Cooling history of the eastern Svecokarelides: whole-rock and mica Rb-Sr and hornblende K-Ar ages in the areas of Pihtipudas-Iisalmi and Joroinen-Sulkava, Finland
Bulletin of the Geological Society of Finland
HAUDENSCHILD, UELI, 1990. Cooling history of the eastern Svecokarelides: whole-rock and mica Rb-Sr and hornblende K-Ar ages in the areas of Finland. Bull. Geol. Soc. Finland, 62, Part 1, Rb-Sr age determination on mica and whole-rock samples and K-Ar age determination on hornblendes were carried out to get information on the time of metamorphism and early cooling in two Proterozic areas of the Finnish Svecokarelides. In the Joroinen-Sulkava area (southern Finland) a whole-rock isochron of 1812
... k isochron of 1812 + 45Ma on metapelitic gneisses reflects the time of amphibolite-facies metamorphism. A 1875 + 17Ma whole-rock isochron on a quartz-diorite of the granulite-facies Pielavesi area (central Finland) may date an earlier high-grade metamorphic event, the low 87Sr/86Sr initial ratio of 0.7018 can be explained only by a shift or rotation of the isochron postdating its age. The disturbation of the whole-rock system could be related to the 1800-1820Ma metamorphism dated in the Joroinen-Sulkava area. A lower limit for the age of the isochron-shift is given by 1750-1790Ma biotite ages. The 1800-1820Ma metamorphic event seems to have been weaker in central than in southern Finland while the 1875Ma metamorphism seen in central Finland (Pielavesi) is absent or completely overprinted in the south. Muscovite Rb-Sr and hornblende K-Ar ages date the cooling of the areas to about 500°C. Muscovite ages vary between 1730Ma and 1770Ma for different localities. Five fractions of two hornblende samples define an isochron of 1778Ma on the 40Ar/36Ar versus 40K/36Ar diagram. In one locality (Vieremä) the hornblende ages (1995-2143Ma) are remarkably higher than the age of the muscovite (1828Ma). The hornblende ages were interpreted as having a pre-metamorphic memory, being of Jatulian or late Archean origin. The blocking-temperature for argon in horblende therefore should be somewhat higher than the blocking-temperature of muscovite in respect to the Rb-Sr system.