Early Proterozoic stratigraphy of the Salahmi-Pyhäntä area, central Finland, with an emphasis on applying the principles of lithodemic stratigraphy to a complexly deformed and metamorphosed bedrock

Abstract

1988: Early Proterozoic stratigraphy of the Salahmi-Pyhäntä area, central Finland, with an emphasis on applying the principles of lithodemic stratigraphy to a complexly deformed and metamorphosed bedrock. Bull. Geo!. Soc. Finland 60, Part 2, 79-106.The medium-highly metamorphozed and intensely deformed early Proterozoic supracrustal rocks of the study area, located at the boundary zone between the Karelian and Svecofennian zones in central Finland, are classified following the recommendations of the North American Stratigraphic Code into 16 lithostratigraphic and lithodemic units.The lithostratigraphic procedures can be applied only to the southeastern corner of the area where the late Archaean basement complex is rimmed by less than 100 metres thin fluvial metapsammite formations locally underlain by a thin palaeosol of sericite and chloritoid schists. These formations, which are correlated with the Kainuu tectofacies, are overlain by at least 1 km thick sequence of turbiditic metapsephites and metapsammites of the Haajainen and Rotimojoki formations of the western Kaleva tectofacies.The major central part of the area is cored by two gneiss complexes, earlier considered to be the Archaean basement, but now classified mostly as lithodemic derivatives of the Kaleva tectofacies. These complexes are mantled by gneisses and schists which represent western, more metamorphosed distal equivalents of the Haajainen and Rotimojoki formations. The western margin of the area is composed of gneisses and schists part of which show Svecofennian affinities.Excluding the Archaean basement, the area is intruded and migmatized by Svecokarelidic intrusive rocks.

Structural observations and U-Pb mineral ages from igneous rocks at the Archaean-Palaeoproterozoic boundary in the Salahmi Schist Belt, central Finland: constraints on tectonic evolution

“…Within the Raahe-Ladoga zone, the relationship between the Karelian platform intracratonic to craton margin metasediments and the metaturbidites of the Svecofennian Orogen is of particular interest (Laajoki & Luukas 1988). Because granulite facies metamorphism associated with a migmatizing event in the southeastern part of the Raahe-Ladoga zone has mostly erased original features, it is better to restrict such studies to the central and northwestern parts of the Archaean-Palaeoproterozoic boundary zone.…”

“…In this respect, the Salahmi Schist Belt, consisting of tectonically deformed metasedimentary rocks unconformably overlying Archaean basement, is one area in central Finland in which original features are well-preserved (Savolahti 1965, Korkiakoski & Laajoki 1988. According to Laajoki and Luukas (1988), all Karelian units are present, being overlain by Svecofennian metasediments. Mafic-felsic igneous rocks, which are presumed to be of Palaeoproterozoic age, intrude all the metasedimentary rocks.…”