Growth-related 1.85-1.55 Ga magmatism in the Baltic Shield; a review addressing the tectonic characteristics of Svecofennian, TIB 1-related, and Gothian events

During the late Palaeoproterozoic, western Baltica was characterised by con vergent-margin tectonics that resulted in the formation of N-S trending cru stal units: the Transscandinavian Igneous Belt (TIB) and the westward young ing Gothian growth zones. Together, these occupy a 200-300 km wide belt bet ween Svecofennian (c. 1.9 Ga) crust in the east and the Skagerrak Sea/Oslo Rift in the west. Spatial and temporal constraints for 83 U-Pb dated rocks previously included in the TIB allow recognition of age groups at 1850, 181 3-1766, and 1723-1657 Ma. However, the c. 1.85 Ga rocks should preferably n ot be included in the TIB because they form a tectonically distinct crustal unit (the Asker-sund suite). The 1.81-1.77 Ga age group is the major contr ibutor to the TIB and corresponds to TIB 1. Westward younging of the TIB ma gmatism is demonstrated by the 1.72-1.66 Ga age group. Integration of the p etrogenetic and spatial aspects for these rocks suggests an older magmatic phase at c. 1.72-1.69 Ga (TIB 2) and a final phase at c. 1.68-1.66 Ga (TIB 3). Recent structural, geochemical and isotopic data provide spatial and te mporal constraints for the Svecofennian and Gothian developments, and for a bridging, c. 1.81-1.75 Ga tectonic stage we term the Smalandian. This tect onic stage includes the N-S trending, 1.81-1.77 Ga TIB 1 magmatism and mani fests a marked shift by 1.81 Ga from previous north-directed convergence du ring the Svecofennian arc-accretionary event to east-directed convergence t hat also characterised the following Gothian evolution (c. 1.75-1.55 Ga). T he early, c. 1.75-1.70 Ga Gothian evolution is still enigmatic but the subs equent development was due to westward stepping orogenesis, resulting in di stinct c. 1.69-1.65, 1.62-1.58, and 1.56-1.55 Ga igneous belts between the TIE rocks and the Oslo Rift. The prolonged period of semi-continuous crusta l growth during the Svecofennian-Smalandian-Gothian course appears to have ceased after the postulated docking of pre-1.60 Ga crust ("proto-SW Norway" ) with Gothian crustal units of Baltica at c. 1.55 Ga. At the very least, t he subsequent 1.50-1.20 Ga evolution lacks evidence for continental-margin processes east of the Oslo Rift.