U-Pb zircon geochronology of selected Archaean migmatites

• Main Authors: In Eastern Finland, Asko Käpyaho, Pentti Hölttä, Martin J. Whitehouse
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Corresponding author; karelia*; karelian
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.571.5798; http://www.geologinenseura.fi/bulletin/Volume79/Kapyaho_etal_2007.pdf

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 proto-liths were generated at broadly the same time as metamorphosed volcanic rocks in the ad-jacent Kuhmo-Suomussalmi-Tipasjärvi greenstone belt, suggesting that at least some of the mesosomes may represent migmatised equivalents of the greenstone belt rocks. Two sepa-rate 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 sug-gested by the zircons dated from the leucosomes. Scattered age data, inheritance and the large number of discordant data prevented establishing accurate leucosome generation ag-es. Consequently, the age data are considered to support the previously proposed hypoth-esis that the Archaean Karelian craton was formed after accumulation of separate tecton-ic entities having variable ages.