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

摘要

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 Karra Granite and >= 1304 +/- 6 Ma for the Gota Granite. In addition, oscillatory zoned zircon grains in a pegmatite dyke in the Karra 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 Karra 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 Trollhattan in the north. Other possible members are the 1.32 Ga Ursand Granite, the 1.33 Ga Hastefjorden Granite, the 1.31 Ga Veddige Augen gneiss, the 1.33 Ga Stravalla 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 Tjarnesjo 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 Karra 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.