SHEAR ZONE-HOSTED QUARTZ AND RUTILE RECORD VARIABLE P-T-D HISTORY ACROSS THE WESTERN GNEISS REGION, NORWAY

Collision and deep burial (>90 km) of continental crust to ultrahigh-pressure (UHP) conditions is associated with the formation of Earth’s largest mountains. Although evidence of UHP metamorphism is relatively common in eroded collisional orogens, little is known about how large volumes of HP/UHP rocks respond to exhumation. The Western Gneiss Region (WGR) of Norway is one of Earth’s largest UHP terranes, and it contains numerous meters-scale shear zones that show evidence of activity during exhumation. Therefore, these shear zones are potentially useful targets for trace element thermobarometry aimed at reconstructing conditions of deformation across a large, exhumed (U)HP terrane.

The WGR exposes three UHP domains. From south to north the domains are: Nordfjord, Sørøyane, and Nordøyane, all located south of the northeast-southwest trending, strike-slip More-Trøndelag Fault Complex (MTFC). Previous thermobarometry efforts indicate that the highest UHP P-T conditions are recorded in the inferred down-slab Nordøyane domain and decrease southward to the Nordfjord domain. Roan, north of the MTFC, shows no evidence of UHP metamorphism. New P-T results for conditions associated with exhumation derived from trace-element thermobarometry (Ti-in-quartz, Zr-in-rutile) show a different trend.

Quartz-bearing gneisses and variably retrogressed eclogites were collected from meters-scale shear zones, and concentrations of Titanium in quartz were measured using EPMA. When paired with previously collected Zr-in-rutile concentrations, the results indicate that exhumation-related deformation occurred at higher pressures and lower temperatures in the Nordfjord domain compared to the sites in closer proximity to the MTFC. This research explores the significance of the variable P-T conditions associated with deformation across the WGR.