GEOCHRONOLOGY OF QUARTZO-FELDSPATHIC AND AMPHIBOLITIC MIGMATITE GNEISSES IN THE EASTERN SEGMENT OF THE SVECONORWEGIAN OROGEN, SOUTHWEST SWEDEN

The Stensjö association crops out for approximately 7 km along the coast in Halland, southwest Sweden, within the Eastern Segment of the Sveconorwegian Orogen. It consists of intercalated sillimanite-free and sillimanite-bearing migmatitic quartzo-feldspathic gneisses, migmatitic garnet amphibolite gneiss, and garnet-poor migmatitic amphibolite gneiss with calc-silicate lenses. U-Pb-Th SIMS analysis of zircon has been carried out on zircon grains separated from the sillimanite-bearing and mafic migmatites. Zircon from all rock types record metamorphic and igneous activity during the late Hallandian thermal event at ca 1400 Ma. Migmatization coincident with F₂ folding was responsible for the last episode of zircon growth during the late Sveconorwegian orogeny at ca 970 Ma. Sugary leucosome in sillimanite gneiss occurs both as thin, centimetric layers parallel to the gneissic S_0-1 foliation, accumulated in F₂ fold hinges, and cross-cutting F₂ folds. Zircon from sillimanite gneiss with thin S₁-parallel leucosome has rounded, oscillatory zoned cores 1.7 Ga and rims too thin to date. Zircon from 5-10 cm granitic leucosome, interpreted as synchrononous with F₂ folding, contains similar 1.7 Ga cores surrounded by a reworked zone 1.4 Ga and an outer euhedral overgrowth 0.97 Ga. The 1.7 Ga zircon cores are interpreted as detrital grains derived from a source within the Transscandinavian Igneous Belt, whereas the 1.4 Ga zones reflect Hallandian metamorphism and the 0.97 Ga euhedral rims formed during Sveconorwegian F₂ folding and synchronous migmatization.

Zircon from a pegmatite which cross-cuts the compositional layering and which has been stretched and recrystallized during L₁ and folded by F₃ includes two generations 1.4 and 0.97 Ga, again demonstrating penetrative deformation and melt conditions at 0.97 Ga. Zircon from the mafic migmatites has cores 1.4 Ga and euhedral rims 0.97 Ga. The rims are markedly thicker in the leucosomes than in the amphibolitic host implying that the 0.97 Ga rims formed during migmatization.

Late undeformed and cross-cutting granitic pegmatite dykes mark the end of ductile penetrative deformation in the Eastern Segment at 0.95 Ga (Christoffel et al. 1999. Prec. Res. 98, 173–195; Möller et al. 2007. J. met. Geol. 25, 727–750).