Rb-Sr isotopic relations of a tonalitic intrusion on Tjörn Island, south-western Sweden

Abstract

Eight samples from the Rönnäng tonalite–granodiorite intrusion on Tjörn Island, south-western Sweden, were investigated in a Rb-Sr geochronological study. The whole-rock Rb-Sr system is disturbed and the eight samples describe a 1615-Ma age-reference line, which does not qualify as an isochron. However, an isochron age of 1710 ± 115 Ma (λ = 1.39 × 10⁻¹¹ a ⁻¹) at a low, initial, ⁸⁷Sr/⁸⁶Sr ratio is obtained when only four of the samples poorest in Rb are employed. This age is interpreted as dating the intrusion. The event which disturbed the whole-rock Rb-Sr system was identified from combined mineral and whole-rock data. An isochron of 1010 ±105 Ma and a reference line of 1015 Ma were obtained. This age coincides with the age of Sveconorwegian intrusions and metamorphism. A compilation of Rb-Sr data from southwestern Sweden identifies a zone of late Sveconorwegian regeneration of the whole-rock isotopic systems.