Geologian tutkimuskeskus, Tutkimusraportti 207 – Geological Survey of Finland, Report of Investigation 207, 2014 Lauri, L. S., Heilimo, E., Leväniemi, H., Tuusjärvi, M., Lahtinen, R. & Hölttä, P. (eds) THE problem WITH THE age of THE Central Puolanka Group KEEPS fighting US by Asko Kontinen1, Hannu Huhma2, Yann Lahaye2 and Hugh O’Brien2 1 Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland 2 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland E-mail:
[email protected] The Central Puolanka Group (CPG) along the western margin of the Palaeopro- terozoic Kainuu schist belt presents an unresolved problem in terms of its age of deposition, being inferred either as early Proterozoic 2.35–2.20 Ga (Laajoki 2005) or late Archean >2.70 Ga (Kontinen et al. 1996, Huhma et al. 2000). This state of affairs is badly hampering the modelling of the Archaean to Proterozoic tectonic transition in Kainuu and southern Lapland (cf. Laajoki 2005). The reason is that the CPG lacks easily datable syngenetic magmatic rocks. Our efforts to address the problem include U-Pb dating of detrital zircon grains from all the main units (from oldest to youngest: Puolankajärvi, Akanvaara and Pärekangas Fms) of the CPG within the Oikarila structure in the western-cen- tral part of the KSB (Fig. 1). The studied samples contain only Archaean zircons, in most cases dominated by ca. 2.70 Ga grains, which in several cases form the youngest, clearly detrital population. This also concerns samples from the rocks previously interpreted (Huhma et al. 2000) as ca.