The Drosendorf Unit in the Austrian part of the Bohemian Massif: Does it host the oldest rock fragments of Variscan Europe?

Remnants of pre-Cadomian rocks are scarce in the Variscides including the Bohemian Massif. In the latter, numerous Variscan metamorphosed Mesoproterozoic and Early Neoproterozoic sediments and granitoids are contained in the Drosendorf Unit (DU) in Lower Austria. Here, we present U-Pb zircon ages for two orthogneisses from this unit showing even older magmatic formation ages of 2.10 Ga and 2.05 Ga. These granitoid gneisses with volcanic-arc and within-plate characteristics belong to the oldest rocks known from the Central European Variscides: the Gaberkirche Gneiss (~2.1 Ga), occurring as a relatively small, ~0.3 km² orthogneiss body near Drosendorf, and the Schallaburg Gneiss (~2.05 Ga) located in the south-eastern outskirts of the Bohemian Massif near Melk, where it forms two small bodies with a total area of ~2.5 km². Although tectonically incorporated into the Moldanubian Zone (Armorica) during the Variscan orogeny, the DU likely represents a part of the Brunovistulian Terrane (BT), which lay north of the Rheic Ocean before the Variscan collisional events. Rare Palaeoproterozoic remnants have also been identified in other parts of the BT in the Velké-Vrbno Dome and the Rzeszotary Horst, the latter being interpreted as a tectonic splinter from north of the Tornquist Line. However, the Meso- to Neoproterozoic rocks of the DU typically show a detrital and inherited Palaeoproterozoic zircon signal, and may thus have been originally associated with a Palaeoproterozoic basement. This could be an important new aspect for future palaeogeographic interpretations.