Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Goteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major N-S trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolcanic rock (the Mjosjo dacite) yields an age of 1643 +/- 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 + 15 Ma. Together with a recent age for a volcanic rock from the Amal region farther north (1.61 Ga, Lundqvist and Skiold, 1992), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the name Amal-Horred Belt is proposed. East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of similar to 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in the Amal-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish the Amal-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as the Ostfold-Marstrand Belt (similar to 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt (similar to 1.81-1.65 Ga; Larson and Berglund, 1992). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The present concept of the ''Gothian orogeny'' must be revised since at least two different orogenic episodes at similar to 1.7 and 1.6 Ga can now be distinguished.
