely with first-order implications for the early Earth tectonics and the history of Laurentia. The purpose of the proposed work is to build on region mapping and petrologic work near the zone and to finally constrain the age and significance of the Snowbird tectonic zone. Research will be carried out within the 600-km-long central segment of the Snowbird tectonic zone. Building on regional mapping and petrologic/structural analysis, new data will be collected and compiled from three transects across the zone. Data will include: (1) the isotopic signature of basement rocks within and on either side of the zone, (2) the age and character of cover sequences, (3) timing and style of deformation and metamorphism within and adjacent to the zone, and finally (4) the age and petrogenesis of the Chipman mafic dike swarm, a ubiquitous component of the Snowbird zone. The working hypothesis is that, unlike other boundaries in the Canadian Shield, the Rae-Hearne boundary is an Archean (not Proterozoic) continental suture, a collisional boundary that may provide an early record of horizontal tectonics in North America. The Snowbird Tectonic Zone, however, is a Proterozoic feature, a rare deep crustal exposure of an incipient and abortive rift and magmatic ?hot zone? that served to localize deformation, metamorphism, and partial melting. Consequently, in addition to placing critical constraints on the tectonic history of North America, the Snowbird Tectonic Zone provides unprecedented exposures of the zone of interaction of mantle magmatism, high-T metamorphism, and deformation within the lower crust. It may provide a model for similar processes occurring on Earth today.