Slow subduction and exhumation of a thick ultrahigh-pressure terrane: Western Gneiss Region, Norway

High-precision Lu-Hf and Sm-Nd ages reported here from the Western Gneiss Region (WGR) ultrahigh-pressure (UHP) terrane, Norway, seven of which range from 419.5 +/- 4.3 Ma to 397.1 +/- 4.8 Ma, indicate that long-term UHP metamorphism occurred over a wide area. In contrast, two UHP eclogites yield significantly younger and overlapping Lu-Hf and Sm-Nd ages of 390-370 Ma; these ages are significantly younger than ages that are generally accepted for peak eclogite-facies conditions in the WGR, and may indicate contamination from inherited inclusions and/or late stage cooling following exhumation. Collectively, new and existing geochronologic data for eclogites demonstrate that eclogite-facies metamorphism occurred over a large area (60,000 km2) for an unexpectedly long period of >20 Myr. A thermal model using slow subduction (2-4 mm/yr) of a relatively warm and thick slab can reproduce the observed P-T-t history of the WGR UHP terrane, indicating that UHP terranes that are large and had protracted peak- or near-peak histories probably reflect subduction of relatively thick crustal sections.; U-Pb ages of titanite and rutile were obtained from the central Western Gneiss Region, Norway, to assess the style and timing of exhumation and cooling of the Western Gneiss UHP terrane. Approximately half of the titanite ages are concordant, the majority of which yield a limited age range from 393-390 Ma. The balance of the data are discordant, and define a discordia array with an upper intercept of either ∼938 Ma or ∼1.6 Ga, and a lower intercept of ∼389 Ma. Concordant rutile analyses range from 385-392 Ma. Both titanite and rutile ages young WNW toward the core of the orogen and are ∼5 Myr older than 40Ar/39Ar muscovite ages, corresponding to a cooling rate of ∼70°C/Myr. A well-defined boundary between concordant and discordant titanite ages, in combination with the WNW-increasing P-T gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west, suggests that the WGR remained coherent throughout its exhumation history, and was progressively unroofed from east to west. A 390.2 +/- 0.8 Ma, large titanite in the Soroyane UHP domain indicates that exhumation occurred at a vertical rate of ∼5-6 mm/yr for ∼12 Myr. These rates are much slower than estimates from smaller UHP terranes, but similar to other large UHP terranes, suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large UHP terranes that undergo protracted subduction and exhumation, and smaller UHP terranes that undergo rapid subduction and exhumation.