Evolution of the Archaean Karelian Province in the Fennoscandian Shield in the light of U-Pb zircon ages and Sm-Nd and Lu-Hf isotope systematics

In situ Lu-Hf (laser ablation microprobe-inductively coupled plasma mass spectrometry (LAMICPMS)) and U-Pb (LAM-ICPMS, secondary ionization mass spectrometry (SIMS)) analyses of zircon, and whole-rock Sm-Nd isotope analyses were performed on rocks formed during magmatic events in three Archaean complexes in the Karelian Province of Fennoscandia (Pudasjärvi, Koillismaa and Iisalmi). These complexes have U-Pb ages ranging from 3.5 to 2.6 Ga. In Pudasjärvi, sparse xenocrystic cores give ages of 3.6-3.7 Ga and initial ¹⁷⁶Hf/ ¹⁷⁷Hf suggesting influence of a crustal component T ≥ 4.0 Ga (assuming a CHUR-like mantle source). Ages and Nd and Hf isotope patterns indicate magmatic events at 3.6-3.7 Ga (Siurua, Pudasjärvi with ≥4.0 Ga precursor), 3.2 Ga (Iisalmi, Koillismaa), 2.8 Ga (Pudasjärvi) and 2.7 Ga (Pudasjärvi, Iisalmi). In the Meso- and Palaeoarchaean events, there is no evidence of sources equivalent to present-day depleted mantle; such sources were, however, involved in the 2.8-2.7 Ga events. Hf and Nd are strongly correlated. Contrasts between the Archaean complexes indicate that they evolved separately until c. 2.7 Ga. The age and Hf pattern of ≤2.8 Ga rocks in the Karelian Province is compatible with a scenario in which the Karelia, Superior, Yilgarn and Slave cratons were part of a late Archaean supercontinent, but does not constitute proof of the existence of such a supercontinent.