Timing of migmatization in east-central Sweden
Åke Johansson
This project aims to elucidate the detailed magmatic and metamorphic evolution in the Svecokarelian (a.k.a. Svecofennian) orogen in east-central Sweden through careful U-Pb SIMS geochronology of zircon.
Project description
This project forms a continuation of the study by M. Stephens and J. Andersson at SGU (Geological Survey of Sweden) on the timing of migmatization in the southern part of the Svecokarelian (Svecofennian) orogen (south of Stockholm), and is also related to similar studies on deformation and metamorphism at Forsmark, in the Bergslagen area, and further north around the Ljusdal batholith. The purpose is to use the NORDSIM ion microprobe for U-Pb dating of zircons in order to study the age of metamorphism in supracrustal rocks and associated plutonic rocks affected by migmatization in four areas in Uppland (the area north of Stockholm): (1) the previous investigated Rimbo area, (2) the area around the Hedesunda complex, and (3-4) the strongly migmatized coastal area north of the Singö zone around (3) Skutskär and (4) Forsmark . The metamorphism in these areas may be comparable with that in more high-metamorphic areas in the north (Ljusdal batholith in Hälsingland) and in the south (Södermanland, Östergötland and northeast Småland), where evidence for two metamorphic episodes (1.87 to 1.85 Ga and 1.83 to 1.79 Ga) have been recorded. To some degree, the project may also give additional information about provenance ages and maximum depositional ages for rocks of sedimentary origin. The purpose is to find out if sediments with different depositional ages (before and efter 1.85 Ga) exist, and if the range of metamorphic ages so far recorded in previous studies represents a longer continuous period of metamorphism, or two or more discrete metamorphic episodes, and if so which geographic distribution these may have.
This project is carried out in close cooperation with Michael Stephens at the Geological Survey of Sweden (SGU), and was supported economically by SGU during 2012-2014.
Results and publications
Orthogneisses of plutonic or volcanic origin from all sub-areas investigated provided magmatic ages between 1880 amd 1910 Ma, with the latter age (from two plutonic orthogneisses from the Rimbo area) being the oldest age recorded from intrusive orthogneisses in central Sweden. A paragneiss from the Rimbo area is inferred to have been deposited penecontemporaneously with this magmatic activity, since it contains detrital zircons of similar age, and was metamorphosed and migmatized around or prior to 1846 Ma (age of crosscutting leucosome). Other metamorphic ages also fall in the 1850-1870 Ma period, although results are often not straightforward to interpret due to the presence of mixed or incompletely reset ages, or U-rich metamict overgrowths yielding highly discordant data. No unequivocal evidence for a younger metamorphic episode (at 1.79-1.83 Ga) were, however, recorded from any of the areas investigated, suggesting that this part of the Bergslagen lithotectonic unit was dominated by the older metamorphic episode (1.85-1.87 Ga).
Preliminary results were reported in a presentation at the Lithosphere conference in Turku, Finland, in November 2014. The full results were published in an article in International Journal of Earth Sciences in 2016:
- Å. Johansson & M.B. Stephens, 2016: Timing of magmatism and migmatization in the 2.0–1.8 Ga accretionary Svecokarelian orogen, south-central Sweden. International Journal of Earth Sciences (Geologische Rundschau) 106, 783-810. DOI 10.1007/s00531-016-1359-3
