A possible basement to the Mesoproterozoic quartzites on Hardangervidda, South-Central Norway: zircon U-Pb geochronology of a migmatitic gneiss

Abstract

On Hardangrvidda west of the Mandal-Ustaoset fault zone in South Norway, some 80-90% of the crust consists of orthogneisses and foliated and massive granites in this plutonic environment the Mesoproterozoic quartzites of the Hettefjorden Group are found as rafts all over Hardangervidds. At Viuvatnet, a migmatic gneiss is interlayered with and possibly underlies the quartzites. This migmatic gneiss has no intrusive contact with the quartzites and could possibly represent their basement. Backscattered electron images and U-Pb ion probe data were acquired on zircon from the gneiss. The main population of zircons gives a weighted average of 1468+- 12Ma. This is interpreted to probably reflect a migmatisation event (evidence for which is observed in the field and in the sample) and points to a high-grade metamorphism occurring after deposition of the Hettefjorden Group. A minor zircon population is represented by a zoned zircon at 1667+- 22Ma. This data could correspond to the age of crystallisation of the magmatic protolith of the gneiss; and it is similar to the age of a foliated granite in the region, the Mårsbrotet granite. A featureless zircon overgrowth at 1167 +- 104Ma is believed to provide evidence for Sveconorwegian metamorphic overprinting in the region. Alternative interpretations of the data set are possible; and more geochronological data are necessary in order to understand the regional significance at the 1.67 and 1.47 Ga events.