MOBILIZATION OF RADIOGENIC PB DURING DYNAMIC RECRYSTALLIZATION OF TITANITE AND DATING THE SYN-MAGMATIC EXTENSIONAL COLLAPSE OF THE SEVIER HINTERLAND

Dynamically recrystallized titanite has the potential to be used to directly date deformation. Titanite is common in rocks and often a robust U-Pb geochronometer, but also incorporates common Pb, has low U concentrations, and is easily recrystallized by metamorphic fluids. The nature of dynamic recrystallization and chemical mobility in titanite during deformation is not well understood. Two samples containing titanite, nondeformed granodiorite and mylonite, were collected from the Wasatch Fault zone where it crosscuts and has exhumed the Oligocene Little Cottonwood stock (LCS) from 11.5 km depth. Hydrothermal sericite from the fault produced a K-Ar date of 17.6 ± 0.6 Ma, and is closely related but does not directly date deformation. Titanites were characterized by EBSD and LASS-ICP-MS to date movement across the fault. Titanites from the nondeformed sample exhibit minimal to no intracrystalline deformation. Titanite porphyroclasts from the mylonitic sample exhibit twin-boundary migration and subgrain rotation recrystallization; quartz porphyroclasts in the same sample exhibit bulging and subgrain rotation recrystallization, which indicate ~300–400 ºC deformation conditions. Neither quartz or titanite shows signs of static or fluid-aided recrystallization after deformation. U-Pb dates (n = 124) from the nondeformed titanites range from ~37–31 Ma with two populations at ~35 and ~32 Ma. U-Pb dates (n = 246) from the titanite porphyroclasts range from ~35–22 Ma with a mode at ~30 Ma. These dates could have resulted from early, protracted deformation in the Wasatch Fault zone, partial re-equilibration during a younger deformation event, or a combination of both. The timing of deformation (~32–22 Ma) is consistent with collapse of the Sevier hinterland and emplacement of the LCS (~35–25 Ma). The nearby Deer Creek Fault, interpreted as late Eocene, bounds the LCS to the south and was later exploited as a right-lateral step between Wasatch Fault segments. Extension on both a proto-Wasatch and Deer Creek Faults would efficiently make space for the LCS. Some amount of partial re-equilibration during young Wasatch Fault movement is also likely and consistent with the dates. No matter the efficiency, these data demonstrate that radiogenic Pb is mobilized in titanite during low temperature dynamic recrystallization.