Bulletin of the Geological Society of Finland (Jun 2007)
U-Pb zircon geochronology of selected Archaean migmatites in eastern Finland
Abstract
U-Pb SIMS data on eight samples (four mesosome-leucosome pairs) from migmatites in gneiss complexes are presented from the western part of the Archaean Karelian craton, eastern Finland. Most zircon grains in the mesosomes yield ages between 2.84 Ga and 2.79 Ga, with cores of 2.94 Ga in one sample. Age data imply that migmatite mesosome protoliths were generated at broadly the same time as metamorphosed volcanic rocks in the adjacent Kuhmo-Suomussalmi-Tipasjärvi greenstone belt, suggesting that at least some of the mesosomes may represent migmatised equivalents of the greenstone belt rocks. Two separate events of metamorphic zircon growth are recognised from different samples: the first occurred between 2.84 Ga and 2.81 Ga, and the second between 2.73 Ga and 2.70 Ga. The latter event is considered to have led up to a prolonged period of partial melting, as is suggested by the zircons dated from the leucosomes. Scattered age data, inheritance and the large number of discordant data prevented establishing accurate leucosome generation ages. Consequently, the age data are considered to support the previously proposed hypothesis that the Archaean Karelian craton was formed after accumulation of separate tectonic entities having variable ages.
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