Isotopic age determinations in South Norway: II. The problem of errorchron ages from Telemark rhyolites.
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Two sets of Telemark Supracrustal acid metavolcanics of the Rjukan Group were investigated by the Rb-Sr whole-rock method. Eleven smples within a sampling range of about 10 m were taken at Kvamsstøl at a distance of about 30 km from the Sveconorwegian reset Telemark Gneisses. This set defines a Sveconorwegian woorochron of 0.98 \u00B1 0.05 Ga with an exetremely high initial 87Sr\/86Sr ratio of 0.0849\u00B10.0162 (MSWD = 9.3). The second set was taken from a 1 m thick layer along Vråvatn, 1.5 km from the Telemark Gneisses with a sample spacing of about 10 cm. This set defines a pre-Sveconorwegian (Gothian) errorchron of 1.29\u00B10.06 Ga with an initial 87Sr\/86Sr ratio of 0.7064\u00B10.0036 (MSWD = 13.3). A plot of the Kvamsstøl samples in a Hughes igneous spectrum diagram suggests K, and therefore probably also Rb metsomatism. The high initial 87\/Sr\/86Sr ratio might thus provide evidence of Sveconorwegian resetting of volcanics that were metasomatized in pre-Sveconorwegian, Gothian times. It is not improbable that the appearent 1.29\u00B10.06 Ga age of the Vråvatn samples is a reasonable age approximation for Rjukan Group acid volcanism. According to Beswick & Soucie's graphical procedure to quantify metasomatic alterations for one of the Kvamsstøl samples, enrichment of 60% in SiO2, 125 % inK2O and depletions of 65% in Na2O, 97 % in CaO and 40 % in Fm(FeO + MgO +Mno) have been calculated. For one of the Vråvatn samples, enrichment of 60% in SiO2, and depletions of 60 % in CaO and 60 % in Fm were found.