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Mafic rocks in east-central Sweden

Mafic dyke belonging to the Herräng dyke swarm intruding granitic gneiss at Herräng, Uppland.

Mafic dyke belonging to the Herräng dyke swarm intruding granitic gneiss at Herräng, Uppland.

Summary

This project aims to characterize mafic dykes and other mafic intrusive rocks in east-central Sweden geochemically, in order to better understand their formation, the origin of their magmas, and the composition of the underlying mantle.

The Roslagen basic to ultrabasic intrusions

Basic and ultrabasic rocks are messengers from the mantle. Their chemical and isotopic composition yields information about the mantle source from which their magmas originated, as well as their tectonic setting. While several studies have been made on younger mafic rocks from Fennoscandia, few modern studies exist of early Svecofennian mafic rocks, such as the gabbro massifs of the Roslagen region in east-central Sweden. These gabbro massifs, with a probable age around 1.9 Ga, have been described thoroughly in the older literature when it comes to field relations, petrography and mineralogy, but modern geochemical studies have been totally lacking.

The purpose of this project was to sample and analyse these grabbros geochemically, including their Sr and Nd isotope composition, in order to characterize their source and tectonic setting, as well as the composition of the early Svecofennian mantle. Geochemical analyses were done externally, whereas the isotope analyses were done in our own laboratory.

One of the main results of this investigation is that the above-mentioned gabbro massifs, which range in composition from basic to ultrabasic, in fact are dominated by cumulate rocks, hence explaining many of their unusual geochemical and mineralogical characters.

The Avesta-Östhammar belt of mafic intrusions

In a follow-up study, a belt of ca. 1.9 Ga old gabbroic or dioritic intrusions, now metamorphosed into amphibolites, stretching through northern Uppland (east-central Sweden) from Avesta to Östhammar, was investigated geochemically.

The Dannemora and Herräng dykes

A related geochemical study on altered mafic Svecofennian dykes (greenstone dykes) in the Dannmora area of northern Uppland has been carried out together with Peter Dahlin (Ph.D.-student at Uppsala University) and Ulf Bertil Andersson (LKAB, Kiruna), as part of Peter Dahlin´s Ph.D.-project.

As a further follow up, samples from the so called Herräng dykes, to which the Dannemora dykes are assumed to be related, were taken from three sub-areas (the Herräng mine field itself, the islet Riddarskäret between Väddö and Singö, and the area around lake Limnaren south of Norrtälje) during 2014 for geochemical studies. The analytical work is now completed, and compilation and interpretation of results are underway.

This research is funded by Magnus Bergvalls stiftelse, and formerly also by the Geological Survey of Sweden (SGU)

Project participants at NRM

External project participants

  • Ulf Bertil Andersson, LKAB, Kiruna

Publications

  • Johansson, Å., Andersson, U.B. & Hålenius, Å., 2012: Petrogenesis and geotectonic setting of early Svecofennian arc cumulates in the Roslagen area, east-central Sweden. Geological Journal, vol.47, p. 557-593. http://onlinelibrary.wiley.com/doi/10.1002/gj.2416/abstractexternal link, opens in new window
  • Johansson, Å. & Hålenius, Å., 2013: Palaeoproterozoic mafic intrusions along the Avesta-Östhammar belt, east-central Sweden: mineralogy, geochemistry and magmatic evolution. International Geology Review, vol.55, p. 131-157. http://www.tandfonline.com/doi/abs/10.1080/00206814.2012.684455external link, opens in new window
  • Dahlin, P., Johansson, Å. & Andersson, U.B., 2014: Source character, mixing, fractionation and alkali metasomatism in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden. Geological Magazine vol. 151,
    p. 573-590. http://dx.doi.org/10.1017/S0016756813000551external link, opens in new window