Middle Proterozoic anorogenic magmatism in Sweden and worldwide

Abstract

Compilation of geochronological data from southeastern Sweden indicates widespread anorogenic intrusive activity between 1.40 and 1.35 Ga ago. This activity was part of a major igneous event in a belt extending from Siberia and the Urals in the U.S.S.R. across southern Scandinavia, southern Greenland, and Labrador to western North America. It was characterized by high-level granites, sometimes rapakivilike, intruded under anorogenic conditions. The source granite melts were probably derived by the fusion of lower crust, i.e. older calc-alkaline Proterozoic granitoids, by mantle magma. These composite melts were mixed at the time of intrusion and gave rise to granitoids showing mixed I- and S-type features. The causes for the igneous activity were probably related to changes in the configuration of the continents with ensuing rifting and associated igneous activity. The 1.40-1.35 Ga old intrusions reset older isotope systems, especially the KAr one in southeastern Sweden. Between 1.25 and 1.20 Ga ago, there was a second event of smaller magnitude, characterized by the intrusion of acid and basic dykes. These dykes probably correspond to an initial stage of the Grenvillian (Sveconorwegian) orogeny soon to be followed by a 90° rotation of the Baltic shield.