Bulletin of the Geological Society of Finland (Dec 2012)
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.%), K2O (~3–8 wt.%) and FeOtot (8–15 wt.%) contents, low CaO and Na2O 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.
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