Palaeoproterozoic mafic and intermediate metavolcanic rocks in the Turku area, SW Finland

Abstract

Twenty two samples were analysed from mafic and intermediate volcanic rocks from the Turku area along ~60 km long north-south geotraverse. Geographically and lithologically the samples were divided into the Pargas, Turku and Vahto groups. The Pargas and Turku groups consist of mafic units and intercalations within migmatitic gneisses. Their geochemical compositions are very similar. In average, they show quite flat REE curves and the multielement diagrams show moderate LILE enrichments, minor negative Ta-Nb anomalies and straight HFSE curves. We infer these as transitional between MORB and VAB. The Vahto group consists of thicker volcanic sequences in connection with synorogenic plutonic rocks. The data show enriched LREEs, LILEs, Th and P combined with negative Ta-Nb anomalies, i.e. the Vahto group shows subduction component. We interpret that the Vahto volcanic rocks belong to the Häme belt. The geochemical data indicate that the Pargas and Turku groups were formed in extensional, possibly back-arc or intra-arc setting while the Vahto group was formed at continental volcanic arc.

Keywords

• metavolcanic rocks
• geochemistry
• trace elements
• tectonics
• back-arc basins
• volcanic arcs
• Paleoproterozoic
• Svecofennian
• Parainen
• Turku
• Vahto
• Finland