| Summary: | Mapping and isotopic work in the Instrumentberget area has shown that a granitic gneiss, previously referred to the Bangenhuk Formation, is unconformably overlain by a younger meta-sedimentary unit with a basal conglomerate horizon, previously included in the Rittervatnet Formation. In this paper it is suggested that the granitic gneiss, named the Instrumentberget Granitic Gneiss, occurs at a lower structural level than the Bangenhuk Formation and that this meta-sedimentary unit is the basal part of the Polhem Formation. Isotopic work on zircons from the Instrumentberget Granitic Gneiss and one boulder in the overlying basal conglomerate has shown that some of the conglomerate clasts are derived from the underlying Instrumentberget Granitic Gneiss the latter having a U-Pb zircon age of 1737±43 Ma. Evidence for this is three fold; firstly, the morphology of the zircons in one of the boulders is similar to zircons in the underlying granitic gneiss; secondly, a single U-Pb analysis from one conglomerate boulder falls right on the discordia line of the granitic gneiss; and thirdly, age analyses by the direct Pb-Pb evaporation method on single zircons from the granitic gneiss and the conglomerate boulder agree with the upper intercept age of the U-Pb analysis. A second conglomerate boulder has a quite different zircon morphology to the underlying granitic gneiss and a single U-Pb analysis falls significantly above the discordia line of the granitic gneiss, suggesting an exotic origin. Direct single zircon Pb-Pb evaporation analyses of this second boulder suggests that the Polhem Formation may be younger than 1000 Ma.
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