Provenance of the Asker Group, Oslo Rift : A detrital zircon U-Pb and Lu-Hf study

Abstract

To identify possible protosources for the Asker Group 573 in situ LA-MC-ICPMS uranium-lead and 509 lutetium-hafnium analyses were performed on detrital zircon grains from seven samples. The samples from the Kolsås and Skaugum Formations did not yield enough zircons (2 and 5, respectively) to be statistically viable, while for each of the five other samples, where three belong to the Tanum formation and two were taken from an area where no formational subdivision has been done, ∼ 100 zircon grains were analyzed. Major peaks and age clusters in the recorded 313 ± 4 to 2844 ± 14 Ma age span coincides with virtually every significant period of magmatism in Fennoscandia – including Archean, Svecofennian, TIB, Gothian, Sveconorwegian and Caledonian magmatism. This is also confirmed by the initial 176Hf/177Hf ratios. One major peak at c. 350 Ma does not correspond with any known magmatism in Fennoscandia, and is thus likely from the Variscides of Central Europe or the British Isles. Comparison of the recorded data with U-Pb and Lu-Hf data from the Ringerike Group and the Orsa sandstone suggests that the bulk zircon detritus of the Asker Group were derived from recycled Silurian sandstone cover sequences outside the Oslo Rift area, with a significant contribution from the Variscan mountains.