Collaborative Research: Growth, preservation, and modification of Neoarchean to Paleoproterozoic crust, an example from the Wyoming province, Montana

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Earth's continental crust is thought to have formed over discrete intervals of geologic time, where particular geologic processes resulted in enhanced crust formation and growth. These time intervals in the Precambrian also reflect a transition to modern plate tectonic processes, resulting in collision and assembly of smaller crustal fragments into continents and supercontinents. Rocks at the northwest margin of the Wyoming province in southwestern Montana provide a record of these processes during formation of the North American continent. This project will utilize an integrated approach by applying petrologic, geochemical, and geochronologic tools to establish relative contributions of crustal growth versus subsequent modification by tectonic processes. The major outcomes of this work will be a better understanding of the balance between crustal preservation and growth during continental assembly. This project will promote teaching and training by supporting graduate and undergraduate research, and results of this study will be incorporated into undergraduate class field trips and laboratory curricula. Infrastructure for research and education will be enhanced through the development and refinement of new geochronological techniques, and by providing training for students in state-of-the-art analytical equipment. New age data from this project will be integrated into current Montana state geologic mapping efforts to better interpret the overall ages of rock units and structural relationships. Outreach efforts will include presenting informal science talks for students and faculty from schools nationwide that conduct summer field courses in southwest Montana.Two intervals in the Neoarchean and Paleoproterozoic represent major periods in the growth and assembly of North America as well as continents worldwide. These periods have been inferred to correspond to times of enhanced production of continental crust. This project will examine the processes that have operated during these time intervals to produce new continental crust as well as the processes operating to rework and assemble this crust. The northwest margin of the Wyoming province in southwestern Montana is an ideal natural laboratory for this study because it is comprised of a diverse suite of metasedimentary garnet-bearing lithologies that are amenable to Pressure-temperature-time studies, as well as containing interlayered meta-igneous rocks that are poorly dated. The proposed work will apply an integrated approach of metamorphic phase equilibria and geochronology of Neoarchean to Paleoproterozoic rocks in the Laramide basement-cored uplifts of the Montana Metasedimentary Terrane to establish relative crustal growth versus modification during these time intervals. Data collected will determine the conditions, timing, duration, and spatial extent of magmatism and metamorphism through metamorphic phase equilibria, Uranium-Lead (U-Pb) geo- and thermochronology of zircon, monazite, and titanite, Hafnium (Hf) and Neodymium (Nd) isotopic analysis of zircon and monazite, and Lutetium-Hafnium/Samarium-Neodymium (Lu-Hf/Sm-Nd) dating of garnet. This integration of tools will provide temporal resolution magmatic and metamorphic histories and directly constrain the growth and modification history of the northwestern Wyoming province.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.