Zircon and monazite geochronology of deformation in the Pielavesi Shear Zone, Finland: multistage evolution of the Archaean–Proterozoic boundary in the Fennoscandian Shield

The Raahe–Ladoga Shear Complex is a major crustal structure representing the Archaean–Palaeoproterozoic boundary in the Fennoscandian Shield. The complex developed during the Svecofennian Orogeny (c. 1.9 – 1.8 Ga) beginning with regional thrust tectonic phases D₁ and D₂, followed by large-scale shearing events D₃ and D₄. The Pielavesi Shear Zone is a vertical north–south-trending shear zone within the Raahe–Ladoga Shear Complex formed during regional D₃ shearing and later reactivated during the regional D₄ phase. Three north–south-trending elongate granitoid intrusions were selected as representative of silicic melts that intruded the transtensional Pielavesi Shear Zone during the regional D₃ phase. The oriented magmatic fabric of the granitoids indicates that they intruded coeval to the deformation event. The zircon U– (Th) –Pb secondary ionization mass spectrometry (SIMS) ages of these intrusions (1888 ± 4, 1884 ± 6 and 1883 ± 5 Ma) overlap within error and provide a direct age for the regional D₃ deformation. ε_Hf(T) (–1.1 to +3.4) and ε_Nd(T) (–1.2 to +0.4) values from these granitoids are both consistent with a predominantly juvenile source affected by a minor Archaean component. U– (Th) –Pb SIMS analyses of metamorphic monazite formed within a crosscutting blastomylonite provide an age for the regional D₄ phase and associated fluid activity of 1793 ± 3 Ma.