Fennoscandian evolution and the position of Baltica in earlier supercontinents

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.

Two of the latest published papers concern the origin of the Hedesunda granite complex and the mafic 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.

Under the heading "Cleaning up the record" I have recently redone older and in part inaccurate U-Pb zircon ages from southern Sweden with new and refined methods, and published the revised results. See more below.

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).

In addition, I have investigated rocks from drill cores from Poland and Lithuania in cooperation with geologists from these countries. These drill cores penetrate the younger sedimentary cover rocks and go into the underlying Precambrian basement. Material from the drill cores have been studied in order to determine how the Precambrian basement in those areas is related to the exposed Precambrian crust in southeast Sweden. Some results from these studies have been published by Krzeminska et al. (2021) and Skridlaite et al. (2021) in the journal Precambrian Research (see publication list below).

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 results have been published in Geological Journal in 2012 (Johansson et al. 2012, see publication list below)

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 results have been published in an article in International Geology Review in 2013 (Johansson & Hålenius 2013, see publication list below)

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. The results were published in Geological Magazine in 2014 (Dahlin et al. 2014, see publication list below)

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 results have been published 2020 in an article in GFF (Johansson & Karlsson 2020, see publication list below).

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 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 been obtained. Thin sections have been studied, and all samples have been analysed for major and trace elements. Sr and Nd isotopes have also been analysed from a selection of samples for further characterization and discrimination of magma sources. Furthermore, zircons from the previously dated samples have been reanalysed for U-Th-Pb with the Nordsim ion microprobe, in order to test and confirm the above age results, and for Lu-Hf by LA-ICP-MS at the Vegacenter in order to further determine the origin of the magma.

The results have been published in an article in International Journal of Earth Sciences in 2020 (Johansson 2020, see publication list below).

Cleaning up the record

Old age U-Pb zircon age determinations by TIMS of rocks from the southern part of the Protogine Zone and the Eastern Segment in southern Sweden are redone with more accurate SIMS technique that can separate magmatic ages from inherited gains and metamorphic overprinting. These revised U-Pb ages are also complemented by new Lu-Hf isotope data, obtained by LA-ICP-MS, in order to better understand the origin of the magmas forming these rocks. The results have been published in the Swedish geological journal GFF in 2021 (Johansson 2021, see below).

Stockholm archipelago

At present, geological studies are conducted in the southern part of the Stockholm archipelago (Ornöhuvud and Fjärdlång with adjacent islets) in cooperation with Stefan Claesson from the Swedish Museum of Natural History and Karin Högdahl from Uppsala University.

The research projects above have been funded by Magnus Bergvalls stiftelse, the Geological Survey of Sweden (SGU), or "Riksmusei vänner".

Project participants at the museum

• Åke Johansson (principal investigator)
• Ulf Hålenius
• Andreas Karlsson
• Stefan Claesson

External project participants

• Ulf Bertil Andersson, LKAB, Kiruna
  
• Karin Högdahl, Uppsala University
• Ewa Krzeminska, Polish Geological Institute, Poland
• Grazina Skridlaite, Vilnius University, Lithuania

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/abstract
• 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/10.1080/00206814.2012.684455
• 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/S0016756813000551
• Johansson, Å & Karlsson, A., 2020: The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance. GFF vol, 142, p. 1-22. https://www.tandfonline.com/doi/full/10.1080/11035897.2019.1708450
• Johansson, Å., 2020: The Paleoproterozoic Hedesunda granite complex, east‑central Sweden, a composite intrusion. International Journal of Earth Sciences vol.109, p. 1991-2022. https://doi.org/10.1007/s00531-020-01885-x
  
• Johansson, Å., 2021: Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden. GFF vol.143, p. 328-359. https://doi.org/10.1080/11035897.2021.1939777.
• Krzemińska, E., Johansson, Å, Krzemiński, L., Wiszniewska, J., Williams, I.S., Petecki, Z., Salwa, S., 2021: Basement correlation across the southernmost Baltic Sea: Geochemical and geochronological evidence from onshore and offshore deep drill cores, northern Poland. Precambrian Research vol. 362, no. 106300, 20 pp. https://doi.org/10.1016/j.precamres.2021.106300.
• Skridlaite, G., Siliauskas, L., Whitehouse, M.J., Johansson, Å., Rimsa, A., 2021: On the origin and evolution of the 1.86–1.76 Ga Mid-Baltic Belt in the western East European Craton. Precambrian Research vol. 367, no. 106403, 21 pp., https://doi.org/10.1016/j.precamres.2021.106403