Rapakivi-related basement structures in the Baltic Sea area; a regional approach

Abstract

The largest massifs of rapakivi granites and related rocks in the western part of the East European Craton are located within the junction area of the Baltic Sea and the Gulfs of Bothnia, Finland and Riga, and along the Gulf of Finland. The rapakivi massifs are of Palaeoproterozoic to Mesoproterozoic (Subjotnian) age, ranging from about 1.67 to 1.5 Ga. In the framework of the Palaeoproterozoic basement tectonics, all major and minor occurrences of rapakivi magmatism are areally limited to within the boundaries of the Svecofennian juvenile crustal domain and its frontiers. The most voluminous of the intrusive bodies are located within the originally thickest (55–65 km) primary part of Svecofennia, where they are coupled with crustal thinning to 40–45 km and mafic underplating anomalies. Only minor rapakivi bodies have intruded the originally thinner (40–45 km) peripheral Palaeoproterozoic crust. The largest igneous subprovinces, namely the Wiborg and Riga-Åland rapakivi massifs, are furthermore connected to specific regional Bouguer gravity anomalies. These anomalies exhibit complex patterns due to the rapakivi-related reworking of the upper mantle and the crust. The bimodal rapakivi and related upper crustal magmatism was systemised in such a way that satellite bodies with minor stock and dike suites became located concentrically around the main igneous bodies. Considering the different ages and locations of the igneous suites, an internal subprovincial distribution occurs in the province. The time of formation for each subprovincial cluster is no longer than 50 m. y. The timing of birth and extinction of the different subprovinces rarely coincide with each other.