Cambrian-Ordovician continental magmatic arc at the northern margin of Gondwana: Insights from the Schladming Complex, Eastern Alps

Abstract

The Austroalpine Unit is one of the major tectonic units in the Alps and contains distinct Pre-Mesozoic basement units, which underwent in part pre-Variscan (Ordovician), Variscan and Alpine events and preserved information of pre-Variscan magmatism and ocean slab relics. The Schladming Complex is an important part of the Austroalpine basement unit, its orthogneiss and paragneiss were intruded by a large number of the Cambrian to Ordovician magmatic rocks, which recorded the tectonic evolution related to the Proto-Tethys, but for which there have been very few studies carried out, especially in geochronology and geochemistry. In this study, for the first time we systematically investigated the magmatic rocks of the Schladming Complex by zircon U–Pb chronology, zircon Lu–Hf isotopic and geochemical to understand the tectonic setting and evolution of the Austroalpine Unit during the Early Paleozoic. Our study shows that the granodioritic gneisses (539–538 Ma) and the fine-grained amphibolite (531 ± 2 Ma) represent a bimodal magmatism. Geochemically, the granodioritic gneisses belong to A2-type granite and originated from the lower crust, the fine-grained amphibolites have an E-MORB affinity and derived from the lithospheric mantle. This association further implies that the Schladming Complex formed in a back-arc rift tectonic setting in the Early Cambrian. A medium-grained amphibolite gives an age of 495 ± 5 Ma, exhibits ocean island basalt-like geochemical features and positive εHf(t) values (+5.3 ~ +10.9) indicating that the medium-grained amphibolite derived from a mantle source. The zircon U–Pb dating of monzonite granitic gneiss and plagioclase gneiss yields ages of 464 ± 4 Ma for and 487 ± 3 Ma, respectively. The monzonite granitic gneiss derived from the mixing of melts derived from pelitic and metaluminous rocks. The protolith of plagioclase gneiss is aplite, which has positive εHf(t) values of +5.9 ~ +7.9, indicating it derived from the lower crust sources. The monzonite granitic gneiss and plagioclase gneiss exhibit S-type and I-type geochemical features, respectively. They are geochemically similar to the volcanic arc granite. We propose that the Schladming Complex in the Eastern Alps has been a part of the active continental margin of Gondwana during Cambrian to Ordovician. With the subduction of the Proto-Tethyan Ocean to the south, a back-arc rift developed along the northern margin of the Gondwana in the Early Cambrian, subsequently, resulted in the opening of the Crypto-Rheic Ocean and the break-off of the Cadomian terrane from Gondwana in the Late Cambrian. Meanwhile, the I-type granites, related to the subduction, intruded into the northern margin of the Cadomian terrane. In the Early Ordovician the Cadomian terrane collided back to the Gondwana with the closure of the Crypto-Rheic Ocean due to the continuing southward subduction of the Proto-Tethyan Ocean. Subsequently, a number of S-type granites intruded due to the post-collisional extension during Early to Late Ordovician.