Permian-Triassic granites of the Schladming complex (Austroalpine basement): Implications for subduction of the Paleo-Tethys Ocean in the Eastern Alps

Abstract

Permian magmatic activity is widespread in the Austroalpine Unit stretching from Eastern Alps to the Western Carpathians and Pannonian region, the link between magmatism and the subduction of Paleo-Tethys Ocean is not well constrained up to now. Here, we report, for the first time, a Middle Triassic granite in the Austroalpine basement, which is at odds with a common opinion of the passive margin setting of this unit. The Tweng and Schladming Complexes representing a Neoproterozoic to Early Paleozoic magmatic arc were intruded by Permian and Triassic granites, related to back-arc extension triggered by the subduction of Paleo-Tethys Ocean. To reveal the tectonic affinity of these granitic rocks, we systematically analyzed their zircon geochronology and whole-rock geochemistry. The zircon U-Pb data show that the Tweng granitic gneisses and leucogranites were formed at 261 Ma and 244 Ma, respectively. The Schladming granitic gneisses were formed at 244–241 Ma, their εHf(t) values (-0.9 to + 5.6) indicate their derivation from the lower crust and involved continent material. Geochemically, the granitic rocks from the Tweng Complexes have common features with subduction-related continental arc andesitic affinity. The granitic gneisses of Tweng and Schladming Complexes show A-type granite-affinity with high zircon crystallization temperatures of 784–835 ℃ and 781–818 ℃, respectively, whereas the Tweng leucogranites exhibit I-type granite characteristics and low zircon crystallization temperatures of 720–724 ℃. The geochemical features suggest that the Tweng granitic gneisses and leucogranites originated from the lower crust, while the Tweng leucogranites are explained by remelting of Tweng granitic gneisses, and interacted with subduction-related fluids. Therefore, we propose that the Permian-Triassic granitic rocks of Tweng and Schladming Complexes were formed in a back-arc setting during the subduction of the Paleo-Tethys Ocean, which finally resulted in the opening of the Meliata oceanic back-arc basin during Middle Triassic times.