Geology, geochemistry, age and geotectonic evolution of the Dala granitoids, central Sweden

The Dala granitoids and their associated volcanic products are part of a ma jor Palaeoproterozoic igneous complex (the 1.85-1.67 Ga Transscandinavian I gneous Belt; TIB) in the Fennoscandian Shield. TIE constitutes the south-we stern border of the Svecofennian Domain, which was formed by an orogenic ep isode at 1.93-1.83 Ga. Earlier plate tectonic models proposed that TIE is a postorogenic batholith complex, which was formed in a compressional tecton ic regime representing an Andino-type environment, whereas an extensional t ectonic regime has been proposed for the anorogenic (1.65-1.51 Ga) rapakivi granites in the Svecofennian Domain. In this paper, a key segment of TIE ( the Dala granitoids) is discussed in order to focus attention on the post- to anorogenic magmatic evolution in the Svecofennian Domain. Three types of granitoids (Jarna, Siljan and Garberg) can be distinguished within the Dal a granitoid complex. The Jarna granitoids are the most primitive and were e mplaced at a significant depth in the crust at ca 1.79 Ga. The 1.70-1.68 Ga Siljan and Garberg granites are more evolved and were emplaced at a shallo w level in the crust, closely associated in space, time and origin with the volcanic Dala porphyries. Field relationships, as well as geochemical and geochronological data, indicate magma genesis within a compressional tecton ic regime for the 1.79 Ga Jarna granitoids. However, most of the Dala grani toids appear to have formed in an extensional tectonic regime, which was in itiated during the last phase of TIE magmatism. This extensional igneous co mponent is represented by the 1.70-1.68 Ga high-level Siljan and Garberg gr anites and their closely associated volcanic products (the Dala porphyries) . An improved model for the formation of the post- to anorogenic magmatism in the Svecofennian Domain suggests that the Dala granitoids represent a po st- to anorogenic igneous key complex, which reflects the transition from a compressional to an extensional tectonic environment along a plate boundar y. (C) 1999 Elsevier Science B.V. All rights reserved.