Late‐orogenic Svecofennian deformation in SW Finland constrained by pegmatite emplacement ages

ABSTRACT Late stages of the Svecofennian orogeny in SW Finland were related to a tranpressional stress field and dextral movements along crustal scale shear zones under decreasing temperature and pressure conditions. In the Kemiö area, a minimum estimate for the time span of movements along one of these shear zones is obtained from the ages of 1840 to 1830 Myr‐old microcline granite sheets, related to early ductile deformation, and by dating structurally late rare‐mineral granite pegmatites, related to brittle deformation. One pegmatite was emplaced when the rheological conditions in the gabbro changed from ductile to semi‐ductile and brittle. It has U‐Pb ferrotapiolite ages ranging between 1807.0 ± 2.9 Myr (2 s̀) and 1803.1 + 2.9/ ‐2.0 Myr (2 s̀). Another pegmatite emplaced under brittle conditions has an U‐Pb ferrotapiolite age of 1802.9 ± 1.3 Myr (2 s̀). These pegmatites were emplaced preferentially in gabbroic rocks, that showed brittle to semi‐ductile deformation at a time when their more felsic host‐rocks still showed ductile deformation. The age range bracketed by the microcline granite sheets and the structurally late pegmatites indicates that ductile deformation related to the transpressional Late Svecofennian tectonic regime in southwestern Finland, persisted for at least 30–40 Myr.