Magmatism of Kozara Mts. (N. Bosnia and Herzegovina) revisited: Zircon U-Pb geochronology and Sr-Nd-Pb-Hf isotope geochemistry

For decades, the geotectonic models of the Balkan Peninsula presumed a unified Late Jurassic termination for the Neotethyan oceanic domain traditionally called Vardar Ocean. However, the identification of Late Cretaceous ages of a magmatic suite within the Kozara Mts. in N. Bosnia and Herzegovina, triggered a major paradigm shift, introducing the concept of Late Cretaceous ophiolites to the region. Consequently, researchers began reinterpreting these formations as evidence of a still-open oceanic realm called the Sava Ocean, extending the regional subduction-collision timeline to the Late Cretaceous-Paleogene. The Kozara suite consists of gabbros, dolerites, pillow basalts, and rhyolites. Previously, published U-Pb zircon ages are 81.4 Ma for dolerites and 81.6 Ma for rhyolites. In this contribution, we re-investigate this critical locality, using advanced isotopic techniques combined with new geochronological constraints.

New U-Pb analyses of zircons from rhyolites reveal inherited Permian-Triassic and Variscan zircon populations, providing direct evidence for continental crustal involvement in the evolution of the felsic magmas. Notably, inherited zircon populations have also been reported from Late Cretaceous magmatic rocks of the Slavonija–Srijem Depression and the Slavonian Mountains, suggesting that this was a regional characteristic of Late Cretaceous Sava Zone magmatism.

Kozara basalts display LREE-enriched REE patterns, enriched MORB affinities on the Nb/Yb-TiO₂/Yb discrimination diagram, and no significant HFSE depletion on primitive mantle-normalized multi-element diagrams, indicating derivation from an enriched asthenospheric mantle source. Whole-rock Sr-Nd-Pb-Hf isotope signatures for basalts show ⁸⁷Sr/⁸⁶Sr = 0.704934-0.707886, ¹⁴³Nd/¹⁴⁴Nd = 0.512882-0.512952, ²⁰⁶Pb/²⁰⁴Pb = 18.92-19.28, ²⁰⁷Pb/²⁰⁴Pb = 15.63-15.67, ²⁰⁸Pb/²⁰⁴Pb = 38.76-39.10, and ¹⁷⁶Hf/¹⁷⁷Hf = 0.282973-0.283029, whereas the rhyolites exhibit distinct isotopic ratios with ⁸⁷Sr/⁸⁶S = 0.706232-0.706462, ¹⁴³Nd/¹⁴⁴Nd = 0.512666-0.512923, ²⁰⁶Pb/²⁰⁴Pb = 19.10-19.03, ²⁰⁷Pb/²⁰⁴Pb = 15.66-15.68, ²⁰⁸Pb/²⁰⁴Pb = 38.93-39.01, and ¹⁷⁶Hf/¹⁷⁷Hf = 0.282871-0.282893. The rhyolite isotopic arrays are consistent with the involvement of continental crust in their petrogenesis, as also independently inferred from the zircon record. Collectively, these data argue against a purely oceanic origin and instead support the evolution of the Kozara magmatism within an intracontinental setting.