The Paleoproterozoic Nattanen-type granites in northern Finland and vicinity: A postcollisional oxidized A-type suite

The ~1.8 Ga Nattanen-type granites in the Finnish Lapland and western Kola Peninsula are found as several relatively small, high-level, discordant plutons that are easily discernible as weak maxima on aeromagnetic maps. We present U–Pb mineral isotope data on the Finn-ish plutons. The concordia ages are in the 1.79−1.77 Ga range and there is little evidence for inheritance. Initial radiogenic isotope compositions (our common-Pb data combined with previously published whole-rock Nd and Hf data) imply a major, yet varying, Archean source component. Elemental geochemical data on five Finnish intrusions (the Nattanen stock, the Tepasto and Pomovaara complexes, the Riestovaara and Vainospää batholiths), as well as, associated dyke rocks allow the Nattanen-type granites to be classified as oxidized A-type granites. Their petrogenesis may be related to partial melting of the lower crust by mafic underplating (extensional setting) or as a result of thermal relaxation in thickened crustal setting.