Collaborative Research: Precambrian Crustal Evolution in Western Laurentia: Implications from the Sawtooth Metamorphic Complex

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Earth scientists from Louisiana State University and the University of Florida are collaborating on an investigation of a unique segment of Precambrian (550 million years old, m.y.o.) continental crust brought to the earth?s surface by the ~50 m.y.o. Sawtooth batholith in eastern Idaho. This crust is unique because it lies between some of the oldest crust in North America (exposed in SW Montana) and some of the most recent additions to the North American continent in western Idaho (Blue Mountains). This research will test a wide range of hypotheses for the formation and evolution of this crust: Is it an independent terrane accreted to North America in the recent past, or does it show geochemical ties to the old (3000 m.y.o.) crust exposed in SW Montana? Does it record evidence of a ?vanished? episode of mountain building similar in age to the Grenville orogeny that formed much of eastern North America 1000 m.y. ago? If accreted to North America in the Precambrian, does it provide insight the fragmentation of North America that occurred 500-700 m.y.o ago? Specifically, the team will investigate the origin of metamorphosed sedimentary rocks that carry information about the nature of continental crust exposed at the time they were deposited. The temperature and pressure history of this crust, which will constrain the time at which it was appended to North America and the mechanism by which it was appended. Elemental abundances in the metamorphosed igneous rocks will give important clues to the environment in which they formed, while isotopic compositions will constrain the amount of new crust vs. recycled crust that is preserved. A large fraction of the crust that lies between the Archean (2500 m.y.o.) crust of SW Montana and the 200 m.y.o. crust of western Idaho is buried beneath younger rocks, making the Sawtooth terrane a very important window into crustal evolution in western North America. This research will be carried out by a team that includes members from underrepresented groups and women who will be trained in important quantitative techniques applicable across a range of STEM disciplines. The team will also participate in the public-private partnership between LSU Geology & Geophysics and Marathon Petroleum Corporation (Geoscience Diversity Enrichment Program) that is focused on increasing STEM participation. The public will also benefit by our partnership with the Sawtooth National Recreational Area (U.S. Forest Service), leading to better educational materials distributed in the Sawtooth National Recreational Area. In detail, this team (faculty and students, undergraduate and graduate) will employ and provide training in the following: 1) We will conduct U/Pb radiometric dating of zircons to determine original crystallization ages of igneous rocks and the origin of sediment deposited in protoliths to the metasedimentary rocks using laser ablation to sample individual crystals in the ICP-MS laboratory at the University of Florida; 2) Elemental analyses by a combination of x-ray fluorescence and ICP methods at the University of Florida; 3) The electron microprobe laboratory at LSU will be used to measure elemental abundances in individual minerals, which can then be used to determine the burial conditions and pressure-temperature path of the Sawtooth crust (thermobarometry); 4) U-Th-Pb (radiometric) dating of texturally and petrologically constrained monazite and titanite will provide a definitive age for the metamorphic event(s) recorded in the Sawtooth crust needed to develop Pressure-Temperature-time path(s) and determine the tectonic setting of metamorphism using facilities at the University of Florida and the University of Massachusetts; and 5) Measurement of K-Ar systematics (40Ar/39Ar) will help assess the most recent, post-metamorphic, thermal history of the terrain. Students from both universities will cross-train in these techniques as part of their thesis and dissertation research, and will then be well prepared to meet the technologically challenging opportunities afforded members of the country?s STEM workforce.