U-Pb ages for PGR dykes, KFP, and adjacent older leucosomic PGRs from ONKALO underground research facility, Olkiluoto, Eurajoki, SW Finland

Zircon LA-MC-ICP-MS and monazite TIMS ages have been determined for three PGR dykes (A2018, A2020, and A2070) and a KFP (A2022) from the ONKALO underground research facility, Olkiluoto, Eurajoki, SW Finland. For purposes of comparison, leucosomic PGRs (A2019, A2021, and A2023) crosscut by the PGR dykes and the KFP were also dated. Minimum ages for the PGR dykes and the A2022 KFP were determined by the monazite U-Pb ages. PGR dykes reveal partially overlapping ages of 1826 ± 7 Ma (A2018), 1811 ± 5 Ma (A2020), and 1817 ± 3 Ma (A2070). Monazite age of 1808 ± 6 Ma for the KFP is coeval with the youngest PGR dyke monazite. The ∼1.81-1.80 Ga ages for the supposed PGR dyke zircon agree within the error limits with the monazite ages. For the KFP, obvious co-magmatic zircon was not identified. The metamorphic zircon rims and domains found from the PGR dykes reveal multiphase overprinting, the ages ranging from 1.86 Ga to 1.80 Ga. Subsequently, the high-U PGR dyke zircon suffered major lead loss during the Rapakivi event at 1.58 Ga and finally, a few grains show early Devonian age of ∼400 Ma. In the KFP, many structurally homogeneous zircon domains and rims plot on a ∼1.80 Ga discordia line, while the concordant data are ∼1.83 Ga. Leucosomic PGRs A2019 and A2021 crosscut by the PGR dykes enclose 1.87-1.85 Ga zircon resembling and being contemporaneous with the tonalite zircon in the Olkiluoto area. The concordia ages of 1836 ± 13 Ma (A2019) and 1807 ± 11 Ma (A2021) for the metamorphic zircon domains in the leucosomes correlate perfectly with the supposed ages of the crosscutting PGR dykes. The youngest metamorphic zircon phases in leucosome A2023 are dated at 1.82-1.80 Ga. That is in good accordance with the monazite age 1808 ± 6 Ma from the KFP A2022 crosscutting the leucosome A2023. The samples contain also abundant older inherited zircon cores and grains. Archaean ages vary from 3.1 Ga to 2.7 Ga and the older Palaeoproterozoic between 2.1 Ga and 1.9 Ga. The older ∼1.83-1.82 Ga and the younger ∼1.81-1.80 Ga ages from PGR dykes, KFP, and leucosome PGRs plausibly date the D3 and D4 shearing events, respectively. (orig.)