Slow exhumation of UHP terranes: Titanite and rutile ages of the Western Gneiss Region, Norway

Abstract

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 to 390 Ma. The other titanite data are discordant, and define discordia arrays with upper intercept ages of either ∼ 938 Ma or ∼ 1.6 Ga, and a lower intercept of ∼ 389 Ma. Concordant rutile analyses range from 385 to 392 Ma. Both titanite and rutile ages young WNW toward the core of the orogen and are ∼ 4 Ma older than 40Ar/ 39Ar muscovite ages, corresponding to a cooling rate of ∼ 90 °C/Ma. 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 titanite in the Sørøyane UHP domain indicates that exhumation occurred at a vertical rate of ∼ 7 mm/yr for ∼ 12 Ma. These rates are 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.