Geochronology of paleoproterozoic augen gneisses in the western gneiss region, Norway: evidence for sveconorwegian zircon neocrystallization and caledonian zircon deformation

The Western Gneiss Region, western Norway, consists of Paleoproterozoic crust of Baltica ancestry (Baltican Basement), partly subducted to high- and ultrahigh-pressure (HP-UHP) conditions during the Scandian Orogeny between 415 and 395 Ma. The dominant felsic gneisses carry little evidence for the HP-UHP history but were affected by amphibolite-facies reworking during exhumation. Laser ablation-ICP-MS and secondary ion mass spectrometry (SIMS) zircon U-Pb data collected in augen gneiss samples constrain the magmatic and metamorphic geochronology in this crust. Five samples from the eclogite-bearing HP-UHP basement near Molde yield intrusion ages ranging from 1644±6 to 1594±10 Ma. Two samples of the structurally underlying eclogite-free basement yield ages of 1685± 18 and 1644±13 Ma, and a sample from the infolded Middle Allochthon Risberget Nappe yields an equivalent age of 1676±18 Ma. Two samples of the eclogite-bearing basement contain low Th/U neocrystallized zircon with an age of 950±26 Ma. This zircon provides the northernmost direct evidence for at least amphibolite-facies Sveconorwegian metamorphism in unquestionable Baltica crust, close to the known "Sveconorwegian boundary" in the Western Gneiss Region. The Western Gneiss Region (1686-1594 Ma magmatism), the Eastern Segment of the Sveconorwegian Orogen (1795-1640 Ma magmatism), and the Idefjorden terrane hosting the type Gothian active margin magmatism (1659-1520 Ma) probably represent three distinct Proterozoic growth zones of Baltica into which Sveconorwegian reworking propagated. Samples of the eclogite-bearing basement lack Scandian neocrystallized zircon but do show partial recrystallization of zircon. Paired cathodoluminescence and electron backscatter diffraction images indicate that zircon crystals underwent crystal-plastic deformation during the Scandian subduction-exhumation cycle. They illustrate a relationship between crystal-plastic deformation by dislocation creep, fading of oscillatory growth zoning, and loss of radiogenic lead.