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Emplacement ages of the mid-Proterozoic Kungsbacka Bimodal Suite, SW Sweden

2007; Taylor & Francis; Volume: 129; Issue: 3 Linguagem: Inglês

10.1080/11035890701293227

2000-0863

Eric Austin Hegardt, David H. Cornell, Fredrik Hellstrøm, Inger Lundqvist,

Geochemistry and Geologic Mapping

Abstract Ion-microprobe U-Pb geochronological data of zircon grains from three Mesoproterozoic deformed granite plutons in the Western and Median Segments of the Sveconorwegian Province, SW Sweden give intrusion ages of 1336±10 Ma for the Askim Granite, 1311±8 Ma for the Kärra Granite and ≥1304±6 Ma for the Göta Granite. In addition, oscillatory zoned zircon grains in a pegmatite dyke in the Kärra Granite were dated at 1325±8 Ma, i.e. identical within analytical uncertainty with the age of the host granite. The zircon grains from the pegmatite are interpreted as xenocrysts from the Kärra Granite. The plutons are part of a 1.34-1.30 Ga bimodal magmatic suite which we call the Kungsbacka Bimodal Suite, forming a N-S trending linear belt from Kungsbacka in the south to Trollhättan in the north. Other possible members are the 1.32 Ga Ursand Granite, the 1.33 Ga Hästefjorden Granite, the 1.31 Ga Veddige Augen gneiss, the 1.33 Ga Stråvalla Augen gneiss and the 1.33 Ga Chalmers Mafic Intrusion. The suite probably formed in a continental rift environment. This study demonstrates that the Kungsbacka Bimodal Suite, which is restricted to the Western and Median Segments, is younger than the 1.38 Ga Tjärnesjö and Torpa Granites in the Eastern Segment of the Sveconorwegian Province. This difference indicates that the Mylonite Zone may be interpreted as a first order Sveconorwegian tectonic boundary, separating crustal segments with distinct pre-Sveconorwegian histories. Metamorphic overgrowths on zircons from the Kärra pegmatite are dated at 1043±11 Ma and confirm the age of Sveconorwegian metamorphism at 1.04 Ga in the Western Segment. This study also documents the occurrence of post-1.3 Ga penetrative deformation and high-grade metamorphism including partial melting in the Western and Median Segments. It also proves that in some areas it is impossible to distinguish post-Gothian from older units, based only on their degree of deformation.