The 1.23 Ga Fjellhovdane rhyolite, Grossae-Totak: a new age within the Telemark supracrustals, southern Norway. Norwegian Journal of Geology

The Grossa-Totak supracrustal belt is part of the several-kilometre thick Telemark supracrustal sequences that are exposed in southern Norway. Deposition of the Telemark supracrustals spans the period between Telemarkian continental growth at ∼1.52-1.48 Ga and Sveconorwegian orogenesis associated with continental collision at ∼1.1-0.9 Ga. The timing of deposition is largely constrained by U-Pb geochronology of detrital zircons in sedimentary units, and igneous zircons within felsic volcanics. A younger Supergroup that has been referred to as the Sveconorwegian Supergroup comprises depositional ages younger than 1.16 Ga; units of the Grossae-Totak belt have been mapped as part of this Supergroup. This study presents a new U-Pb age of 1233 ± 29 Ma for the Fjellhovdane rhyolite, one of the lowermost units within the Grossa-Totak belt; this age suggests that at least the lower part of this sequence is not part of the Sveconorwegian Supergroup, but formed in an earlier volcano-sedimentary basin that is correlative in age to the Sasvatn-Valldal and Setesdal supracrustal belts that occur to the west and south respectively. The geochemistry of the Fjellhovdane rhyolite is compatible with crustal melting of previously-formed supra-subduction rocks, as has been advocated for the Sasvatn-Valldal rhyolites.