Ordovician zircons as detrital markers in the Ötztal Nappe (Austroalpine, Italy)

The Austroalpine Ötztal Nappe shows pervasive Eoalpine and local Variscan high-pressure metamorphism and deformation in its southeastern end, which obscure prior structures. We used magmatic and detrital zircon U-Pb dating by laser ablation ICP-MS to identify the precursor units of the Ötztal Nappe and the relationships among them.

Magmatic protolith dating of granitoid othogneisses in the Ötztal basement yielded Ordovician ages (450 – 470 Ma). The zircons of the Ordovician magmatism are important markers in the detrital zircon record. The paragneisses of the Ötztal basement, in which the Ordovician granitoids intruded, show no Ordovician zircons. The partly calcareous metasediments of the Schneeberg Complex and the Laas Series record some Ordovician detrital zircons. While the Schneeberg Complex is in tectonic contact to the Ötztal Nappe (Klug & Froitzheim, subm.), the Laas Series is the post-Ordovician sedimentary cover of the Ötztal basement. A Permo-Triassic basal metasandstone of the Brenner Mesozoic shows next to a strong Ordovician zircon age population some Variscan and Permo-Triassic zircons.

Zircon dating allowed to identify pre-Ordovician basement with Ordovician intrusions covered by post-Ordovician-pre-Variscan and Permo-Mesozoic sediments. This supports the concept of a non-tectonic unity in the southeastern Ötztal Nappe outside of the Schneeberg Complex.