Abstract
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.
Highlights
The break-up of Pangaea and Permo-Triassic rifting led to lithospheric thinning and subsidence at the southern European margin
We argue that the magmatic protolith of the Glockner Nappe metagabbro and its disrupted ophiolite sequence formed during the Late Jurassic in the eastward extension of the Liguria–Piemont branch of the Alpine Tethys
Based on textural and compositional characteristics, we conclude that the coarse-grained prasinite body in the Froßnitztal originated from a coarse-grained gabbroic intrusion into a segment of oceanic crust which later formed the Glockner Nappe
Summary
The break-up of Pangaea and Permo-Triassic rifting led to lithospheric thinning and subsidence at the southern European margin. Late Cretaceous to Oligocene closure of the Alpine Tethys due to subduction of the oceanic basins (Penninic nappes) and parts of the European continental margin (Subpenninic nappes) resulted in variable metamorphic overprint of older units and merged the complex Alpine nappe system (e.g., Agard 2021; Balestro et al 2019). In this framework, both the timing of sea-floor spreading and the mantle sources for mafic–ultramafic igneous rocks are important data for reconstruction of the tectonic processes before the onset of the Alpine orogeny (e.g., Handy et al 2010; Kurz 2006; Schmid et al 2004)
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