Over half of the active subduction zones on Earth are eroding in a process whereby pieces of the overriding plate are removed and transported down the subduction channel. Subduction erosion is very difficult to recognize in the rock record, but eclogite-bearing continental crust from the Yukon-Tanana arc terrane in the Canadian Cordillera is a good candidate. Locally well-exposed, high-pressure rock units of the Yukon-Tanana arc are being studied to establish the possible settings for eclogite formation. The ultimate goal of this project is to advance our understanding of processes that occur along the interface between the subducting slab and the overriding plate in collisional orogens. The specific goals are to identify the genealogy, metamorphic evolution and tectonic setting of the coherent eclogites and their host rocks, as opposed to mÃ©langes, in Yukon-Tanana; and to evaluate the results in terms of plausible models for subduction and exhumation channels. Detailed mapping is an essential part of the project; the crux of the problem is to demonstrate shared metamorphic histories for the eclogites and their host rocks. Quantitative mineral chemistry and zoning data collected on the electron microprobe at Rensselaer Polytechnic Institute are being combined with the results of phase equilibrium modeling to produce detailed pressure-temperature paths. U-Pb geochronology utilizing the USGS-Stanford sensitive high-resolution ion microprobe (SHRIMP) yields zircon, titanite and monazite ages that are then linked to various points along the subduction and exhumation path. Models for eclogite formation via subduction erosion will be evaluated in light of the results. Our proposed idea that the eclogites in Yukon-Tanana terrane formed via subduction erosion challenges the assumptions that all high-pressure metamorphism forms in the subducting plate and that subduction polarity in accreted terranes can be deduced from the mere presence of eclogites.
Our collaboration involves contributions from researchers from the United States, Canada, and German, and involves important scientific and logistical support from the Canadian Geological Survey and Yukon Geological Survey. The research results directly pertain to the EDGES project, a collaborative effort among the federal, state and provincial geological surveys of Canada to enhance mineral resource exploration by understanding the terranes of the northern Cordillera of North America. The project is providing important training opportunities for graduate and undergraduate students at the University of Iowa in a STEM discipline, is contributing to the broadening of underrepresented groups in science, and is providing outreach efforts for K-12 students through the University of Iowa's Natural History Museum.
Intellectual Merit Over half of active subduction zones on Earth are eroding in a process whereby parts of the overriding plate are removed and transported down the subduction channel. This process, called subduction erosion, is notoriously difficult to recognize in the rock record, but may play a fundamental and underappreciated role in causing high-pressure (HP) metamorphism of continental crust. We investigated the little known HP rocks of the St. Cyr area in southern Yukon as a potential candidate for formation by tectonic erosion. Our mapping, geochemistry, petrology and geochronology confirms that eclogites (mafic HP rocks) in the Yukon-Tanana terrane of the Canadian Cordillera are abundant layers and lenses that share a common deformational history with their quartzofeldspathic host rocks. They form fairly large (up to 30 km long and 6 km thick) coherent slices of continental arc crust, and are distinct from adjacent lower grade imbricates of oceanic crust. The presence of the typical HP mineral, phengite, and Permian age HP metamorphic zircon in both eclogites and surrounding schists indicates that the mafic rocks and host rocks were metamorphosed together as a coherent unit. The schists have both igneous and sedimentary precursors. U-Pb analysis of zircon shows that tonalites related to arc plutonism formed at 335 Ma. Detrital zircon ages from the metasedimentary rocks tie them to the composite arc of the overriding plate, consistent with a subduction erosion mechanism. Phase equilibria determined from thermodynamic modeling of mineral compositions and assemblages show that the St. Cyr eclogites reached UHP conditions (3.2 GPa and 610°C). This is the first example of ultrahigh-pressure (UHP) metamorphism of crustal rocks reported in North America. The results corroborate growing evidence from around the world that UHP metamorphism can occur in accretionary settings, and not just during continent-continent collision by deep subduction erosion of the overriding plate. Broader Impacts This project significantly enhanced teaching and learning at the University of Iowa. It provided research opportunities for a Ph.D. student, a M.S. student and two undergraduates, all of who participated in fieldwork and/or laboratory work off campus, and had the chance to interact with international collaborators. Both graduate students are women and both graduated in May 2014. The project supported a Research Assistantship for Ph.D. candidate M. Petrie, a young woman with minority status, and S. Isard on a M.S. project, and thus enhanced the inclusion of underrepresented groups in a STEM discipline. In addition, samples collected by Petrie were utilized in a class project in the PIâ€™s Fall 2011 upper level course on "Advanced Structural Geology—Microstructures." Each of the 9 students worked with a different oriented sample of a deformed metamorphic rock in order to learn techniques of microstructural analysis. For example, the class visited the Scanning Electron Microscopy lab at the University of Minnesota to acquire orientation (EBSD) data on quartz-rich samples with the help of technician N. Seaton; each student did a separate report interpreting the data. The students presented their results to the class and then wrote an abstract synthesizing everyoneâ€™s results. The class presented a poster on the project at the North-Central Geological Society of America meeting in Dayton, OH (Petrie et al. 2012). The PI taught a similar course as a Directed Study for 2 more M.S. students the following year. Two of the M.S. students are currently employed by CTL Group, studying the petrology and microstructures of concrete. S. Isard is using her newly acquired skill sets (geological mapping, sample preparation, GIS, petrology) in her current position at a state geological survey. The project fostered a high degree of international collaboration. The Geological Survey of Canada contributed approximately $60,000 to the fieldwork, and the Yukon Geological Survey CanNOR program provided $10,000. Cees van Staal (GSC) served on Petrieâ€™s Ph.D. committee, visited her in the field, funded additional dating samples, and has collaborated on many aspects of the project. Maurice Colpron facilitated much of the collaboration with YGS. H.-J. Massonne (Univ. Stuttgart) ran a workshop in Germany on thermodynamic phase equilibrium modeling and visited the University of Iowa. These collaborations, together with Bill McClelland (Univ. Iowa) on the timing of geologic events, have produced a synergy that concerns projects in the Cordillera well beyond the scope of this one. Results of the project have been broadly disseminated at meetings and in peer-reviewed journals. The PI and Petrie have presented the work at 4 conferences, 3 workshops and 3 universities. One paper is published and both dissertations are available online from the University of Iowa library. One paper is submitted and 3 papers are in preparation. The map is available from the YGSâ€™s GIS database and will be incorporated into the Geological Map of Yukon by Colpron. We have showcased the work in outreach programs for the Cedar Valley Rocks and Minerals Society and Women in Science and Engineering groups.