DEFORMATION-ENHANCED DIFFUSIVE LOSS OF TITANIUM IN QUARTZ DURING GREENSCHIST-FACIES CRYSTAL-PLASTIC DEFORMATION, WASATCH FAULT ZONE, UTAH

The Wasatch Fault Zone (WFZ) crosscuts the Little Cottonwood Stock (LCS) and has exhumed remnants of mylonite in its footwall. Titanite and apatite U-Pb petrochronology from a mylonite and adjacent nondeformed protolith require that the shear zone was active between 31–22 Ma, concurrent with the emplacement of the LCS, but prior to the onset of Basin and Range extension at 18–12 Ma. Zr content of titanite porphyroclasts was not reset during ductile deformation, leading to uncertainty about the thermal conditions of the shear zone. Quartz porphyroclasts were characterized optically and by cathodoluminescence (CL) imaging, EBSD mapping, analysis of Ti content by EPMA, and diffusion modeling to determine the temperature of deformation in the shear zone and examine Ti mobility in quartz during low temperature crystal-plastic deformation. Protolith quartz crystals show sparse undulose extinction, but are generally undeformed. Mylonite quartz ribbons contain bulging and subgrain rotation recrystallization microstructures. Quartz crystals in the protolith have gradational CL zoning interpreted to record magmatic crystallization with subsequent high temperature hydrothermal alteration. EBSD maps of quartz porphyroclasts in the mylonite show highly deformed grains made up of subgrain families. Quartz porphyroclasts have bright CL cores that become gradationally darker towards the rim. Additionally, subgrains contain bright cores and dark rims that resemble loss of Ti by volume diffusion. Protolith quartz crystals contain 50–125 ppm titanium while mylonite quartz grains contain ≤50 ppm titanium. Core-to-rim profiles of CL brightness were extracted and modeled with a finite source–infinite sink 1D volume diffusion model using the titanium diffusivity from Cherniak et al. (2007). Isothermal diffusion models had the highest goodness of fit (R2 ~ 0.9) for a max diffusion temperature of ~450 ºC and a max duration of ~5–15 Myr, consistent with observed quartz deformation mechanisms and previous thermochronology and titanite and apatite petrochronology. Diffusion models that incorporated cooling did not yield reasonable outcomes. These results demonstrate that Ti was mobile in deforming quartz at greenschist facies and corroborates the thermal conditions within the proto-WFZ and decoupling of Zr-in-titanite thermometry from the U-Pb and other trace elements.