DATING DYNAMIC RECRYSTALLIZATION: MICROSTRUCTURAL GEOCHRONOLOGY OF TITANITE

Directly dating high-temperature deformation remains an unsolved challenge in structural geology and geochronology. Titanite is common in a variety of bulk compositions, has a high Pb closure temperature (≥750 °C), incorporates trace elements as a function of the evolving pressure and temperature in the host rock, and preserves deformation microstructures, making it the ideal candidate for a deformation chronometer. Combined microstructural analyses, U–Pb geochronology, and trace-element geochemistry from titanite in the Coast shear zone, British Columbia, provide evidence for Pb loss as a result of subgrain formation during crystal-plastic deformation. EBSD maps of ~500 µm-long titanite show up to 70° of intragrain misorientation, with subgrains exclusively present at grain tips. U–Pb LASS dates range from ~60–50 Ma and correlate with intragrain microstructure. The oldest dates (~60 Ma) are associated with undeformed cores; subgrains at titanite tips have the youngest dates (54–50 Ma). The U–Pb dates have a weak or zero correlation with distance to the edge of the titanite grain, suggesting that volume diffusion was not significant. Similarly, there is no straightforward correlation between titanite trace-element composition and U–Pb date, implying that the variations in U–Pb date do not reflect titanite growth. The correlation between date and microstructure and the absence of or weak correlation between date and any other variable imply that the 54–50 Ma dates record the timing of crystal-plastic deformation during slip on the Coast shear zone. This interpretation is consistent with independent constraints on the timing of Coast shear zone deformation, including the 70–55 Ma crystallization age of the Great Tonalite Sill, which intruded before and during slip along the Coast shear zone. All of the dated titanite subgrains are younger than grain cores, suggesting that the migration of dislocations into subgrain walls or enhanced diffusion along subgrain boundaries may drive Pb loss during dynamic recrystallization.