WESTWARD ACCRETION OF THE BALTIC SHIELD - IMPLICATIONS FROM THE 1.6-GA AMAL HORRED BELT, SW SWEDEN

Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Goteborg, SW Sweden. This is a region of th e Southwest Scandinavian Domain, within which a major N-S trending she ar zone and tectonic boundary, the Mylonite Zone, juxtaposes comparati vely weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolc anic 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 age s 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. Ea st 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 i ndicate that more deformed and more strongly metamorphosed equivalents of the rocks in the Amal-Horred Belt may occur also to the east of th e Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish the Amal-Horred Belt a s a major geological unit younger than most other crustal components i n 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; Larso n and Berglund, 1992). The configuration of the crustal units in SW Sw eden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The pre sent 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.