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Karin Appelquist, University of Gothenburg (Sweden)
Thomas Eliasson, Geological Survey of Sweden (Sweden)
Bergström Ulf, Geological Survey of Sweden (Sweden)
Åsa Pettersson, University of Gothenburg (Sweden)
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The Transscandinavian Igneous Belt (TIB) constitutes a ∼1500 km long province in the Fennoscandian Shield, East European Craton. It is a Paleoproterozoic province with coeval rocks within the Labrador Belt in Canada and consists mainly of ∼1.85-1.65 Ga granitic to gabbroic plutonic rocks and associated volcanic rocks. Two hypotheses have dominated the interpretations of its tectonic setting; one favours the idea of an active continental margin setting, whereas the other advocates interaction of crustal and mantle derived material in an extensional ensialic environment.
Ongoing mapping by the Geological Survey of Sweden has shown that the Malmbäck succession in the southwestern part of TIB comprises mafic to felsic volcanic rocks and syn-eruptive volcaniclastic deposits. These include effusive and resedimented rocks with grain sizes from mudstones to conglomerates. Conglomerates have angular to rounded polymict lithic clasts up to 40 cm and are sometimes graded. Lithofacies include massive, laminated and clast-supported rocks whereas components include crystals (porphyritic and phenocrystic), lithics and fiammes. Field observations indicate terrestrial to lacustrine or fluvial deposits - with ignimbrites and massive to highly vesicular lavas, ashes, redeposited volcanic rocks and conglomerates. Poor sorting and angular to rounded clasts suggest short transport for parts of the succession.
New U-Pb SIMS analysis of zircon from a rhyolite of the Malmbäck succession, within the Smland-Värmland batholith, yields a concordia age of 1796±7 Ma. It is interpreted as the crystallization age of the rhyolite and the age of deposition of the Malmbäck succession. The age agrees well with other ∼1.8 Ga zircon ages of Smland-Värmland volcanic rocks. The Malmbäck volcanic rocks occur as xenoliths in and are intruded by the ∼1.8 Ga surrounding TIB plutonic rocks, supporting the idea of a co-magmatic relationship between the intrusive and extrusive rocks within the Smland-Värmland batholith.
Three groups of volcanic rocks from the Malmbäck succession can be distinguished geochemically; primitive basalts to basaltic andesites, dacites-andesites and rhyolites, with a wide range of trace element concentrations. The groups constitute a calc-alkaline to shoshonitic series. These show LILE and LREE enrichment, combined with depletion in HREE and HFSE; particulary in Nb (and Ta), characteristic of typical arc rocks. La/Nb ratios range between 2 and 10, indicating a subduction-related environment. Discriminant diagrams also suggest emplacement in an active continental margin. This supports the hypothesis that the TIB was emplaced in an Andean type active continental margin setting.
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