Collaborative Research: Constraining the Timing and Nature of Proterozoic Metamorphism in the Northwest U.S. Cordillera

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The goal of this project is to test the hypothesis that garnet-bearing amphibolites and pelitic schists from the northwest U.S. Cordillera preserve a record of late Mesoproterozoic high pressure metamorphism that can be attributed to significant crustal thickening along the western margin of Laurentia before or during the assembly of the supercontinent Rodinia. To this end a research team from Washington State University, West Virginia University, and University of Idaho are undertaking an integrated geochronologic and metamorphic analysis of garnet-bearing metamorphic assemblages in the northwest U.S. Cordillera in order to: 1) determine the ages of metamorphism using Lu-Hf and Sm-Nd geochronology of garnets and U-Th-Pb geochronology of monazites in order to determine the extent of Proterozoic metamorphism throughout the northwest U.S. Cordillera; 2) establish the pressure-temperature conditions of mineral growth in these metamorphic assemblages; 3) compare the results from the garnet and monazite chronometers and integrate this information with the metamorphic fabrics using detailed petrography; and 4) link both the geochronologic and pressure-temperature information to the penetrative deformational fabrics preserved in the rocks. Establishment of a linkage between late Mesoproterozoic metamorphism and the high temperature, high-pressure assemblages that exist in these rocks over a considerable area will show that a major period of crustal thickening during convergent tectonism occurred on the western margin of Laurentia at the end of the Mesoproterozoic. The presence of an approximately one billion year old contractional belt in the northwestern U.S., if confirmed, would imply that the history of metamorphism and deformation in the Cordillera is more complicated than previously appreciated. These results would provide important piercing points for late Mesoproterozoic tectonic reconstructions. In particular, results of this project will provide additional constraints on correlations of western Laurentia with other cratonal fragments such as Australia, Siberia, Antarctica, and China and will ultimately impact various reconstructions of Rodinia.The Grenville orogeny was an important event that substantially modified the North American continent approximately one billion years ago. The metamorphic rocks produced in this event can be found in wide belt that extends from eastern Canada to southwestern U.S. and northern Mexico. Preliminary geochronological data yielded the unexpected result that metamorphism of this age also occurred in the northwestern U.S. To confirm these data, this project is using new geochronological methods to determine the ages of minerals that are commonly produced during the metamorphism that occurs during mountain belt formation. If the preliminary results are born out, current understanding of the nature of the Grenville event will require revision. Evidence for a billion year old metamorphic event in the northwest U.S. would also require modification of current ideas about the configuration of continents at that time.