SVECONORWEGIAN (-GRENVILLIAN) OROGENESIS IN THE SW BALTIC SHIELD–TIMING AND TECTONIC FRAMEWORK IN LIGHT OF COMPLEX ZIRCON GEOCHRONOLOGY

The Sveconorwegian orogen in SW Baltica (the tectonic counterpart to the Grenville orogen of North America) forms an about 500 km wide mobile belt in S Norway and SW Sweden. It is composed of several Proterozoic gneiss segments metamorphosed in amphibolite to high-pressure granulite facies. Refined dating techniques involving high-spatial resolution ion microprobe in situ analyses of complex zircons allow direct dating of igneous protoliths, high-grade metamorphic recrystallisation and partial melting of high-grade gneisses in the region. These data form the basis for a revised model for the 1.2-0.9 Ga Sveconorwegian orogeny that involves extensive crustal block reconfiguration in the SW Baltic Shield. The new chronological data, field relations and petrography, demonstrate that: 1) Sveconorwegian (-Grenvillian) reworking involved gneissification and partial melting of major parts of SW Baltica, and; 2) the orogen is composed of several distinct lithological and metamorphic terranes that differ in terms of timing and character of tectonic events. Direct datings of high-grade metamorphism in different lithotectonic units indicates that a sequence of compressional events took place over a time period of at least 150 M.Y. Early compression and partial melting is dated at 1.13 Ga in the Bamble segment in the central part of the orogen and was preceded by subduction of the continental margin at around 1.20 Ga (Knudsen and Andersen, 1999, J. of Petrology 40). Regional deformation and partial melting occurred in both the western and eastern part of the orogen between 1.03-0.97 Ga, and may be considered as the main Sveconorwegian phase. The latest event is dated at 0.97 Ga and involves, in addition to amphibolite metamorphism and partial melting, high-pressure granulite and eclogite facies metamorphism in the parautochthonous basement in the easternmost part of the orogen. The differences in timing of metamorphism, uplift and cooling in the different lithotectonic units, together with differences in the age and character of protoliths, suggest large-scale displacements along roughly N-S striking prominent late-Sveconorwegian deformation zones prior to final tectonic adjustments at about 0.92 Ga.