Is the Estonian Alutaguse Section of Eastern Fennoscandia a continuation of the Southern Svecofennian Finnish Terranes, or is it akin to the Swedish Bergslagen region?

This research explores the geochemistry of Paleoproterozoic metasedimentary and metavolcanic units in the Alutaguse region of North Estonia and the South Svecofennian (SS) zones, including Ladoga, Saimaa, Häme Belt, and Uusimaa Belt, to better understand the tectonic evolution of the Svecofennian Orogeny in Eastern Fennoscandia. Metasedimentary units consist of micaceous gneisses (± Grt ± Crd ± Sil), while metavolcanics include amphibolites and pyroxenic gneisses. Historical and new data show that High-SiO₂ (>63 wt%) metasediments have felsic origins similar to the Upper Continental Crust (UCC), whereas Low-SiO₂ (≤63 wt%) metasediments, resembling graywackes and shales, indicate mafic to intermediate origins similar to post-Archean Australian Shale (PAAS). Various weathering indices, including CIA, PIA, CIW, and ICV for metasediments, and AI, CCPI, WIP, and SI for metavolcanics, were applied to reveal these geochemical trends. The metavolcanics are classified as sub-alkaline, with geochemical signatures pointing to asthenospheric mantle origins for Alutaguse and subducted oceanic crust origins for SS. Tectonic affinity analyses indicate a predominant oceanic arc setting across both regions. High CaO and MnO concentrations in Alutaguse and Uusimaa metasediments suggest a genetic link, positioning Alutaguse as a 1.90–1.89 Ga back-arc to the Uusimaa belt, followed by the accretion of Uusimaa and Häme belts around 1.87 Ga, marking the closure of the Svecofennian ocean. The Alutaguse zone likely developed as a back-arc to the Tallinn-Uusimaa belt after the accretion of the Bergslagen microcontinent. This interpretation is supported by geophysical anomalies correlated with Zn-Pb-Fe mineralisation. The assemblages found in Alutaguse province comprises high proportions of highly deformed sulphides (pyrite, pyrrhotite, arsenopyrite) and sphalerite disseminated in graphitic amphibolitic-gneisses, which shows similarities with Bergslagen's VMS (SEDEX?) provinces and warrants further investigation.

 

Figure 1. Crustal structure in the central and southern parts of the Svecofennian orogen as integrated across the Baltic Sea, after Bogdanova et al. (2015) and Geochemical relations from the Alutaguse and SS metasedimentary units include major elemental tectonic discriminant functions.