Sveconorwegian orogenesis in the southwestern Baltic Shield - Zircon geochronology and tectonothermal setting of orthogneisses in SW Sweden
Abstract: The Mylonite zone (MZ) is a prominent late-Sveconorwegian lithological terrane boundary in the SW Baltic Shield along which large-scale crustal block movements took place during the Sveconorwegian orogeny. The southern section of the MZ also defines a conspicuous metamorphic break separating upper amphibolite and high-pressure granulite facies rocks in the parautochthonous Eastern Segment (ES), from middle amphibolite facies rocks in the overlying Western Segment (WS). Zircon geochronology and structural and metamorphic data for orthogneisses across the southern section of the MZ do not support previous interpretations for the MZ to have originated as a pre-Sveconorwegian (>=1.55 Ga) amalgamation zone of Gothian terranes. East of the MZ, in the parautochthonous ES, the oldest orthogneisses are dated at 1.70-1.68 Ga supporting the concept that these rocks are reworked equivalents to the youngest rocks of the 1.85-1.66 Ga Transscandinavian Igneous Belt of the Sveconorwegian foreland. West of the MZ, in the allochthonous southern WS, the oldest orthogneisses are dated at 1.59 Ga, which confirms that these rocks represent an igneous event, not recognised east of the MZ in the Sveconorwegian parautochthon. Datings of augen gneisses in the southern WS at 1.33-1.31 Ga also contradict previous “stitching granite” correlation with 1.40-1.38 Ga granite-monzonite magmatism in the underlying ES, across the MZ. Investigation of field relations, deformational fabrics and metamorphic character, combined with zircon and titanite chronology of orthogneisses in the southern ES and the WS demonstrate that these rocks underwent regional penetrative ductile deformation, high-grade metamorphism and anatexis during the Sveconorwegian orogeny. The high-grade metamorphism and anatexis is dated at 0.99-0.96 Ga. Ductile deformation internal to the southern ES is dated at 0.96 Ga, and a lower age limit for this deformation is set by post-tectonic pegmatite-granite dyke intrusions dated at 0.95 Ga. In the southern ES, regional, pre-Sveconorwegian high-grade metamorphism, anatexis, and felsic vein injections are dated at 1.46-1.42 Ga. The tectonothermal character of this early event is unclear due to penetrative late Sveconorwegian tectonic transposition and metamorphic recrystallisation in the upper amphibolite to high-pressure granulite facies, followed by amphibolite facies retrogression. Evidence for high-grade metamorphism and anatexis prior to 1.46 Ga is lacking. A difference in timing and character of Sveconorwegian metamorphism and deformation within and between the ES and the WS indicates a complex build-up of lithological and/or metamorphic terranes. Models of the Paleo- and Mesoproterozoic geological evolution of this part of the Baltic continent must therefore be based on an understanding of the large-scale Sveconorwegian crustal block movements taking place prior to final juxtaposition in late Sveconorwegian time.
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