Durations and Rates of High Temperature Metamorphism During Archean Orogenesis: Implications for Early Earth's Tectonics

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The Archean represents a significant period in Earth's history during which large volumes of continental crust were generated and modern plate tectonic processes are thought to have initiated. Archean cratons contain stable blocks of continental lithosphere that form the foundation from which Earth?s continents have grown and evolved, and petrologic evidence suggests that their earliest histories generally involved at least one period of extreme heat flow (with crustal temperatures in excess of 900 °C). Metamorphism at such high to ultrahigh temperature (HT/UHT) conditions helped to produce the dense, strong, relatively anhydrous rocks comprising most of the Earth's cratonic crust, and the mechanisms that lead to extreme heating may thus be required for craton stabilization. Different mechanisms for crustal heating occur at diagnostic depths, over various timescales, and are associated with distinguishable geologic features. Metamorphic rocks formed by extreme crustal heating provide an interpretable record of these characteristics and this project seeks to use this information to elucidate the processes that resulted in the formation of Earth's early, stable continents.We aim to quantify the absolute depths, temperatures, and durations of metamorphism in two Archean terranes with contrasting geological characteristics. Rocks from the Wyoming and Superior cratons, exposed respectively in the Beartooth Range, MT, USA, and the Pikwitonei Granulite Domain, MB, Canada, both record HT/UHT conditions at ~2.7 billion years ago, but metamorphism occurred over different length-scales, and possibly at different depths and over different durations. This study will combine several geochronological techniques (Lu-Hf and Sm-Nd in garnet and U-Pb in monazite and zircon) with trace element characterization and diffusion speedometry in order to constrain the timescales of crustal heating. Thermodynamic modeling will determine the depths at which maximum temperatures were attained and reveal segments of the paths that were followed upon cooling and/or exhumation. Integration of the geochronology and thermodynamic modeling will place constraints on the durations of these pressure (depth)-temperature paths. These results will provide insights into the mechanisms responsible for HT/UHT metamorphism in each terrane, and the style of plate tectonic processes that operated in the Archean. This project will support one graduate student and one undergraduate, and will initiate new research partnerships between the PIs, both of whom are early career scientists, and premier NSF-funded laboratories.