Implications for the pre-Alpine evolution of the Eastern Alps – a U/Pb zircon study on the Austroalpine Schladming Nappe

The Schladming Nappe, as a part of the Silvretta-Seckau Nappe System of the Eastern Alps, comprises pre-Alpine remnants of crystalline basement rocks which give important information for reconstructing the Variscan and even pre-Variscan history of the Alps.

The Schladming Nappe mainly consists of paragneisses being intruded by subsequently overprinted granitoids. U-Pb zircon ages were acquired through LA-MC-ICPMS to determine the magmatic emplacement of the metagranitoids and constrain the tectono-metamorphic history of the Schladming Nappe.

Within these meta-granitoids, several intrusive events can be distinguished: (1) a Cambrian event with ²⁰⁶Pb/²³⁸U zircon mean ages between 496±6.5 and 501±7 Ma, (2) a Late Devonian/Early Carboniferous event with zircon mean ages between 350±5 Ma and 371±5 Ma and (3) a Permian event with zircon mean ages between 261±3 Ma and 263±3.5 Ma. The youngest age group is only found in metagranitoids from the southeastern part of the Schladming Nappe. The tectonic contact to the metapelites of the Wölz Nappe system and therefore the affiliation of these Permian granitoids to the Schladming Nappe, however, is still enigmatic.

The various age groups can also be differentiated by their whole rock geochemistry. While all of the metagranitoids are peraluminous, the Cambrian age group exhibits higher SiO₂ values compared to the Late Devonian age group. The Late Devonian age group shows higher contents of CaO, MgO, FeO, Al₂O₃, as well Sr and Ba and can be further divided into two subgroups, with one depicting a distinct negative Eu-anomaly (EuN/Eu*=0.44-0.69) and the other subgroup lacking one (EuN/Eu*=0.82-1.08). The Permian age group often displays high contents of K₂O, Nb and Y.

The Late Cambrian to Early Ordovician metagranitoids can be classified as part of a magmatic arc system, probably belonging to the northern Gondwana margin. The early Variscan granitoids can also be interpreted as part of an active margin. The Permian granitoids show a within plate granite affiliation and can further be interpreted as A-type granitoids, probably related to post-Variscan lithospheric extension.