Protolith and metamorphic age of the Sieggraben Eclogites: Implications for the Permian to Cretaceous Wilson cycle in the Austroalpine unit

Abstract

The Austroalpine eclogite-bearing Sieggraben Complex, exposed at the eastern margin of the Eastern Alps, is considered to represent a subducted and then exhumed fragment of the continental rift, which led to the formation of the Meliata oceanic basin. Combined zircon U-Pb dating, whole rock geochemistry and Hf isotope analysis of the Sieggraben Complex revealed that the eclogite sample shows a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) and displays tholeiitic basalt/N-MORB geochemical characteristics. Associated ultramafic rocks as part of oceanic or subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during Cretaceous. A trondhjemite dike cutting through eclogite gives a crystallization age of 82.19 ± 0.4 Ma and formed by partial melting during decompression. The host metasedimentary rocks are interpreted as old continental crust close to the margin of the Meliata basin were affected by Permian migmatitic metamorphism. The results of this study combined with previous results from the Sieggraben Complex are used to present an updated model for the tectonic evolution of the distal Austroalpine unit associated with the oceanic Meliata relics The Austroalpine Sieggraben Complex represents a location on the distal continental margin during Permian rifting. This piece of continental crust of an inferred paleotectonic position adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times. • A Middle Triassic ptotolith age was found for eclogites of the Sieggraben Complex. • A Cretaceous-aged trondhjemite dike report formed by decompressional melting after eclogite metamorphism. • Paragneiss hosts a Panafrican detrital zircon memory and was affected by Permian migmatization. • A new tectonic model for the distal Austroalpine unit relates with the Meliata Ocean in a Wilson cycle.