Late Cretaceous magmatism in the Balkans: geochronological and isotopic constraints on the newly defined Central Balkan Upper Cretaceous Magmatic Province

The Sava Zone (SZ) forms a major tectonic boundary between Europe-derived and Adria-derived continental units in the central Balkans, hosting a discontinuous belt of Late Cretaceous volcanic and plutonic rocks. The origin and geodynamic significance of this magmatism remain controversial. Initially, these rocks were interpreted as the youngest remnants of the Neotethyan Ocean, leading to the proposal of the Late Cretaceous Sava Ocean. However, an increasing body of evidence challenges this interpretation, suggesting that much of the Sava Zone magmatism developed within an intracontinental tectonic setting.

We present new zircon U - Pb ages from the Bela Reka basalts (90.60 ± 0.40 Ma; n = 77), the Jelica basalts (87.30 ± 0.80 Ma; n = 11), the Resnik trachyandesites (85.57 ± 0.49 Ma, n = 62; 84.18 ± 0.53 Ma, n = 35), the Karađorđevo Formation andesites (76.34 ± 0.93 Ma; n = 14), and the Struganik tuff (80.33 ± 0.40 Ma; n = 20). Importantly, the 87.30 ± 0.80 Ma age for the Jelica basalts represents the first robust U - Pb zircon age from Adria-affinity magmatism, demonstrating that Late Cretaceous magmatism was contemporaneous on both the Europe and the Adria margins of the Balkans.

The magmatism encompasses tholeiitic to alkaline basalts together with compositionally diverse felsic rocks. Two contrasting magmatic domains can be recognized. Adria-affinity localities are dominated by tholeiitic to transitional basalts with N- to E-MORB geochemical characteristics derived from relatively depleted spinel-bearing mantle sources, whereas the associated felsic rocks are predominantly restricted to A2-type rocks. In contrast, Europe-derived terranes contain enriched within-plate basalts, lamprophyres, and compositionally diverse felsic rocks, including A1-, A2-, and S-type granitoids, consistent with contributions from metasomatized lithospheric mantle extending into the garnet-spinel transition zone.

New whole-rock Sr-Nd-Pb isotope data further distinguish these domains. Trachyandesites yield ⁸⁷Sr/⁸⁶Sr = 0.70744-0.70746, ¹⁴³Nd/¹⁴⁴Nd = 0.51243-0.51245, ²⁰⁶Pb/²⁰⁴Pb = 18.80-18.83, ²⁰⁷Pb/²⁰⁴Pb = 15.65-15.66, and ²⁰⁸Pb/²⁰⁴Pb = 38.90-39.05. Europe-affinity basalts are characterized by ⁸⁷Sr/⁸⁶Sr = 0.70670-0.70761, ¹⁴³Nd/¹⁴⁴Nd = 0.51222-0.51266, ²⁰⁶Pb/²⁰⁴Pb = 18.59-18.96, ²⁰⁷Pb/²⁰⁴Pb = 15.62-15.73, and ²⁰⁸Pb/²⁰⁴Pb = 38.69-39.37, whereas Adria-affinity basalts display ⁸⁷Sr/⁸⁶Sr = 0.70568-0.70932, ¹⁴³Nd/¹⁴⁴Nd = 0.51256-0.51293, ²⁰⁶Pb/²⁰⁴Pb = 18.49-19.15, ²⁰⁷Pb/²⁰⁴Pb = 15.59-15.67, and ²⁰⁸Pb/²⁰⁴Pb = 38.44-38.77.

Available geochronological, geochemical, and isotopic evidence indicates that Sava Zone magmatism developed between ca. 91 and 76 Ma within an intracontinental tectonic setting rather than in an active Late Cretaceous oceanic basin. The systematic differences between the suites are best explained by contrasting lithospheric architecture and mantle source characteristics rather than fundamentally different tectonic environments. Because our data demonstrate that a significant portion of this magmatic activity occurred outside the spatial limits of the Sava Zone sensu stricto, we propose redefining this system as the Central Balkan Upper Cretaceous Magmatic Province. This new nomenclature replaces the restrictive "Sava Zone magmatism" concept, properly reflecting a regionally extensive magmatic system that transcends the boundaries of the suture zone, originating from shared intracontinental processes.