Insights into the tectonic evolution of the Svecofennian orogeny based on in situ Lu-Hf dating of garnet from Olkiluoto, SW Finland

Abstract. The Southern Finland granites and associated migmatitic rocks have a complex metamorphic history, being affected by multiple events during the ca. 1.88–1.79 Ga Svecofennian orogeny. In this study, the prolonged tectonic evolution of migmatites and associated rocks in SW Finland has been studied using the new in situ Lu-Hf method. Results reveal detailed temporal constraints for the tectonic evolution that can be linked to major events in adjacent tectonic blocks in both Finland and Sweden during the Svecofennian orogeny. The metamorphic peak at the Olkiluoto site occurred at 1834 ± 7 Ma based on in situ Lu-Hf dating of garnet. The P-T path for the rocks indicates a prograde evolution, with peak P-T conditions of 3–5 kbar and approximately 700 °C. The metamorphic constraints and age presented in this paper enhance our understanding of the geological and tectonic evolution in SW Finland, coupling the Olkiluoto site to the Häme metamorphic and tectonic belt in Finland and highlighting tectonic and metamorphic similarities with the Ljusdal Block of Sweden.