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Abstract
Two orthogneisses in the southwestern part of the Fennoscandian Shield have been investigated with respect to geochemistry, age and lead isotopic composition in feldspar. Both orthogneisses host gold-bearing quartz veins in the Mj⊘sa-Vänern ore district. The alkali-calcic Brustad augen gneiss of granitic composition is situated within the Mylonite Zone (MZ), close to Mj⊘sa in southeastern Norway. The calc-alkaline Harnäs gneiss of granodioritic composition is situated 50 km west of the MZ, northwest of Vänern in southwestern Sweden. U-Pb zircon dating yielded protolith ages of 1674±10 Ma for the Brustad augen gneiss and 1595+24 −17 Ma for the Harnäs gneiss, indicating affinity to the Transscandinavian Igneous Belt (TIB) and the Gothian Åmål granitoid suite, respectively; the extension of TIB units to the northwestern margin of the MZ is thus confirmed. Both rocks were deformed during Sveconorwegian orogenesis (c. 1.15–0.9 Ga). Their feldspar lead isotopic compositions are significantly more radiogenic than expected for initial lead and no vestiges of more primary lead could be detected in pre-deformational grains. Resetting of feldspar lead is suggested to have taken place at around 1000 Ma, in conjunction with Sveconorwegian ductile deformation. A revision of the domain boundaries within the Fennoscandian Shield is proposed, such that the Torsby and Borås Segments are conceptually excluded from the Southwest Scandinavian Domain. Thereby, the eastern boundary of the Southwest Scandinavian Domain is outlined by the MZ. It is further proposed that the Southwest Scandinavian Domain be renamed as the Gothian Domain.