Linking deformation, migmatite formation and zircon U–Pb geochronology in polymetamorphic orthogneisses, Sveconorwegian Province, Sweden C. MO ¨ LLER, 1 J. ANDERSSON, 2 I. LUNDQVIST 3 AND F. HELLSTRO ¨ M 2 1 Geological Survey of Sweden, Kiliansgatan 10, SE-22350 Lund, Sweden (charlotte.moller@sgu.se) 2 Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden 3 Geological Survey of Sweden, Geovetarcentrum, Guldhedsgatan 5A, SE-413 20 Go ¨ teborg, Sweden ABSTRACT Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron-bright, U-rich (800–4400 ppm) with low Th/U-ratios (generally 0.01–0.1), unzoned or Ôoscillatory ghost zonedÕ, and occurs as up to 100 lm-thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U ¼ 0.5–1.5, U < 300 ppm) in meso- somes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south- vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre-Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW-plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite– pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high-pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent– continent collision. Key words: dating deformation; dating migmatization; Mesoproterozoic orogenesis; Sveconorwegian orogenesis; zircon geochronology. INTRODUCTION High-grade metamorphic terranes are commonly characterized by a multiphase metamorphic and tec- tonic history. Metamorphic and deformation phases are not homogeneously distributed throughout these terranes but generally partitioned into specific lithol- ogies, specific areas or specific shear zones. Successful dating of individual phases of deformation or meta- morphism requires not only appropriate samples from localities where these phases are well expressed but also reliable linking between the geochronological data from specific minerals and the geological, petrological and structural data. U–Pb dating of zircon is com- monly the method of choice for high-grade terranes, but linking individual zircon generations to large-scale processes may prove difficult. In this work, we address the tectonometamorphic evolution of the Eastern Segment of the Sveconorwe- gian Province in south Scandinavia, a tectonic coun- terpart to the Grenville Province of North America. Available data indicate the existence of two high-grade metamorphic events in the Eastern Segment, one pre-Sveconorwegian at c. 1.43 Ga and one Sveconor- wegian at c. 0.97 Ga. The relative importance and geochronology of tectonometamorphic events in the segment are controversial and have been discussed at length in scientific publications. The primary aim of J. metamorphic Geol., 2007, 25, 727–750 doi:10.1111/j.1525-1314.2007.00726.x Ó 2007 Blackwell Publishing Ltd 727