U-Pb age evidence for repeated Palaeoproterozoic metamorphism and deformation near the Pajala shear zone in the northern Fennoscandian shield

Abstract

This paper presents U-Pb ages for an extended period of geological evolution between about 1860 and 1740 Ma in the northern part of the Fennoscandian Shield. It involved magmatism, ductile deformation and metamorphism along the border between Sweden and Finland. The study area separates into a western and an eastern domain roughly along a north-south trending structural boundary defined by the western margin of the Pajala shear zone. Approximately 1860-1850 Ma zircon ages from two granitoid rocks record contrasting post-emplacement deformation histories on either side of this boundary. While metamorphic monazite records c. 1850 Ma events in rocks of the western domain, metamorphic monazite and titanite as welI as zircon overgrowths in the eastern domain verify deformation and high-grade metamorphism in the 1820-1780 Ma interval. This post- I 820 Ma phase is most probably related to shear zones and to nearby intrusions. A titanite age at c. ) 740 Ma in the eastern domain is suggested to exemplify a tectonic phase, which elsewhere in northern Fennoscandia is marked by fracturing, generation of pegmatites, and hydrothermal activity.