Geochemistry and age of two orthogneisses in the Proterozoic Mj⊘sa-Vänern ore district, southwestern Scandinavia

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-Vanern 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 Harnas gneiss of granodioritic composition is situated 50 km west of the MZ, northwest of Vanern 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 Harnas gneiss, indicating affinity to the Transscandinavian Igneous Belt (TIB) and the Gothian Amal 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 radio...