In situ Microstructural and Mechanical Analysis of Zircon and Monazite Using Type Sections for U-Pb Strain Chronometry

English Abstract:

Direct radiometric dating of deformation in the lithosphere has been a longstanding goal in geochronology. Moser et al. (2009) have demonstrated that zones within textured zircon grains from the lower crust beneath the Vredefort impact structure in South Africa date a lower-crustal flow event. The material setting(s) that gave rise to zircon deformation and Pb loss are yet not well understood. We explore the relationship between in-situ material properties, textural features in zircon and monazite grains and their petrologic setting in these type sections of the mylonitic mafic lower-crust. Methods used include EBSD mapping, and texture analysis with commercial and MTEX software. In one thin section we have identified 64 zircons ranging in size between 4 and 190 microns. The largest grains are enclosed in garnet macrocrysts and more abundant smaller grains are in the matrix and garnet coronae. A total of 145 monazites were identified between 4 and 49 microns in size. Monazites were mostly restricted to the matrix. Textures in BSE imaging have proved useful as a predictor of deformation microstructures. Using this proxy, deformed grains were found exclusively in the matrix and are typically larger, anhedral and non-equiaxed. Low-angle grain boundaries were measured in these grains using EBSD. Monazites are less textured than zircons. It remains to be determined whether these are primary or secondary phases. Comparisons will be made with samples from other high-grade terranes including the Central Gneiss Complex of Norway and the Kapuskasing Uplift in Ontario. LA-ICP/MS will be used to investigate spatial relationships of age-resetting and texture. We hope that this work will increase our predictive capacity for locating ideal rock types and environments for U-Pb strain chronometry.