The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy

Because of the relatively low metamorphic grade, low strain and well-preserved early structures in volcanic and volcaniclastic rocks, the Orijärvi triangle (in the center of the larger Orijärvi area) is one of the few locations in the Svecofennian orogen of southern Finland where lithostratigraphy has been determined. The geochemistry of the picritic- basaltic metavolcanic rocks of the Salittu Formation, located in the northeastern part of the Orijärvi area, has been characterized but otherwise the bedrock and structures have been barely touched. After remapping we give an interpretation of structural evolution, provide new age data to constrain duration of volcanism at Salittu, and refine the stratigraphy in the Orijärvi area. The original stratigraphy is visible at Salittu: metabasalt overlies migmatitic gneisses, and metapicrite is on top. The rocks were folded during early Svecofennian D 1 –D 2 deformations, and the large synformal structures developed as D 2 –D 5 interference structures formed during late Svecofennian D 5 deformation. The structural pattern at Salittu is much the same as in the Orijärvi triangle. The new age data, combined with earlier published data, constrains the Salittu volcanism at ca. 1875 Ma. The stratigraphy in the Orijärvi area consists of the early (1.90–1.89 Ga) volcanic Orijärvi Formation, overlain by the sedimentary Vetio Formation, the volcanic Kisko Formation, the volcanic-sedimentary Ahdisto Formation, the volcanic Toija Formation, and on top the Salittu Formation, all emplaced at 1.88–1.87 Ga. We propose a model in which the Orijärvi Formation represents magmatism at the margin of a microcontinent, and the overlying package represents sedimentation and magmatism above a subduction zone during an initial stage of microcontinental accretion. D 1 deformation occurred in an advanced stage of accretion, after emplacement of the volcanic rocks of the Salittu Formation.