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Fennoscandian evolution and the position of Baltica in earlier supercontinents

Åke Johansson

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

An overarching theme for my research has been the formation and evolution of the Fennoscandian shield during the Proterozoic. For this purpose, U-Pb dating of different rocks is combined with geochemical and isotope (strontium, neodymium) studies in order to understand their origin and tectonic setting. A related question is how the geological evolution of Fennoscandia fits into the larger plate tectonic pattern, how Fennoscandia and Baltica (the northeastern, Precambrian part of Europe) have fitted into different early supercontinents (Columbia/Nuna, Rodinia) during this time period, and how they have been moving across the globe.

My present research concerns the origin of the Hedesunda granite complex and the basic Herräng dykes in east-central Sweden (see below). The latter project is a continuation of similar studies of other basic intrusive Svecofennian rock complexes in the same area (The Roslagen basic to ultrabasic intrusions; The Avesta-Östhammar belt of mafic rocks; The mafic Dannemora dykes; see below), which may give information of the tectonic environment in the area 1900 to 1850 million years ago, when these rocks formed, and the nature and composition of the underlying mantle.

Previous projects, now finished, have been concerned with The Proterozoic bedrock of Blekinge and Bornholm (in southern Fennoscandia) and Timing of migmatization in east-central Sweden (central Fennoscandia). When it comes to reconstructions of Precambrian supercontinents, I have written about Baltica’s connection to Amazonia and West Africa during the Proterozoic (the so-called SAMBA model).

Currently, I am also working with earlier obtained Sr and Nd isotope data from drill cores into the Precambrian basement in Poland and the southern Baltic Sea, which may have important implications for the continuation of the exposed crust of the Fennoscandian shield in southern Sweden beneath the Baltic Sea and the sedimentary rock cover in the Baltic republics and Poland. In addition, during the last couple of years I have been preoccupied with the writing of the Precambrian part of a planned popular science book about the geology of Sweden.

The Hedesunda granite complex

The Hedesunda granite complex is straddling the boundary between Uppland and Gästrikland. It has the shape of a somewhat skewed rectangle extending E-W for about 40 km and N-S about 20 km, i.e. covering an area of ca 800 square kilometres, and dominantly consists of coarse-grained granite, little or moderately deformed.

There has been considerable discussion about the age and tectonic position of the Hedesunda granite. U-Pb dating has given two sets of ages, 1.86 – 1.87 Ga (billion years) from the southeastern part of the complex, and 1.78 Ga from its central part. This has led to the subdivision of the Hedesunda complex into two intrusions. However, the geographic subdivision of the granite complex into these two intrusions remains uncertain, with different divisions proposed on different maps from SGU.

The purpose of the present project is to make a geochemical study of the whole Hedesunda granite complex, and if possible use geochemical discrimination to help in delineating the two types, as well as determine their geochemical variation and tectonic setting. 27 samples covering most of the area in question have already been obtained. Thin sections will be studied, and samples will be analysed for major and trace elements. Sr and Nd isotopes will be analysed from a selection of samples for further characterization and discrimination of magma sources. Furthermore, it might be useful to reanalyse zircons from the previously dated samples with the Nordsim ion microprobe, in order to test and confirm (or modify) the above age results.

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