Geochronology of Paleoproterozoic Augen Gneisses in the Western Gneiss Region, Norway: Evidence for Sveconorwegian Zircon Neocrystallization and Caledonian Zircon Deformation

Abstract

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 to Ma. Two samples of the structurally underlying eclogite-free basement yield ages of and Ma, and a sample from the infolded Middle Allochthon Risberget Nappe yields an equivalent age of Ma. Two samples of the eclogite-bearing basement contain low Th/U neocrystallized zircon with an age of 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.