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

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, th e remaining crustal thickness is 50-65 km. In the marginal parts of the 1,0 00,000 km(2) Svecofennian Domain, the Moho depth reached 40-75 km, which ex ceeds 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 rapakiv i magmatism of the Fennoscandian Province was limited to the Svecofennian D omain. The rapakivi igneous structures are confined to areally isolated sub provinces, which each have distinct 20-60 Ma long age spans of formation. T he petrologic sequences of the subprovinces are alike, although similar pet rological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous po lyphase 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 starred in juvenile crust which was in a late stage of e rosional thinning, 150-300 Ma after the period of maximal thickening. The m ost extensive rapakivi igneous activities were associated with crustal thin ning down to 45-50 km. As a result, the thinner marginal parts of Svecofenn ia and the large interior rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained o nly in areas void of rapakivi magmatism. No major events destroyed the Svec ofennian and rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive rapakivi igneous activity sub stantially thinned and stabilised the overthickened portions of the Svecofe nnian crust. (C) 1999 Elsevier Science B.V. All rights reserved.