From shallow into deep sea: Sedimentary facies and U-Pb zircon ages in the early Paleozoic Noric Group at Veitsch (Eastern Greywacke Zone, Austria)

Abstract

The low-grade metamorphic early Paleozoic basement of the Veitsch area presents a wide variety of sedimentary facies domains. The first domain consists of thick metadacites of Middle Ordovician age (Blasseneck Porphyroid), overlain by fine-grained metaclastics of the Rad Formation (Late Ordovician to Silurian) and Devonian limestones and calcitic marbles (Kaiserstein and Kaskögerl Formation, respectively). Rhyolitic to dacitic magmatism initiated at ca. 479 Ma (LAMC-ICP-MS U-Pb zircon data) and lasted until ca. 444 Ma. The second domain comprises metaclastics of the Stocker Formation (Early Ordovician to Silurian), characterized by thin volcanics and volcaniclastics of andesitic and rhyolitic composition. U-Pb zircon data give Middle Ordovician age (463 Ma – 468 Ma). The third domain, exposed northwest of Veitsch, consists of thick metadacites (Blasseneck Porphyroid, ca. 478 Ma), followed by (siliceous) phyllites which grade into turbiditic metasediments (Sommerauer Formation, Late Ordovician to Devonian?). Clastic sediments of the Stocker and Sommerauer Formations were sourced from northern Gondwana showing a prominent Pan-African detrital zircon peak at ca. 640 Ma. Middle to Upper Ordovician volcanics (ca. 462 Ma – 448 Ma) represent the second source. Tectonic reconstruction leads us to the arrangement of three facies domains. A shallow marine shelf facies is located in the present days southwest. A marginal basin with volcanic islands on a sloping continent, and a deep-water environment containing turbidites are situated further to the northwest. The present arrangement of these facies domains is explained by eo-Alpine and Variscan thrust tectonics.

The role of the proto-Alpine Cenerian Orogen in the Avalonian- Cadomian belt

“…In the Ordovician, subduction/accretion processes likely dominated at the outboard flank of the CENO, while back-arc rifting and contemporaneous passive margin type sedimentation occurred inboard and on the Gondwana facing side. In fact, Nievoll et al (2022) describe Ordovician shelf sedimentation in the eastern Greywacke Zone of the Alps that supports the hollowing of a rift basin. Felsic volcanism of Ordovician age in the same area (Blasseneck porphyry) is also considered extension-related (Haas et al, 2020;Nievoll et al, 2022) with a possible petrogenetic connection to slab roll back (Best et al, 2016;Lewis et al, 2022).…”

“…In fact, Nievoll et al (2022) describe Ordovician shelf sedimentation in the eastern Greywacke Zone of the Alps that supports the hollowing of a rift basin. Felsic volcanism of Ordovician age in the same area (Blasseneck porphyry) is also considered extension-related (Haas et al, 2020;Nievoll et al, 2022) with a possible petrogenetic connection to slab roll back (Best et al, 2016;Lewis et al, 2022). Likewise, the numerous Cambro-Ordovician mafic complexes in the Alps (Chamrousse, Speik, etc.)…”

“…Finally, widespread Ordovician granitic magmatism, which is seen in both the Alps and the extra-Alpine Variscides, deserves further attention. Although it is mainly of S-type to I/S transitional character (Schulz et al, 2004;Zurbriggen, 2015;René and Finger, 2016;Žak et al, 2023;Nievoll et al, 2022), some geologists working in the Alps have, understandably, related this igneous event to arc magmatism associated with Cenerian subduction and slab roll-back (Siegesmund et al, 2023). However, similar felsic magmatism of the same age also occurs in equal intensity in the CADO, where no Ordovician subduction activities are documented.…”

“…We concur with the position of Zurbriggen (2015Zurbriggen ( , 2017 that the CENO likely had the character of a sediment-rich, Alaskan-type subduction-accretion orogen, in which crustal thickening and topographic elevation remained moderate. Fast erosion of this orogen could account for the voluminous Ordovician clastic sediments in the Alps (Nievoll et al, 2022;Neubauer et al, 2022).…”