Structure, stratigraphy and ore-forming processes in Bergslagen: implications for the development of the Svecofennian of the Baltic Shield

Abstract

Bergslagen is the ore-bearing part of the 1.9-1.86 Ga Svecofennian of central Sweden, characterized by large thicknesses of felsic metavolcanics and sediments, with an intricate association of local and regional hydrothermal alterations and mineralizations related to the development of mostly submarine rift basins. Mineralization types show a change with time in response to the evolving geotectonic setting. The change in lithology from predominantly volcanic Bergslagen to more sedimentary in the easterly Stockholm region is accompanied by an increase in metamorphic grade and change in deformational style. The main geological features of the three granitoid magmatic regions, and intervening sedimentary areas of the Svecofennian are summarized and related to the processes affecting the northern Archean block during Proterozoic times. A two stage model for the evolution of the 1.9-1.86 Ga Svecofennian of the Baltic Shield is proposed. New crust of felsic to intermediate composition was generated in the period 2.6-2.1 Ga by multiple subduction from the SSE. In this period the Archean block underwent deformation as an active margin. Accretion of the newly formed crust occurred during a collision type orogeny with the Archean block in the period 2.1-2.0 Ga. Reworking of the accreted crust in Central Finland, Skellefteå-Kiruna and Bergslagen occurred in the period 1.9-1.86 Ga, with elongate volcanic basins developing in transtensional strike slip zones, where deep faulting penetrated to the lower crust.