Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield

摘要: The southern section of the Mylonite Zone (MZ) is a major lithotectonic terrane boundary in SW Baltic Shield. It separates parautochthonous Eastern Segment Sveconorwegian orogen from allochthonous Western Segment. Complex zircons migmatised and banded orthogneisses along MZ were investigated by ion microprobe analyses guided backscattered electron imaging to directly date partial melting associated penetrative ductile deformation this zone. In eastern part (Eastern Segment), resorbed igneous zircon cores stromatic orthogneiss are 1686 +/- 11 Ma old, whereas extensive overgrowths abundant newly formed simple 969 13 old. Migmatised K-feldspar megacrystic granite has 1359 26 protolith 968 grains. Both rock units contain amphibolitised mafic pods with remnants garnet-clinopyroxene-bearing, high-pressure granulite facies parageneses. western (Western orthogneiss, dated at 1585 new growth 980 Ma. Enclosed rocks have middle amphibolite Secondary all three samples occurs as distinct, sub-idiomorphic or crystals low Th/U ratios ( < 0.07). morphology high modal abundance secondary (25-50% total volume zircon), absence early- pre-Sveconorwegian zircon, field relations provide evidence for that anatexis took place between These late-Sveconorwegian ages contradict previous interpretations pre-1.55 Ga age principal layering MZ. As consequence, role an important amalgamation zone crustal Shield during 1.70-1.55 Gothian orogeny questioned. Instead, data suggest lithologic metamorphic which middle-amphibolite supracrustals (protolith ages: 1.64-1.58 Ga) juxtaposed against upper-amphibolite 1.70-1.66 late time. By implication, relation west margin continent Baltica remains be established. Understanding tectonic evolution, particularly amount displacement MZ, therefore, pre-requisite modelling early evolution block configurations