Shear zone-hosted titanite, rutile, and quartz capture a detailed exhumation history of an ultrahigh-pressure terrane: Otrøya, Western Gneiss Region, Norway

The Western Gneiss Region (WGR) of Norway is one of Earth's largest ultrahigh-pressure (UHP) metamorphic terranes. Thermobarometric studies of the WGR delineate metamorphic conditions resulting from Caledonian continental subduction, structural studies highlight multistage deformation associated with exhumation, and geo/thermochronology investigations show that exhumation-related deformation was associated with Caledonian titanite growth or recrystallization of inherited grains. However, an unresolved question concerns the relationship between deformation, pressure-temperature conditions, and variable Caledonian U Pb titanite dates; i.e., is the range of these dates related to different steps along the exhumation path? The northern coast of Otrøya in the deepest-exposed (Nordøyane) UHP domain of the WGR has abundant outcrop evidence for syn -exhumation deformation in variably retrogressed and deformed eclogites and gneisses. Tetravalent cation thermobarometry, outcrop-scale mapping, and U Pb geo/thermochronology indicate fabric formation in retrogressed eclogite at ~825 °C and ~ 2.0 GPa at ~402 Ma, followed by ~392 Ma titanite growth associated with partial melting of eclogite at ~740 °C and ~ 1.3 GPa. Subsequently, ~382–375 Ma dates from zircon, monazite, and titanite correspond to recrystallization under constrictional strain at ~740 °C and ~ 1.0 GPa. Younger, more localized deformation followed at temperatures <600 °C. These results explain the variable Caledonian U Pb titanite dates reported in the WGR and highlight the power of trace elements to identify geologically significant titanite (re)crystallization events in nondispersed U Pb datasets. The results support a two-stage exhumation history for the WGR. • Titanite, rutile, and quartz in shear zones track exhumation from high-pressures. • Common Pb composition varies between Caledonian titanite populations. • Multiple generations of exhumation-related titanite are differentiated and dated. • Results support a two-stage exhumation history for the Western Gneiss Region.