GEOCHRONOLOGY AND THERMOMETRY OF RUTILE FROM THE WESTERN GNEISS REGION UHP TERRANE, NORWAY

Understanding the style and rates at which continents are subducted, released from the downgoing plate, and exhumed are paramount to understanding virtually all processes related to continental collision, such as surficial erosion, the rearrangement of tectonic plates, and the chemical evolution of the crust and mantle, to name but a few. The Western Gneiss Region (WGR) of western Norway provides the perfect natural laboratory to understand crustal evolution at mid–lower crustal levels during orogenesis, because of the extensive exposure (>60,000 km²) of rocks that preserve their history of subduction into and exhumtion from the mantle. Here we report >20 rutile ages and Zr-in-rutile temperatures of rutile — using laser ablation split stream analysis — from a broad region across the WGR in order to understand the prograde temperature history and to distinguish different stages of exhumation from eclogite-facies depths to the mid-crust in both time and space. Rutile, the primary Ti-bearing phase in HP assemblages, is unique in that it retains a temperature record during prograde growth and yet closes to Pb diffusion at temperatures between that for the closure of white mica to Ar and titanite to Pb; these two characteristics allow one to refine the prograde and exhumation portions of the P-T-t path of high-temperature metamorphic rocks.

Ages across the region range from >400 Ma in the southeast where temperatures are ca. 600–650°C (depending on P; ca. 100 ppm Zr) to as young as ca. 380 Ma in the northwest, where temperatures are as high as ca. 750–800°C (ca. 550 ppm Zr); these temperatures are broadly similar to those of nearby titanites, indicating similar temperatures of prograde and retrograde metamorphism. In the cores of the two northern UHP regions, ages are~20 myr younger than eclogite ages and ~5 myr older than nearby ⁴⁰Ar/³⁹Ar white mica ages, whereas further east and south, ages are ~10 myr older than nearby white mica ages and similar to eclogite-facies ages. This gradient in age differential indicates either that cooling was slower and earlier in the east, or that the more modest temperatures in the east resulted in incomplete resetting of the U-Pb system in rutile. Finally, temperature and age profiles of individual grains can aid in the reconstruction of the P-T-t path of this enigmatic region.