U-Pb geochronology on detrital zircons from FAR-DEEP cores, Fennoscandian Shield - age constraints for events of the Archaean-Palaeoproterozoic transition and provenance

The Archaean-Palaeoproterozoic transition is marked by several environmental events that were important for the evolution of the Earth system and occurred coeval with plate tectonic reorganisations including the break-up of the supercontinent Kenorland. We applied U-Pb-geochronology on detrital zircons by LA-ICP-MS and ID-TIMS to improve age constraints of these events, recorded in the 2.5-2.0 Ga rock successions of the Fennoscandian Shield and including the Huronian glaciation and the Lomagundi-Jatuli positive excursion of δ13C in sedimentary carbonates. The detrital zircons provided ages ranging from 3.5 to 1.9 Ga with dominant age populations in the range of 2.9-2.5 Ga for most samples. Youngest zircon ages from the Seidorechka Sedimentary and Polisarka formations in the Imandra-Varzuga Greenstone Belt and the Neverskrukk Formation in the Pechenga Greenstone Belt constrain, for the first time, the deposition of glacial diamictites on the Fennoscandian Shield to between 2.43 and 2.2 Ga. Similar ages have been reported for glacial deposits in Canada and South Africa, consistent with a global glaciation at that time. The youngest detrital zircon ages derived from the Kuetsjärvi Sedimentary Fm, containing isotopically heavy carbonates, and from the overlying Kolasjoki Sedimentary Fm in the Pechenga Greenstone Belt indicate that deposition of carbonates corresponding to the Lomagundi-Jatuli δ13C excursion took place between 2.3 Ga and 2.06 Ga, in agreement with previous ages from these formations. Detrital zircon populations of c. 2.06 Ga and c. 1.92 Ga characterise the lower and upper parts of the Kolasjoki Sedimentary Fm, respectively. These units are separated by an unconformity of unknown duration, but the age distributions may indicate a significant hiatus because these are considerably younger than the 2.0-1.97 Ga ages reported from the overlying Pilgujärvi Sedimentary and Volcanic fms. Alternatively, the younger cluster of ages may record deformation associated with the formation of a mylonitic thrust fault cutting the top of the Kolasjoki sedimentary units. We verified the youngest ages (1.92 Ga) from this unit by ID-TIMS analysis, which confirmed the LA ICP-MS data. Additional geochronologic work, integrated with new field observations, will be undertaken to assess the origin and significance of the 1.92 Ga population in the Kolasjoki Sedimentary Fm. Provenance areas for zircons with ages of 2.9-2.6 Ga, likely reflect derivation from the surrounding exposed Archaean basement in the Kola and Karelian Province and the Murmansk Craton. From 2.5-2.4 Ga rifting of Archaean crust and accretion of terranes to Fennoscandia started, generating layered mafic intrusions, and continued until 2.1 Ga. These intrusions as well as those surrounding terranes might also be a source for detrital zircons of such ages. The provenance area of the zircons from the Kuetsjärvi and Kolasjoki Sedimentary fms (2.1-1.92 Ga) is possibly in the southern Svecofennian domain and the western Lapland Granulite Belt.