Late Sveconorwegian metamorphism and deformation in southwestern Sweden

Abstract

Abstract The Precambrian of southwestern Sweden is divided into eastern and western crustal segments separated by a major tectonic boundary, the Mylonite Zone. The southern part of the eastern segment is characterized by numerous occurrences of mafic granulites and charnockites, the western segment comprising greenschist to amphibolite facies gneisses of supracrustal and plutonic origin. SmNd mineral ages of the granulite facies metamorphism in the eastern segment range from ∼ 920 to ∼ 880 Ma. Garnet+pyroxene+feldspar+whole-rock SmNd isochrons of the Varberg charnockite and the Traslovslage mafic granulite yield ages of 893 ± 5 Ma and 881 ± 4 Ma, respectively. This late Sveconorwegian (late Grenvillian) granulite facies metamorphism in southern Sweden appears to be confined to the eastern segment, while the last high-grade metamorphic event in the western segment is contemporaneous with the ∼ 1100 Ma old granulite facies metamorphism in the Bamble area of SE Norway. At least three phases of regionally important deformation postdate the late Sveconorwegian metamorphism in southwestern Sweden. Deformation related to movements within the Mylonite Zone was contemporaneous with or later than the granulite facies metamorphism in the Varberg area, and took place mainly under retrograde metamorphic conditions leading to almost complete replacement of garnet and pyroxene by amphibole, biotite and plagioclase. Extensional tectonics were presumably important during the uplift of the granulite terrain east of the Mylonite Zone. Peak P - T conditions during the late Sveconorwegian metamorphic event were considerably higher than those required for the melting of granitoid rocks. The SmNd ages of granulite facies parageneses within the eastern segment are approximately the same as those of numerous late Sveconorwegian granite intrusions in southwestern Scandinavia, suggesting a genetic relationship between high-grade metamorphism at depth and the emplacement of granites at higher crustal levels. The Nd model ages of the Varberg charnockite agree reasonably well with the Nd model ages of 1700-1600 Ma old granites within the same crustal domain, while the model age of the mafic granulite (originally a mafic dyke) corresponds roughly to the model ages of ∼ 930 Ma old mafic dykes in SE Sweden.