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[en] Rb-Sr whole-rock analysis of seven samples of the Fellingsbro-type Younger Granite east of Kopparberg gives an age of 1715 +- 50 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7046+-0.0026 (errors 95% confidence level). This age falls in the age range determined for the Younger Granites of the Vaermland-Smaaland, Dala and Fellingsbro Groups, 1.74-1.64 Ga. Reliable age data from the Malingsbo-type granites in the Kopparberg area are still lacking, however, so it is uncertain whether they can be included in the Fellingsbro Group of Younger Granites. (orig.)

[en] Metavolcanic rocks of the Bergslagen Supracrustal Series in the Grythyttan-Hjulsjoe area, central Sweden, are 1.9-1.8 Ga old according to published zircon U-Pb data. Rhyolitic rocks from Gillershoejden-Kullberget and Norr-Aelgen and metatuffites from Salbosjoen and Kranktorp give Rb-Sr whole-rock ages of 1.6 ± 0.1 Ga and 1.8 ± 0.1 Ga, respectively. The rhyolitic rocks show reset Rb-Sr whole-rock ages due to a thermal event related to the intrusion of 1.7-1.6 Ga old granites, whereas the Rb-Sr whole-rock ages of the metatuffites still approximate the age of emplacement and earliest metamorphism of the supracrustals. (Auth.)

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[en] This paper is an introduction to a series of short communications 'Isotopic age determinations in Bergslagen, Sweden', which will be submitted to this journal. An earlier paper on K-Ar ages of hornblendes in skarns and metabasites in the Hjulsjoe area has been published elsewhere (Moorman et al., 1982). This joint research project of the ZWO-Laboratory of Isotope Geology and the Geological Institute University of Amsterdam forms part of the research program of the 'Stichting voor Isotopen-Geologisch Onderzoek', supported by the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). (Auth.)

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[en] Rb-Sr whole rock data of 17 Horrsjoe granite samples from 5 localities show a best-fit line corresponding to an age of 1.76 +- 0.04 Ga. This result is unsatisfactory in view of the presumed age of the Horrsjoe granite, which is coeval with the 1.84 +- 0.06 Ga Hyttsjoe Suite of rocks. Six samples from one locality give a best-fit line corresponding to 1.86 +- 0.12 Ga; three samples from another locality show a best-fit line corresponding to 1.84 +- 0.44 Ga. These two localities presumably represent relic domains of Horrsjoe granite that have escaped post-emplacement, metamorphic Rb-Sr isotopic resettings of whole-rock samples. The data show a large geological variance in excess of the analytical variance. This may be due to variation in initial ⁸⁷Sr/⁸⁶Sr in the samples, possibly caused by contamination of the granite magma by basic magma and/or seawater. (Auth.)

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[en] Biotite Rb-Sr data from 1.70-1.65 Ga ol rocks of the Gothian Trans-Scandinavian Smaland-Vaermland Granitic Belt and from 1.90-1.84 Ga old rocks of the adjoining Svecokarelian Berglagen 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 oldre 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. Apperently, 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 (author). 34 refs.; 4 figs.; 3 tabs