MICRON-SCALE ELEMENT CHANGES WITH CONTINENTAL IMPACTS: ASSESSING DEFORMATION-RELATED ELEMENT MOBILIZATION IN UHP ZIRCONS FROM THE WESTERN GNEISS REGION, NORWAY

Deformed zircons from small-scale (~20—100 m) shear zones in the Western Gneiss Region (WGR), Norway, provide insight into the ways that crust changes when subducted to, and exhumed from, depths of >100 km. However, geochronology studies of zircon are complicated by deformation-related microstructures, which may act as fast diffusion pathways or element traps, and potentially lead to the loss or sub- to micron-scale localization of elements. The extent of element mobilization must be considered for accurate interpretation of zircon geochemistry and associated tectonic histories. This study investigates potential deformation-related element mobilization in zircon from five samples from the Drage and Finnøya shear zones in the southernmost and northernmost UHP domains of the WGR, respectively. Previous work indicates that the zircons mostly record Proterozoic dates, with only two yielding Caledonian dates consistent with the UHP event. Electron backscatter diffraction was performed to identify microstructures (i.e., low angle boundaries (LABs) and areas of grain bending). Petrologically significant elements in zircon (e.g., Ce, Y, Yb) were analyzed at the micron- to sub-micron scale to assess if elements were mobilized in response to crystal-plastic deformation. Elements that are commonly interstitial in zircon (Al and Ca) were also analyzed to investigate the presence of fluid-induced element mobilization.

The results from this study suggest that multiple trace elements were mobilized due to the formation of different microstructures. In grains that are bent, P, Pb, Th, and Zr increase with bending, Ca concentrations decrease with bending, and Hf and Y concentration trends vary by sample. In zircons with LABs, Pb, Th, U, and Y concentrations increase approaching LABs. Similar element concentration changes in zircons from orthogneisses with variable compositions and from a dioritic metabasite suggest that host lithologies do not significantly impact the effects of microstructures on trace elements in zircon. Given that most of the grains preserve older Proterozoic U-Pb dates, the observed microstructures and element mobilization trends from this study likely reflect the inherited deformation that was not reset during UHP metamorphism and exhumation.