Detrital zircon dating of the Palaeoproterozoic Himmerkinlahti Member, Posio, northern Finland; lithostratigraphic implications

The Himmerkinlahti Member is a thin, poorly exposed metaconglomerate – metaquartzite unit in the SE part of the Palaeoproterozoic Kuusamo Belt, Posio, northern Finland (Figs. 1 & 2). It was originally included into the middle part of the Karelian supracrustal rocks of the belt (Laajoki, 2000), but later on its lithostratigraphic position was considered problematic (Laajoki & Wanke, 2002; Laajoki, 2005). In order to have more light to this question, zircons from a granule-pebbly metaquartzite sample (4/190KL91) with abundant opaques and a sericite-rich (pseudo)matrix (see Fig. 7e in Laajoki, 2000) were separated for U-Pb dating at the Geological Survey of Norway. Dr. Ansgar Wanke picked and mounted them at the University of Oulu. The ion microprobe analyses were performed using the Nordic Cameca IMS 1270 at the Swedish Museum of Natural History, Stockholm. The spot diameter for the 4nA primary O2ion beam was ca. 25 μm and oxygen flooding in the sample chamber was used to increase the production of Pb+ ions. Four counting blocks comprising a total of twelve cycles of the Pb, Th and U species were measured in each spot. The mass resolution (M/DM) was ca. 5000. The raw data was calibrated relative to Geostandards zircon 91500 reference, which has an age of 1065 Ma (Wiedenbeck et al., 1995) and corrected for background at mass 204.2 and modern common lead (T=0; Stacey & Kramers, 1975). For further details of the analytical procedures see Whitehouse et al. (1997, 1999). The excel-programs by Whitehouse were used for data reduction, and the Isoplot/Ex programs by Ludwig (2001) for line fitting and concordia plots. Decay constants follow the recommendations of Steiger & Jager (1977).