Evolution and tectonic history of the Bergslagen volcano-plutonic complex, central Sweden

Abstract

The Proterozoic Svecofennian volcano-plutonic rocks in Bergslagen, central Sweden, exhibit changes in both geochemistry, grade of deformation and metamorphism going from the west toward the eastern and more interior parts of the south Svecofennian volcanic belt. In the west geochemically homogeneous rhyolitic volcanic rocks cover vast areas while towards the east the magmatic variation is larger, and dacitic volcanic rocks become increasingly common. Andesites are locally present. Similarly coeval granitoids in western Bergslagen are granitic, while granodiorite and tonalite plutonic complexes are more common to the east. A change in the magmatic character of the rocks is substantiated by their trace element signatures. The rocks in the westernmost parts of Bergslagen have characteristics similar to modern continental within-plate settings, while those in the easternmost parts of Bergslagen have characteristics similar to volcanic arcs. The change in magmatic character is successive, and resembles the compositional polarity across a Phanerozoic magmatic arc at a destructive plate margin. The more continental character of the Svecofennian rocks in western Bergslagen implies thicker crust. This is substantiated by the more homogeneous magmatic character of the volcanic rocks which can be taken to imply larger magma chambers. Thus, when the Svecofennian crust, shortly after its consolidation, was subjected to an E-W compression, the crust in western Bergslagen acted as a more competent block relative to the thinner more 'primitive' crust in east. This resulted in stronger compression of the crust to the east, which in turn explains more intense folding and higher grades of regional metamorphism in these areas.