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Presented By: Earth and Environmental Sciences

Smith Lecture - Hannah Blatchford, Eastern Michigan University

Accessory minerals and trace elements from a lithologically diverse shear zone capture a detailed exhumation history: Otrøya, Western Gneiss Region, Norway

The Western Gneiss Region (WGR) of Norway is one of Earth’s largest ultrahigh-pressure (UHP) metamorphic terranes. Within the WGR, thermobarometric studies delineate peak pressures and temperature resulting from Caledonian continental subduction, structural studies highlight multistage deformation associated with exhumation, and geo/thermochronology investigations show that exhumation-related deformation is associated with Caledonian titanite growth or recrystallization of inherited grains. However, an unresolved question concerns the relationship between deformation, pressure-temperature conditions, and the variable Caledonian U-Pb titanite dates recovered from areas of the WGR. Can the range of Caledonian U-Pb titanite dates be 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 due to constrictional strain at ~ 740°C and ~1.0 GPa. Younger, more localized deformation followed at temperatures <600°C. The 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 apparently nondispersed U-Pb datasets. The results support a two-stage exhumation history for the WGR.

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