Single or Multiphase Metamorphic History of the Nordfjord Ultrahigh-Pressure Province, Western Norway?

Deciphering the metamorphic evolution of exposed ancient high/ultrahigh-pressure terranes yields important insights into the tectonic processes hidden beneath active continental collision belts. In particular, continuing controversy exists over the age and tectonic significance of multiple, broadly syn-orogenic, eclogite-, granulite- and amphibolite-facies metamorphic episodes that have been identified by numerous workers in both ancient and modern orogens. Do these episodes represent discrete tectonic events, changing boundary conditions, or simply asynchronous equilibrium within a continuous kinematic framework? The Nordfjord area of western Norway was subducted to depths >100 km during the Devonian Scandian orogeny, and exhumed rapidly thereafter. Peak metamorphic grade increases steadily from high-P amphibolite facies south of Nordfjord, to coesite-eclogite facies north and westward. This transition is not disrupted by any significant structural or metamorphic breaks, and thus likely results from in-situ prograde metamorphism. Yet two ages of eclogite-facies metamorphism are known from this region: in southern parts of the transition, TIMS U/Pb dating of multigrain zircon fractions from a fine-grained mafic eclogite (2.3 GPa, 600°C) yield a discord with a lower intercept of 414±4 Ma (MSWD=1.1). Farther north within the UHP zone, however, TIMS dating of two zircon grains from a coarser-grained, higher temperature (∼700°C) eclogite return a concordant weighted mean age of 405±2 Ma (MSWD=0.96). Determining the relative age(s) of metamorphic episodes in the Nordfjord region has relied on dating scattered, rare mafic eclogite. Yet the volumetrically dominant granodioritic and granitic gneiss country rocks, which mostly retain amphibolite-facies assemblages, may offer a more continuous database to elucidate the significance of spatial variations in metamorphic age. Populations of small zircons (<50-80 microns) from seven samples of felsic gneiss were analyzed for U and Pb by laser-ablation multicollector ICPMS. These small grains were chosen to minimize the effects of inheritance from older metamorphic episodes and maximize the potential for Scandian resetting and new growth. Zircons from three high-pressure gneisses (garnet + muscovite ± kyanite) adjacent to or within the UHP zone record significant Scandian overprinting, and yield Discordia with lower intercept ages between 412-405 Ma. Farther south, zircons in garnet + white mica schists (near the 414 Ma eclogite above) were little affected by Scandian metamorphism, retaining mostly Precambrian protolith ages. These preliminary data suggest high-pressure metamorphism was continuous across the Nordfjord area from 414 Ma to at least 405 Ma. Neocrystallization of zircon was limited in colder rocks to the south, whereas gneiss in northern parts grew zircon throughout the duration of the metamorphism. Our early conclusion is that younger eclogite ages in the north do not necessarily signal a discrete event, but may result from ongoing zircon- and uranium-liberating reactions at higher peak temperature.