Inferring protoliths of high-grade metamorphic gneisses of the Erzgebirge using zirconology, geochemistry and comparison with lower-grade rocks from Lusatia (Saxothuringia, Germany)

Protoliths of highly metamorphosed gneisses from the Erzgebirge are deduced from the morphology, age and chemistry of zircons as well as from whole rock geochemistry and are compared with lower-grade rocks of Lusatia. Gneisses with similar structural appearance and/or geochemical pattern may have quite different protoliths. The oldest rocks in the Erzgebirge are paragneisses representing meta-greywackes and meta-conglomerates. The youngest group of zircon of meta-greywackes that did not undergo Pb loss represents the youngest igneous component for source rocks (about 575 Ma). Similar ages and zircon morphology reflect the subordinate formation of new zircon grains or only zircon rims in the augengneiss from Barenstein and Wolkenstein, which probably represent metamorphic equivalents to Lower Cambrian two-mica granodiorites from Lusatia. Bulk rock chemistry, intense fracturing and high U and Th concentrations of zircons suggest deformation-induced and fluid-enhanced recrystallisation of zircon grains. Temperatures during tectonic overprinting—too low to reset zircon ages—indicate mid- or upper crustal levels for shearing recorded in these augengneisses. Lower Cambrian (~540 Ma) granodiorites are widespread in Lusatia but are exclusively represented by the Freiberg gneiss dome in the Eastern Erzgebirge. Ordovician protolith ages were recorded by zircons from the augengneisses of the Reitzenhain–Catherine dome and the Schwarzenberg dome (Western Erzgebirge) documenting significant regional differences between the eastern and the western Erzgebirge (~540 vs. ~490 Ma). In the Western Erzgebirge, most meta-volcanic rocks (muscovite gneisses) and meta-granites (mainly red augengneisses) yield Ordovician zircon ages, whereas in the Eastern part, similar rocks mainly recorded Lower Cambrian protolith ages. Zircon overprinting was highest within discrete tectonic zones where the combination of fluid infiltration and deformation induced variable degrees of recrystallisation and formation of a new augengneiss structure. Variable degrees of Pb loss caused age shifts that do not correspond to changes in zircon morphology but may be associated with U and Th enrichments. Major changes in bulk rock composition appear to be restricted to discrete zones and to (U)HP nappes, whereas gneisses with a MP–MT metamorphic overprint basically show no geochemical modifications.