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

The Dala granitoids and their associated volcanic products are part of a major Palaeoproterozoic igneous complex (the 1.85-1.67 Ga Transscandinavian Igneous Belt; TIB) in the Fennoscandian Shield. TIE constitutes the south-western border of the Svecofennian Domain, which was formed by an orogenic episode at 1.93-1.83 Ga. Earlier plate tectonic models proposed that TIE is a postorogenic batholith complex, which was formed in a compressional tectonic regime representing an Andino-type environment, whereas an extensional tectonic 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 Dala granitoid complex. The Jarna granitoids are the most primitive and were emplaced 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 shallow 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 tectonic regime for the 1.79 Ga Jarna granitoids. However, most of the Dala granitoids appear to have formed in an extensional tectonic regime, which was initiated during the last phase of TIE magmatism. This extensional igneous component is represented by the 1.70-1.68 Ga high-level Siljan and Garberg granites 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 post- to anorogenic igneous key complex, which reflects the transition from a compressional to an extensional tectonic environment along a plate boundary. (C) 1999 Elsevier Science B.V. All rights reserved.