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535326 
Journal Article 
Regional 1.92 Ga tectonothermal episode in Ostrobothnia, Finland: Implications for models of Svecofennian accretion 
Williams, IS; Rutland, RWR; Kousa, J 
2008 
1 
Precambrian Research
ISSN: 0301-9268 
165 
1-2 
15-36 
English 
The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijarvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at similar to 1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92-1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between similar to 1.99 and similar to 1.95 Ga. Three samples of Bothnian sediments lack pervasive similar to 1.91 Ga overgrowths, instead having a variety of detrital zircons as young as similar to 1.95-1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be similar to 1.91 Ga. The Niska granitoid, which intrudes the Evijarvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 +/- 6 Ma. That episode, which established the present relationships between basement and cover, is dated by similar to 1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between similar to 1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (similar to 1.92-1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited. (C) 2008 Elsevier B.V. All rights reserved. 
Finland; Ostrobothnia; Svecofennian; zircon U-Pb ages; metasediments; northern sweden; baltic shield; precambrian evolution; skellefte; district; zircon ages; u-pb; deformation; nd