LEVERAGING SHEAR ZONE-HOSTED TITANITE, RUTILE, AND QUARTZ TO EXTRACT A DETAILED EXHUMATION HISTORY OF THE NORDØYANE UHP DOMAIN, WESTERN GNEISS REGION, NORWAY (Invited Presentation)

The Western Gneiss Region (WGR) of Norway is one of Earth’s largest and most extensively studied ultrahigh-pressure metamorphic (UHP) terranes. Previous work in the WGR delineates peak pressures attained during Caledonian subduction of Baltica beneath the Laurentian continental margin, the multistage exhumation of Baltica basement, and a detailed record of Caledonian titanite growth and recrystallization of inherited (pre-UHP) titanite grains. However, the relationship between deformation, P-T conditions, and complex U-Pb titanite dates and compositions is not well understood.

To investigate the connection between titanite petrochronology and the P-T-d record of deformation, we sampled a lithologically diverse, meters-scale shear zone on Otrøya, within the deepest exposed UHP domain in the WGR (reported pressures of 3.2–3.6 GPa and temperatures of ~850 °C). Evidence for syn-exhumation deformation is found in variably retrogressed and variably deformed eclogites and gneisses. Application of tetravalent cation thermobarometry, outcrop-scale mapping, and U/Pb-geo/thermochronology indicate an early fabric in retrogressed eclogite that formed at ~825°C and ~2.0 GPa at ~402 Ma. At ~392 Ma, early titanite growth was associated with partial melting of eclogite at ~740 °C and ~1.3 GPa. Younger, ~382–375 Ma dates from zircon, monazite, and titanite correspond to recrystallization accompanied by constrictional strain at ~ 740°C and ~1.0 GPa. More localized deformation overprints earlier constrictional fabrics, and the rocks impacted by this late deformation contain quartz that yields Ti-in-quartz temperatures <600°C. The P-T-t-d results add context to the disparate Caledonian U-Pb titanite dates reported for rocks in the WGR and demonstrate the importance of trace elements for identifying titanite (re)crystallization events in apparently nondispersed U-Pb datasets. Furthermore, the detailed P-T-t-d path produced from Otrøya samples support a multi-stage exhumation history for the WGR.