Variscan tectonism overprinting on Ordovician arc in the Eastern Alps: Insights from the Schladming Complex in the Austroalpine mega-unit

The last known supercontinent, Pangea, formed through the Variscan orogeny as a result of diachronous convergence and collision between Gondwana and Laurussia. In many Variscan tectonic models, the Alps are not considered a major paleogeographic constituent. Here we present new zircon U-Pb ages, Lu-Hf isotopic data, and whole-rock geochemistry for rock samples from the Variscan Schladming Complex in the Austroalpine mega-unit to investigate the Paleozoic evolution of the Proto- and Paleo-Tethys oceans and the Variscan orogeny within this mega-unit. Granitic gneisses and biotite-plagioclase gneisses preserve information of Early Ordovician magmatism. Granitic gneisses have protolith ages of 485–483 Ma and record metamorphic age peaks of ca. 420 Ma, ca. 380 Ma, and ca. 350 Ma. Magmatic zircons from the granitic gneisses display positive εHf(t) values (+2.24 to +4.71) and geochemical characteristics similar to continental arc magmas, suggesting the southward subduction of the Proto-Tethys Ocean beneath the northern Gondwana margin during the Early Ordovician. Similar Early Ordovician ages are recorded in meta-sedimentary or metavolcanic/metatuffaceous rocks (biotite-plagioclase gneisses) with peaks of detrital zircon ages of ca. 600 Ma and ca. 485 Ma and some ages at ca. 355 Ma. The mineral composition and the zircon εHf(t) values of most meta-clastics are similar to those of nearby granitic gneisses, indicating that they are proximal deposits adjacent to the Early Ordovician magmatic belt. Tournaisian metamorphic ages (355–350 Ma) of granitic gneisses and biotite-plagioclase gneisses reflect the subduction of the Balkan-Carpathian Ocean and the ongoing Variscan orogeny. An amphibolite, dated at 382 ± 3 Ma, exhibits subduction-related geochemical characteristics. The zircon εHf(t) values (+10.47 to +12.72) and young one-stage model ages (570–490 Ma) indicate derivation from the depleted mantle metasomatized by subduction-related fluids. An amphibolite (359 ± 13 Ma), a hornblende granite (354 ± 1.3 Ma), and a biotite granite (355 ± 2 Ma) formed contemporaneously in a subduction-related tectonic setting. The zircon εHf(t) values (+3.61 to +6.04) and the one-stage model ages (830–770 Ma) of the amphibolite indicate that the magma originated from the depleted mantle source and was affected by subducted material. A two-mica granite exhibits geochemical features of S-type granites with predominantly negative zircon εHf(t) values (−1.29 to +1.0) and old two-stage model ages (1420–1280 Ma), indicating that these granites are partial melt products of Mesoproterozoic crust and are mixed with minor depleted mantle-derived magmas. These Variscan granitic gneisses from the Schladming Complex suggest that the Schladming Complex was part of the Variscan granitoid belt, similar to those in the sub-Penninic and Helvetic basement units as well as to the extra-Alpine Moldanubian zone, representing the upper plate above a subduction/collision system. Therefore, we conclude that the Austroalpine mega-unit constitutes an important part of the Variscan orogeny.