FORMATION AND EXHUMATION OF THE WESTERN GNEISS REGION ULTRAHIGH-PRESSURE TERRANE: GEOCHRONOLOGY, STRUCTURAL GEOLOGY, AND PETROLOGY

Coordinated structural, petrological, and geochronological analysis of the Norwegian HP–UHP terrane, the world’s largest, lends new insight into these remarkable rocks. Newly discovered UHP eclogites, newly discovered HP eclogites, and new ⁴⁰Ar/³⁹Ar ages define three discrete UHP domains that are separated by distinctly lower pressure, but still eclogite-facies, rocks. ⁴⁰Ar/³⁹Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and that the UHP domains comprise antiformal culminations beneath a HP veneer. The shapes of the UHP antiforms overturn the long-held idea that P and T increase monotonically toward the coast. The sizes of the UHP antiforms range from ~2500 km² to >100 km²; if the UHP culminations are part of a continuous sheet at depth, the UHP terrane underlies a minimum of 11,000 km² and is overlain by a HP veneer of 60,000 km². No recognized structure separates the UHP and HP domains, and the transition in the Nordfjord area is defined by a gradual increase in P and T.

Metapelites in the UHP core of the orogen indicate that the (U)HP metamorphism was followed by near-isothermal decompression to ≥1.2–1.7 GPa/~750 ºC and then to ~0.5 GPa/~750 ºC; metapelites in the HP area show a supra-Barrovian overprint at 1.0–1.2 GPa/~750 ºC and then ~0.5 GPa/~750 ºC. New U/Pb zircon, U-Th/Pb monazite, and ⁴⁰Ar/³⁹Ar ages, combined with extant geochronology, require decompression from eclogite-facies conditions at 410–405 Ma to mid-crustal depths in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with a minimum dimension of 20–30 km. ⁴⁰Ar/³⁹Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (~30 ºC/m.y.) at mid-crustal levels caused by large-scale extension.