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

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 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 probabl...