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

Abstract

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.