Bulletin of the Geological Society of Finland, Vol. 94, 2022, pp 109–118 https://doi.org/10.17741/bgsf/94.2.001 Zircon dating of the basalt and felsic dyke in Haveri, SW Finland Jaakko Kara 1* , Markku Väisänen 1 and Hugh O’Brien 2 1 University of Turku, Department of Geography and Geology, 20014 Turku, Finland 2 Geological Survey of Finland, PO Box 96, 02151 Espoo, Finland Abstract The E-MORB type Haveri basalt differs from the volcanic arc type rocks in the Tampere belt showing no subduction signature. It is considered to have formed in a marginal basin prior to the volcanic arc. We present here zircon U-Pb dating on two samples. The basalt and the felsic dyke yielded 207 Pb/ 206 Pb ages of 1902 ± 5 Ma and 1891 ± 2 Ma, respectively, interpreted as crystallisation ages. The basalt also contains older 1.98 Ga grains while the felsic dyke contains older 1.92 Ga, 1.94 Ga, 1.98 Ga and 2.0 Ga grains, which are inferred as inherited. The age dating of E-MORB type basalts can be used to identify the extensional episodes of the accretionary Svecofennian orogeny. Keywords: Svecofennian orogen, age determination, Haveri formation, tectonic switching, extension * Corresponding author (email: jkmkar@utu.fi) Editorial handling: Jarmo Kohonen (email: jarmo.kohonen@gtk.fi) 1. Introduction Absolute age determinations are essential in under- standing and modelling stratigraphic relations and evolution of large-scale events in deformed Precambrian terrains. One such example is the Paleoproterozoic Svecofennian orogen (e.g., Lahti- nen et al. 2005) in the Central Fennoscandian shield where tectonostratigraphic relations are difficult to construct without the information given by the single grain age dating. One intriguing unit in the Svecofennian in Finland is the Haveri formation (e.g., Mäkelä 1980; Kähkönen 1987), located in the northern part of the Tampere belt in SW Finland (Fig. 1b), as the basalts in Haveri show E-MORB type geochemical affinities clearly distinct from the surrounding arc-type volcanic rocks (e.g., Kähkönen et al. 1989; Kähkönen & Nironen 1994; Kähkönen 1999; Strauss 2003). e stratigraphic relations suggest that the basalts in Haveri are the lowermost and thus the oldest known unit in the Tampere belt (e.g., Kähkönen