From sedimentation to multiple tectono-thermal events: U/Pb zircon and allanite dating in the Eastern Alps

The affiliation of the Ennstal Phyllite Zone (EPZ) to either the micaschist units of the Koralpe-Wölz nappe system (KW-NS) to its south or to nappes of the “Greywacke Zone” to its north and east is still debated. Due to similarities with phyllites of the “Greywacke Zone” in the north and phyllonitic micaschists in the south, no clear lithological boundary between these units is observable. Petrographic observations suggest a continuous eoalpine metamorphic gradient with no metamorphic gap between the KW-NS and the EPZ. In order to clear this debate and further constrain the tectonic and temporal evolution of these units, we present new LA-MC-ICP-MS U/Pb age dating results for metapelite samples from the EPZ as well as for the adjacent units of the KW-NS.

Two samples (EA09 and SP02) from the central EPZ and one sample (SP62) from the northernmost part of the Wölz-Complex of the KW-NS were selected for detrital zircon age dating. The distribution of approximately 150 dates per sample reveals major peaks at the Ediacaran-Cryogenian boundary (624 – 646 Ma), a smaller peak at the Neoproterozoic-Mesoproterozoic boundary (~1000 Ma) followed by a hiatus and a smaller peak in the mid-Paleoproterozoic (~2000 Ma). All samples show similar mid-Paleoproterozoic and Neoproterozoic-Mesoproterozoic peaks. Sample SP62 contains one grain of Cambrian age (523 Ma) and one grain of mid-Ordovician age (460 Ma) whereas the youngest zircons from the EPZ samples yield Ediacaran ages of 629 Ma and 625 Ma. The lack of zircons of Ordovician age in samples EA09 and SP02 indicate an affiliation of the EPZ with the basal units of the “Greywacke Zone”.

We also dated metamorphic allanite and REE-bearing epidote rims which are interpreted to form at low pressure and temperature conditions in metapelites. Allanites from the EPZ yield metamorphic ages of 105 ± 3.5 Ma in the northern part of the unit and 279 ± 6 Ma in the southern part. Allanite cores from two micaschist samples from the northern and central Wölz-Complex yield ages of 316 ± 21 Ma and 286 ± 11 Ma. Their respective epidote rims yield eoalpine ages of 98 ± 2 Ma and 96 ± 2 Ma. One micaschist sample from the Rappold-Complex yields ages of 326 ± 9 Ma for the allanite cores and 101 ± 1 Ma for the epidote rims. These ages are interpreted as prograde crystallization of allanite and epidote and give us petrochronological information about three distinct metamorphic events: Variscan, Permian and Eoalpine. By gathering three distinct eoalpine ages within the EPZ and the KW-NS, we can further constrain the metamorphic evolution of the eoalpine lower plate.