Contemporaneous opening of the Alpine Tethys in the Eastern and Western Alps: constraints from a Late Jurassic gabbro intrusion age in the Glockner Nappe, Tauern Window, Austria

Metabasic rocks of the ophiolitic sequences of the Glockner Nappe and Eclogite Zone in the south-central Tauern Window, Austria, reveal important insights into rifting and spreading of the Alpine Tethys. U–Pb dating of magmatic zircons yields a concordant 157 ± 2 Ma crystallization age for the precursor of a coarse-grained metagabbro from the Glockner Nappe. The Late Jurassic intrusion age is coeval with mafic plutonic activity in the Western and Central Alps. Although Penninic ophiolitic sequences in tectonic windows of the Eastern Alps are usually disrupted, an ocean–continent transition setting can be reconstructed for the Glockner Nappe, similar to many ophiolites in the Liguria–Piemont domain in the Western and Central Alps. Together, these observations strongly suggest a formation in the Liguria–Piemont branch of the Alpine Tethys and are inconsistent with a formation in the Valais domain. This finding has important implications for paleogeographic reconstructions of the Penninic realm in the Eastern Alps. Whereas the Glockner Nappe metagabbro and metabasalts clearly reveal their depleted mantle origin, the metabasic rocks of the Eclogite Zone record a more complex formation history involving depleted mantle melting and crustal assimilation in a continental margin setting.