Bulletin of the Geological Society of Finland (Jun 1991)
A pilot Rb-Sr dating of the Suomusjärvi ultramylonite: evidence for major post-Svecofennian deformation in SW Finland
Abstract
The Suomusjärvi shear zone in the West Uusimaa area in the Proterozoic Svecofennides of SW Finland belongs to a group of late, post-peak metamorphic structures comprising vertical shear zones and folds with vertical fold axes. Field and microscopical evidence suggest that these structures can be correlated with a shear zone cutting the Bodom granite with a known U-Pb zircon age of 1641 ± 13 Ma. This paper presents the results of a pilot Rb-Sr whole-rock and biotite dating of the Suomusjärvi shear zone. The shear zone cuts the Suomusjärvi tonalite and is locally developed as a 5 meter thick ultramylonite in which the biotite is completely recrystallized. A sample of undeformed tonalite yields a Rb-Sr biotite age of 1644 ± 16 Ma, the ultramylonite a biotite age of 1533 ± 23 Ma. The tonalite age is interpreted as a cooling age, while the ultramylonite age represents resetting due to the mylonitization process (deformation, recrystallization and fluid flow) below the biotite Rb-Sr blocking temperature of 300‒350°C. The whole-rock data give a meaningless age of 1760± 108 Ma, which is the result of partial whole-rock resetting at the scale of the ultramylonitic banding. It is concluded that the results of the pilot-dating are compatible with the expectations based on other data. This indicates that this group of late deformation structures in the studied area is the result of an important post-Svecofennian event which was previously hardly recognized in south Finland. Structural, isotopical, petrological and fluid inclusion data on the cooling history of the West Uusimaa area are internally consistent, but conflicting with other evidence from southern Finland.
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