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

Abstract

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 Finnish 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.

Geochronology of the Palaeoproterozoic Kautokeino Greenstone Belt, Finnmark, Norway: Tectonic implications in a Fennoscandia context

“…The geotectonic significance of this younger metamorphism between Fig. 8) (Heilimo et al, 2009;Lauri et al, 2012a;Ranta et al, 2015).…”

New high-resolution aeromagnetic and radiometric surveys in Finnmark and North Troms: linking anomaly patterns to bedrock geology and structure

“…This is partly where granitic and pegmatite rocks are exposed; and on the magnetic anomaly map this is shown as positive magnetic anomalies. This zone with combined whitish colours and distinctive magnetic signature is more extensive than the small area of the granitic rocks indicated on the geological maps, and signifies that the granite of the Hetta complex (Heilimo et al, 2009) in nearby Finland extends some 60 km farther north beneath the thin cover of the glacial deposits.…”

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

“…Allanite crystals are partly or fully replaced and overgrown by secondary minerals, mainly REE carbonate phases (bastnäsite and synchysite), chlorite, albite, quartz, and more rarely by xenotime, titanite and apatite (Figure 11a-d Transformation of allanite to secondary REE carbonate minerals (mainly bastnäsite and synchysite) has been described in numerous cases in various magmatic and metamorphic rocks e.g., [45][46][47][48][49]. However, such alteration of allanite is usually associated with clay minerals (kaolinite, montmorillonite), and also with thorite, fluorite, and magnetite without carbonate minerals in some cases [45,50]. In contrast, allanite in the samples from the Tana Belt was replaced mainly by bastnäsite Figure 11.…”

“…Transformation of allanite to secondary REE carbonate minerals (mainly bastnäsite and synchysite) has been described in numerous cases in various magmatic and metamorphic rocks e.g., [45][46][47][48][49]. However, such alteration of allanite is usually associated with clay minerals (kaolinite, montmorillonite), and also with thorite, fluorite, and magnetite without carbonate minerals in some cases [45,50].…”

“…However, such alteration of allanite is usually associated with clay minerals (kaolinite, montmorillonite), and also with thorite, fluorite, and magnetite without carbonate minerals in some cases [45,50]. In contrast, allanite in the samples from the Tana Belt was replaced mainly by bastnäsite and synchysite, and rarely also by ferriallanite and chlorite.…”