Dating Deformation with Titanite: Examples from Exhumed Ductile Shear Zones and Deformation Experiments
Directly dating deformation is critical for understanding the timing and rates of many geologic processes, including plate boundary evolution, fluid–rock reactions, and the formation of critical mineral resources. Because of its physical and chemical properties, Titanite (CaTiSiO 5 ) is one of the best candidates to directly date ductile deformation. We present three examples from exhumed ductile shear zones and high-temperature, high-pressure deformation experiments that advance our understanding of how to best directly date deformation with titanite. In the Coast Shear Zone (British Columbia), the U-Pb dates are correlated with misorientation (“lattice bending”) in titanite crystals, suggesting that the dates record the timing of deformation. However, these microstructures are superimposed on compositional zoning in the titanite, making it unclear what the relative roles of dislocation creep and fluid-mediated recrystallization are in resetting the U-Pb system in deformed titanite crystals. In the Anita Shear Zone (New Zealand), twinned and/or misoriented (“bent”) crystal domains yield Miocene U-Pb dates, directly tying this structure to the development of the modern Pacific–Australian Plate Boundary. However, the wide range of dates in deformed crystals raises questions regarding whether dates are partially or completely reset during deformation. A series of high-temperature, high-pressure titanite deformation experiments were also performed to determine the role that dislocation creep and mechanical twinning play in directly resetting titanite U-Pb dates. Aggregates of titanite and potassium feldspar were deformed in general shear at T=750–950 °C, confining pressure of 1.5 GPa, and strain rates of ~10 -5 s -1 for 2–3 days. The undeformed starting material yielded a U–Pb weighted mean age of 998 ± 5 Ma (MSWD = 1.5), whereas the weighted mean ages from the experimentally deformed samples were 996 ± 10 Ma (MSWD = 0.65), 990 ± 8 Ma (MSWD = 2), and 963 ± 4 Ma (MSWD = 3.9 Ma). Several individual dates from the experimentally deformed titanite were younger than the undeformed starting material outside of analytical uncertainty. The collective results from the exhumed ductile shear zones and experiments suggest that the presence and motion of dislocations resets titanite U-Pb dates over geologic time, and that dates from dislocation-related microstructures in titanite should be interpreted as deformation ages.
