Zircon geochronology in polymetamorphic gneisses in the Sveconorwegian orogen, SW Sweden: ion microprobe evidence for 1.46–1.42 and 0.98–0.96 Ga reworking

Abstract

Ion microprobe U–Th–Pb analyses of zircons in variably metamorphosed and veined orthogneisses in the southern part of the parautochthonous Eastern Segment of the Sveconorwegian (1.20–0.90 Ga) orogen, SW Sweden, broadly define two age groups, oscillatory and sector zoned magmatic zircon cores yield 1.70–1.68 Ga while overgrowths, homogeneous crystals, and recrystallized domains in primary zircon yield 1.46–1.42 Ga. In addition, a late-kinematic pegmatite was dated at 0.96 Ga, while a penetratively deformed granite dyke contains both 1444±8 Ma magmatic and 982±15 Ma metamorphic zircons. The 1.70–1.68 Ga ages date the orthogneiss protoliths and fall in the same age range as well-preserved rocks of the Transscandinavian Igneous Belt that forms a major part of the crust east of the Sveconorwegian orogen. Despite Sveconorwegian penetrative deformation under granulite to upper amphibolite conditions, secondary zircons yielding Sveconorwegian ages are virtually absent in the 1.70–1.68 Ga orthogneisses but are abundant in rocks younger than ca. 1.45 Ga. It is suggested that Zr hosted in magmatic phases was redistributed to form new zircon during the 1.46–1.42 Ga event, resulting in a mineralogy in which the main minerals were depleted in Zr. These data, therefore, imply that high-grade metamorphism may occur without associated growth of new zircon. Furthermore, the absence of secondary zircons with ages >ca. 1.46 Ga suggests a re-assessment of models calling for extensive Gothian deformation and metamorphism in the Eastern Segment.