Linking deformation, migmatite formation and zircon U-Pb geochronology in polymetamorphic orthogneisses, Sveconorwegian Province, Sweden

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 'oscillatory ghost zoned', and occurs as up to 100 mu m-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 = 0.5-1.5, U < 300 ppm) in mesosomes; 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.