Rapakivi-granite–anorthosite magmatism — a way of thinning and stabilisation of the Svecofennian crust, Baltic Sea Basin

Abstract In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of rapakivi magmatism. No major events destroyed the Svecofennian and rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Baltic Sea area; Moho topography; Svecofennian orogen; rapakivi; anorogenic magmatism; Proterozoic