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

• Main Authors: Woodard, Jeremy, Tuisku, Pekka, Kärki, Aulis, Lahaye, Yann, Jarosław Majka, Huhma, Hannu, Whitehouse, Martin J.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Figshare 2016
• Subjects: Geology; FOS Earth and related environmental sciences; Fennoscandian
• Online Access: https://dx.doi.org/10.6084/m9.figshare.c.3498501.v1; https://figshare.com/collections/Zircon_and_monazite_geochronology_of_deformation_in_the_Pielavesi_Shear_Zone_Finland_multistage_evolution_of_the_Archaean_Proterozoic_boundary_in_the_Fennoscandian_Shield/3498501/1

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 D 1 and D 2 , followed by large-scale shearing events D 3 and D 4 . The Pielavesi Shear Zone is a vertical north–south-trending shear zone within the Raahe–Ladoga Shear Complex formed during regional D 3 shearing and later reactivated during the regional D 4 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 D 3 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 D 3 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 D 4 phase and associated fluid activity of 1793 ± 3 Ma.