The Waldbach Complex of Eastern Alps: An early Paleozoic arc system and its significance for Variscan geodynamics

The Waldbach Complex is an amphibolite-grade basement unit within the Lower Austroalpine nappes of Eastern Alps, which differs from other Alpine basement units and which represents a magmatic arc-related tectonic setting. For the first time, twenty samples of magmatic and metasedimentary rocks were studied by the LA-ICP U-Pb zircon dating method supplemented by a geochemical survey. Two units are distinguished: (1) The western structurally Lower Waldbach Unit includes phyllonitic micaschist, paragneiss, and quartzites associated with various orthogneisses including augengneisses. Metasedimentary rocks contain mainly Late Ediacaran (550 Ma) detrital zircon populations. Older zircons are rare and include populations at 2.6 Ga (Late Archean) and 700 Ma (Cryogenian). Youngest ages are at ca. 510 Ma constraining the maximum depositional age. Six granitic orthogneisses from distinct lenses were studied and yield ages between 463.4 ± 3.7 Ma and 492.9 ± 3.1 Ma. Abundant inherited Neoproterozoic zircons suggest their S-type origin by remelting of Neoproterozoic crust. A further, granitic, biotite-gneiss intruded at 340 Ma (Early Carboniferous). All data together suggest a late Cambrian metasedimentary succession subsequently intruded by late Cambrian to Middle Ordovician porphyric granites. (2) The Upper Waldbach Unit  is dominated by various types of amphibolites, hornblende-gneisses, coarse-grained garnet-micaschists and sulphidic micaschists. Associated stratiform massive sulphides are exposed as up to two meter thick synsedimentary layers together with black, carbon-rich micaschists. A hornblende-gneiss is interpreted as a tuff and contains a pronounced population at 455 Ma. Coarse-grained garnet-micaschists include zircon populations with ages at 455 Ma and 505 Ma, and youngest ages at 430 and 410 Ma, respectively. Amphibolites vary in their U-Pb zircon ages between 450 and 340 Ma. Both amphibolites and metasedimentary rocks contain zircons with low Th/U ratios between 330 and 315 Ma supported by a chemical  monazite age  at 304.4 ± 7.8 Ma constraining together the age of amphibolite facies metamorphism of the Upper Waldbach Unit. We interpret the Upper Waldbach Unit as a Late Ordovician to Devonian arc system, which was deposited in an anoxic depositional environment with extensive hydrothermal activity leading to stratiform massive sulphides.

Paleogeographically, the Waldbach Complex was located close to Austroalpine-Penninic interface within the Alpine basement and can be likely traced to Carpathians. Tectonically, it is interpreted as the Late Ordovician to Devonian arc system formed during subduction of oceanic lithosphere as also constrained by Devonian eclogites in adjacent Western Carpathians and Devonian blueschists in Southern Carpathians. Consequently, elements of subduction-related settings allow trace a hitherto unknown subduction zone within the Alpine-Carpathian basement, which is potentially part of the Protogonos arc was recently proposed by A. M. Celal Sengör.