Isotopic age-determinations in Bergslagen, Sweden: VIII. Sveconorwegian Rb-Sr resetting and anomalous radiogenic argon in the Gothian Trans- Scandinavian Smaland-Varmland Granitic Belt and bordering parts of the Svecokarelian Bergslagen Region

Abstract

Biotite Rb-Sr data from 1.70-1.65 Ga old rocks of the Gothian Trans-Scandinavian Småland-Värmland Granitic Belt and from 1.90-1.84 Ga old rocks of the adjoining Svecokarelian Bergslagen Region indicate complete Sveconorwegian resetting 0.90 Ga ago, but a K-Ar analysis of the same biotites and of coexisting hornblendes yields significantly older ages. Some biotites and hornblendes have exceptionally high apparent ages, much older than those of the host rocks, pointing to the presence of excess radiogenic argon. The other K-Ar mineral ages are intermediate between the age of the host rocks and the 0.90 Ga old Sveconorwegian resetting. The hornblendes and biotites with intermediate ages show a weak positive correlation between radiogenic argon content and whole-rock potassium content. It is concluded that there was only limited mobility of radiogenic argon during the Sveconorwegian event. Apparently, the physicochemical conditions during Sveconorwegian recrystallisation were suitable for complete resetting of the Rb-Sr biotite systems, whereas the behaviour of radiogenic argon in the biotites and hornblendes was complex, leading to anomalous K-Ar dates.