LATE-OROGENIC SVECOFENNIAN DEFORMATION IN SW FINLAND CONSTRAINED BY PEGMATITE EMPLACEMENT AGES

Late stages of the Svecofennian orogeny in SW Finland were related to a tranpressional stress field and dextral movements along crustal scal e shear zones under decreasing temperature and pressure conditions. In the Kemio area, a minimum estimate for the time span of movements alo ng one of these shear zones is obtained from the ages of 1840 to 1830 Myr-old microcline granite sheets, related to early ductile deformatio n, and by dating structurally late rare-mineral granite pegmatites, re lated to brittle deformation. One pegmatite was emplaced when the rheo logical 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 sigma) and 1803.1 +/- 2.9/-2.0 Myr (2 sigma). Another peg matite emplaced under brittle conditions has an U-Pb ferrotapiolite ag e of 1802.9 +/- 1.3 Myr (2 sigma). These pegmatites were emplaced pref erentially in gabbroic rocks, that showed brittle to semi-ductile defo rmation at a time when their more felsic host-rocks still showed ducti le deformation. The age range bracketed by the microcline granite shee ts and the structurally late pegmatites indicates that ductile deforma tion related to the transpressional Late Svecofennian tectonic regime in southwestern Finland, persisted for at least 30-40 Myr.