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RESEARCH PRODUCT

Archaean tonalitic gneiss of Finnish Lapland revisited: zircon ion-microprobe ages

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We report single grain and grain-domain U−Pb zircon ages for the Tojottamanselka tonalitic gneiss previously investigated by the whole-rock Rb−Sr, Pb−Pb and Sm−Nd methods, by conventional U−Pb zircon density/size fraction analysis and by Hf-isotopes (Kroner et al. 1981; Patchett et al. 1981; Jahn et al. 1984) and established as one of the oldest known rocks of the Baltic shield. Our data confirm the intrusive age as 3115±29 Ma (standard error), but we also found slightly older xenocrystic zircon cores with 207Pb/206Pb ages between 3161±19 and 3248±10 Ma that may either be derived from earlier phases of the tonalite melt or from pre-tonalite sialic crust. New magmatic zircon growth, probably during a metamorphic event that led to migmatization, is recorded by an age of 2836±30 Ma and may be coeval with widespread tonalite emplacement elsewhere in the northern Baltic shield at about this time.