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

Abstract

The study area in Vieremä, central Finland, contains part of Archaean-Palaeoproterozoic boundary. In the east, the area comprises Archaean gneiss and the Salahmi Schist Belt. The rocks of the schist belt are turbiditic metagreywackes, with well-preserved depositional structures, occurring as Proterozoic wedgeshaped blocks, and staurolite schists, the latter representing higher-strained and metamorphosed equivalents of the metagreywackes. In the west of the area there is an Archaean gneiss block, containing strongly elongated structures, and deformed Svecofennian supracrustal rocks, which are cut by deformed granitoids. These are juxtaposed with the schist belt. The boundaries of these tectonometamorphic blocks are narrow, highly strained mylonites and thrust zones. The metamorphic grade of the supracrustal rocks increases from east to west, the increase being stepwise across the mylonitic block boundaries. The rocks are more deformed from east to west with younger structures overprinting. In the staurolite schists of the Salahmi Schist Belt, the most prominent structure is a lineation (L 2) that overprints the bedding and axial plane foliation. In Sorronmäki quarry, at the western boundary of the schist belt, this Palaeoproterozoic lineation dominates all the structures in tonalite gneiss, which gives a U-Pb age of 2731 ±6 Ma. Southeast of the quarry, at the same boundary, the Salahmi schists have been overturned towards the northeast, suggesting that the Archaean gneiss at Sorronmäki has been thrust towards the northeast over these rocks. In the western part of the study area, the Leppikangas granodiorite that intrudes the Svecofennian supracrustal rocks gives a U-Pb age of 1891+6 Ma. In the granodiorite, a strong lineation formed by the intersection of two foliations, which may be L,, is associated with thrusting towards the northeast. The monazite age of the Archaean Sorronmäki gneiss is 1817+3 Ma, and the titanite age of the Svecofennian Leppikangas granodiorite, 10 km southwest of Sorronmäki, is 1824± 5 Ma. Within the limits of experimental error, these ages are identical and may be associated with a thermal event that accompanied the D, deformation phase. D 3 structures fold D 2 structures in a plastic deformation zone situated between the sample locations. Pegmatitic granite dykes intrude this NW-SE trending zone.

Timing of Palaeoproterozoic crustal shearing in the central Fennoscandian Shield according to U-Pb data from associated granitoids, Finland

“…This suggests that, as in the Salahmi area (Pietikäinen & Vaasjoki 1999), deformation within the shear zones had ceased by 1820 Ma at the latest, and no magmatic activity occurred later than about 1800 Ma ago. The fact that three of the four analysed monazites exhibit ages close to 1800 Ma suggests a fairly wide extent of the thermal pulse responsible for the Takiankangas granite.…”

“…It lies along the Raahe-Ladoga zone (Kahma 1978, Gaál & Gorbatschev 1987, Korsman 1988, and Sm-Nd data on igneous rocks (Lahtinen & Huhma 1997) indicate that it is fairly sharp in Finland. Recent data from the Salahmi schist belt (Pietikäinen & Vaasjoki 1999) indicate that major penetrative movements had ceased by 1882±4 Ma, but that crustal temperatures cooled permanently below c. 500ºC much later, about 1820 Ma ago.…”