The Dobra Gneiss and the Drosendorf Unit in the southeastern Bohemian Massif, Austria: West Amazonian crust in the heart of Europe

Abstract

Abstract U–Pb zircon dating and geochemical investigations were applied to rocks from the Variscan deformed and metamorphosed Drosendorf Unit of the eastern Bohemian Massif, Austria. Data show that this unit contains remnants of a Mesoproterozoic granite (Dobra Gneiss Type A), early Neoproterozoic sediments (paragneiss, marble) and late Neoproterozoic volcanic-arc granitoids from the Avalonian–Cadomian peri-Gondwana Orogen (Dobra Gneiss Type B, Spitz Gneiss). A rock association of such old age is extraordinary in the Variscan Orogen. We interpret that these rocks were originally part of the Brunovistulian foreland plate (i.e. part of Avalonian Europe and the Devonian Old Red Continent, respectively) before being tectonically incorporated into the eastern flank of the Variscan Orogen. This interpretation is considered likely because the Dobra Gneiss Type B turned out to be extremely similar, geochemically and in age, to the Bittesch Gneiss in the Moravian Zone, which is generally accepted as a Brunovistulian (i.e. Avalonian) rock. Based on the zircon ages measured in ortho- and paragneiss samples, the Drosendorf Unit can be excellently correlated with West Amazonia. This supports the long-standing idea that Avalonian Europe contains terranes of Amazonian ancestry. A model is presented to show how West Amazonian rocks could have been transferred to Europe in the Early Paleozoic.