U-PB TITANITE AGES OF DECOMPRESSION MELTING DURING EXHUMATION OF THE WESTERN GNEISS REGION ULTRAHIGH-PRESSURE TERRANE

The Western Gneiss Region of Norway includes one of Earth's giant ultrahigh-pressure (UHP) terranes. Understanding the subduction and exhumation of this >60,000 km² area is relevant to a range of processes, including collisional orogenesis, reworking of the continents, and the global geochemical cycle. Important aspects that remain unanswered include the spatial and temporal variations in the rate of exhumation and the extent of phase transformation-induced reworking during exhumation. To address this issue we have determined the ages of titanites from leucosomes within Western Gneiss Region orthogneiss. Textures of large (up to 1 cm) euhedral titanites imply growth during melt-induced recrystallization. Two of these titanites—one from a high-pressure region, the other from an ultrahigh-pressure region—yield identical concordant ²³⁸U-²⁰⁶Pb ages of 390.2 ± 0.8 Ma; slightly older titanite ages characterize much of the Western Gneiss Region (Tucker et al., 1990). This decompression-related melting occurred ~11 m.y. after peak metamorphism at 401.6 ± 1.6 Ma (Carswell et al., 2003). In contrast to the uniformity in titanite ages, muscovite cooling ages show a pronounced gradient from 399 Ma near the foreland to 375 Ma in the UHP domain (Root et al., 2005; Walsh et al., in press). These differences—coeval closure of titanite and diachronous closure of muscovite—argue for kinematically and geometrically complex exhumation wherein the UHP body reached Moho depths en masse but was then borne up into the shallow crust via successive east-to-west unroofing.