The Petäjäskoski Formation, a new lithostratigraphic unit in the Paleoproterozoic Peräpohja Belt, northern Finland

Abstract

This paper gives the first description of a newly-recognized, basin-wide metasedimentary unit in the Paleoproterozoic (~2.4-1.9 Ga) Peräpohja Belt, northern Finland. The unit, which is named the Petäjäskoski Formation (PFm) after the single location where the rocks are known to be exposed, is situated stratigraphically in the middle part of the Kivalo Group between the quartzites of the >2.22 Ga Palokivalo Formation and the mafic volcanic rocks of the ~2.1 Ga Jouttiaapa Formation.The bulk of the PFm comprises phlogopitic-sericitic and albitic schists with abundant hematite as a diagnostic feature. Quartzite and dolomite interbeds are common. Based on drillcore and geophysical data, the succession is several hundreds of meters thick. The unit has prograde, chiefly lower greenschist facies mineral assemblages and, though being commonly intensely deformed, shows well-preserved sedimentary structures that imply deposition in shallow-water to subaerial environments. Based on the original lithological features, the Petäjäskoski Formation can be defined as a claystone-siltstone-sandstone-dolostone association. On the geochemical and stratigraphic basis, the albite schists likely represent albitized equivalents of the micaceous claystones and siltstones. They are intercalated with stratabound collapse breccias, up to tens of meters in thickness, with clasts composed mainly of bordering albite schist.A mafic sill intruding the Petäjäskoski Formation yielded a U-Pb zircon age of 2140 ± 11 Ma. The older, c. 2220 Ma, differentiated sills are not known to reach the stratigraphic level of the Petäjäskoski Formation and hence, the depositional age of the PFm sediments can be bracketed between c. 2220 and 2140 Ma. Detrital zircon grains dated from a quartzitic sample from the PFm show an Archean (c. 2650-3470 Ma) provenance.The moderate to high MgO (~6-13 wt.%), K 2 O (~3-8 wt.%) and FeO tot (8-15 wt.%) contents, low CaO and Na 2 O contents, and abundant phlogopite and hematite make the mica schists unique among the metasedimentary rocks of the Finnish Karelian supracrustal belts. These features together with the abundance of dravititic tourmaline, the presence of pseudomorphic nodules potentially after gypsum and anhydrite, deposition under shallow-water conditions, and albitization and brecciation of the sedimentary strata suggest an evaporitic depositional environment.The Petäjäskoski Formation, a new lithostratigraphic unit in the Paleoproterozoic Peräpohja Belt, northern Finland

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

“…Finally, it is worth mentioning that the Peräpohja Schist Belt in Finland (Fig. 1) includes a mafic sill, underlying the Jouttiaapa volcanic rocks (Huhma et al, 1990), dated at 2140 ± 11 Ma (zircon U-Pb data; Kyläkoski et al, 2012). Its occurrence suggests that more rocks of the same magmatic province probably exist over Fennoscandia, awaiting further documentation.…”

Epigenetic gold occurrence in a Paleoproterozoic meta-evaporitic sequence in the Rompas-Rajapalot Au system, Peräpohja belt, northern Finland

“…2.4-2.06 Ga. In addition, the Kivalo Group also contains evaporitic rocks of the Petäjäskoski Formation (Kyläkoski et al, 2012) and coarse glacimarine sedimentary rocks (Vanhanen et al, 2015). The upper part of the PB belongs to the Paakkola Group consisting of pillow lava, mafic and felsic tuff, and turbiditic-pelitic rocks with carbonaceous material-rich sulfidic intercalations.…”

The Rompas Prospect, Peräpohja Schist Belt, Northern Finland

“…We believe it is likely that the bulk of the mapped lowermost PSB was deposited on a relatively low gradient surface, either emergent (regionally extensive quartz sandstones) or shallow water (stromatolitic carbonate rocks). Also within the sequence are inferred evaporites of the Petäjäskoski formation (Kyläkoski et al, 2012b). Toward the end of the history of the PSB, however, traction current related sediments, reflecting basinal development, dominated the sequence (Korkiavaara, Pöyliövaara, and Martimo formations).…”

“…We interpret the Rompas prospect to lie in rocks deposited near the top of the middle basin stage, within the platform carbonate and quartzite rocks above the Jouttiaapa basalts, and in close contact with the overlying late basin stage rocks (metamorphosed turbiditic clastic sediments, carbonates, carbon-rich sediments, and mafic rocks). The stratigraphic column shows formation names for the PSB, largely based on Perttunen and Hanski (2003) and Kyläkoski et al (2012b). The Rompas trend is shown within the black ellipse.…”

“…The only active mine is the Kemi chromium mine, and that is within basal-layered mafic and ultramafic rocks (Halkoaho and Iljina, 2012). Kyläkoski et al (2012a) describe quartz-carbonate vein-hosted and dolerite-hosted Cu-Au occurrences in addition to the Vähäjoki deposit ascribed to the iron oxide-copper-gold (IOCG) grouping (see also Eilu et al, 2007). In the northeastern Peräpohja schist belt, small tremolite and serpentine skarn magnetite occurrences are potentially of the IOCG class (Niiranen et al, 2003;Niiranen, 2012, and references therein).…”