A Rb-Sr whole-rock isochron study of the Loftahammar granitic gneisses in the Västervik area, south-eastern Sweden

The Loftahammar granitic gneisses in the Västervik area of south-eastern Sweden yield an eight-point Rb-Sr whole-rock isochron age of 1598 ±35 m.y. (87Rb half-life = 47.2×109 years) or 1694±35 m.y. (87Rb half-life = 50.0×109 years), with initial 87Sr/80Sr ratio = 0.7131 ±0.0041 (errors with 95 per cent confidence level). A sample from a dike of late-syntectonic younger granite cutting the gneisses also fits this isochron. The rather high initial Sr ratio indicates a pre-metamorphic Rb-Sr record, possibly of the order of some 125 million years. It is inferred that all Svecofennian events of deformation, metamorphism and magmatism distinguished in this area may have taken place within this time-span.