Detrital zircon U-Pb-Hf constraints on the Upper Cretaceous evolution of the Sava-Vardar Zone: implications for the waning stage of the Neotethyan Ocean

The Sava-Vardar Zone in the Balkans represents a major Tethyan suture zone whose tectonic history is instrumental to paleogeographic reconstructions of the Adria-Europe collision in the region. In this study we apply coupled detrital zircon U-Pb geochronology and Lu-Hf isotope geochemistry to constrain the provenance and pathways of Upper Cretaceous syn-tectonic sedimentary sequences. By fingerprinting the crystalline basements and matching their unique isotopic signatures to the overlying clastic formations, we evaluate the spatial proximity of the margins during the Cretaceous age.

Crystalline basement characterization reveals contrasting geochronological fingerprints for each continental margin. The Adriatic margin (Dinarides) is characterized by a prominent Permotriassic signal(s) alongside Cadomian and Variscan populations. Importantly, Lu-Hf data from these Adriatic Triassic zircons reveal strongly positive eHf values, reflecting significant juvenile mantle input during regional rifting. In contrast, the European margin basement blocks (Serbian-Macedonian Massif and Carpatho-Balkanides) are dominated by Ordovician (Cenerian) and Carboniferous (Variscan) zircons showing respective crustal and juvenile eHf signatures, completely lacking the Triassic signal.

Provenance study of the Upper Cretaceous flysch and  Lower Cretaceous paraflysch sequences yield critical paleogeographic insights. Clastites from the Upper Cretaceous Sava Zone (Adria margin) and the Lower Cretaceous paraflysch sequences sitting structurally on the European margin (e.g., Kragujevac, Slavkovica, and Kadina Luka) exhibit overlapping detrital zircon age spectra and eHf isotopic arrays. Both sedimentary domains contain the diagnostic Adria-derived Permo-Triassic population(s) carrying the distinctive juvenile eHf mantle signature. On the other hand, Cretaceous sequences located farther east of the paraflysch belt (such as the Bor Basin) completely lack this juvenile Triassic signature, confirming an unmixed Europe-derived sediment supply.

The presence of the distinct Adria-derived Permo-Triassic population and its coupled juvenile eHf signature within the paraflysch sequences on the European margin demonstrates that sediment routing from the Adria side into the European margin basin was already well established by the Early Cretaceous. This cross-suture sediment provenance indicates tight spatial proximity between Adria and Europe. Ultimately, these combined U-Pb and Lu-Hf data exclude the existence of a wide (>300 km) Sava Ocean already during the Early Cretaceous, pointing instead to a narrow oceanic conduit or an advanced stage of continental collision.