Palaeoproterozoic structural evolution of polyphase migmatites in Olkiluoto, SW Finland

In migmatitic environments the behaviour of the system is controlled by the generation and amount of the anatectic melt. Accordingly, migmatites typically show a genetic linkage between the tectonic deformation and melt migration. We investigated this relationship in Olkiluoto (SW Finland) and identified four phases of ductile deformation, which are distinguished by the multiple folding phases, ductile shear events and cross-cutting features associated with pegmatitic leucosomes and/or a specific type of diatexitic migmatite with feldspar megacrysts. U–Pb LA-MC-ICPMS data on zircon cores and rims from migmatites and cross-cutting pegmatites indicate two distinct metamorphic events associated with melt generation and migration at 1.87–1.84 Ga and 1.82–1.78 Ga. These two migmatitic events suggest that the orogenic evolution of the area was long-lasting and characterized by slow cooling. The structural data and the age constraints presented in this paper support the idea of similar tectonic evolution and metamorphic environment in SW Finland and central E Sweden.