Lithostratigraphic correlations in an asymmetrical rift-basin: the Grythyttan area, W. Bergslagen, Sweden

Abstract

The predominantly felsic volcanics and intrusives of the Grythyttan basin in W. Bergslagen, are subdivided into four successive stages illustrating the development of a subaqueous continental rift zone. A change in the type of volcanism from Plinian type eruptions with thick fall deposits (stage 1) into Pelean type with extensive flows (stages 2, 3 and 4) is seen. A rift stage (stage 4) is unconformably superimposed on the former three stages and is characterized by bimodal felsic and mafic magmatism and the presence of turbiditic sediments. Fe-ore rich layers associated with fine ashes and limestones, are found throughout the sequence, with a major concentration in stage 3 preceeding the rift stage proper. This ore deposition is associated with large hydrothermal activity and the ascent of basic material. Mn-rich, Fe-ores are found in the stages 2,3 and 4, indicating the former presence of oxidation/reduction boundaries during ore formation. Ore formation and intrusive and volcanic activity should be considered as inter-related processes, linked to the development of an asymmetrical continental rift, presumably triggered by magmatic underplating.