The Late Mesoproterozoic Sirdal Magmatic Belt, SW Norway: Relationships between magmatism and metamorphism and implications for Sveconorwegian orogenesis

Abstract

The Late Mesoproterozoic Sveconorwegian Province is commonly correlated with the continent-collision related Grenville Province in eastern Canada. Recently, however, the evolution of the Sveconorwegian Province in SW Norway has been strongly debated, casting doubt on a direct correlation between these provinces.Metamorphism in SW Norway has traditionally been interpreted as representing a main collisional event between ca. 1030 and 970Ma, followed by a contact metamorphic event at 930Ma. Magmatism has been grouped into a ‘syn-collisional’ suite at 1050–1035Ma, a ‘post-collisional’ suite at 980–930Ma, and an anorthosite–mangerite–charnockite–granite (AMCG) suite at 930Ma. New detailed mapping and geochronology in the area reveal a very different and much more complex evolution, and require re-evaluation of previously presented models. In this paper, we focus on the introduction and description of a newly discovered, ca. 200km×50km magmatic belt, the Sirdal Magmatic Belt (SMB). Previously mapped as granitic gneisses in many areas, the existence of this large, commonly undeformed and unmetamorphosed granitoid batholith was only recognized a few years ago (Slagstad et al., 2013a). Magmatism in this belt between 1060 and 1020Ma precedes and overlaps the main Sveconorwegian metamorphic event(s) that affected the region.Our observations of cross-cutting relationships between previously metamorphosed gneisses and SMB rocks indicate that at least one episode of amphibolite- to granulite-facies metamorphism occurred in the region during or prior to emplacement. A lack of widespread metamorphic overprinting and common preservation of igneous textures in most of the SMB indicate that high-grade Sveconorwegian metamorphism after ca. 1020Ma was local rather than regional in SW Norway.The orogenic evolution of SW Norway is characterized by emplacement of large volumes of granitic magma and more localized UHT metamorphism, which is quite different from the widespread, long-lasting metamorphic evolution observed in the Grenville Province, and may point to different tectonic regimes for the two provinces.