Late Cretaceous granitic magmatism of the Sava Zone: Geochemical and geochronological insights from the subsurface of the Pannonian Basin in eastern Croatia

The Upper Cretaceous igneous rocks of the Sava Zone consist of tholeiitic and alkaline basalts, rhyolites and minor A-type granites, S-type granites and lamprophyres. Although less voluminous and less significant in terms of economic geology than the contemporaneous Apuseni-Banat-Timok-Srednogorie belt (ABTS), the Sava Zone is crucial for understanding of the geodynamic evolution of southern Europe. Several conflicting scenarios, both intraoceanic and intracontinental, have been evoked to explain this magmatism, resulting in a discussion about the timing of the Neotethys closure in the region. The situation is further complicated by the fact that the Sava Zone is largely covered by deposits of the Pannonian Basin. This study focuses on the Late Cretaceous igneous rocks from the Drava and Slavonia-Srijem Depressions in eastern Croatia. The cores from deep hydrocarbon exploration boreholes were subjected to geochemical, in situ zircon U-Pb dating and Hf in zircon isotope analysis. The basement of the Drava Depression is part of the Tisia megaunit, and the orthogneiss with Late Permian A-type granitic protolith (257±2.6 Ma, εHf(i)=+2.4 to -3.1) indicates that the studied part of the Slavonia-Srijem Depression belongs to the same unit. The Drava Depression hosts calc-alkaline to high-K calc-alkaline I-type granodiorite-monzodiorite (77.3±1.1 Ma, εHf(i)= +3.3 to -3.2), with microgranular mafic enclaves, and hybrid quartz-monzodioritic rocks originating from magma mixing. The I-type granodiorite-monzodiorite is characterized by LILE/HFSE enrichment, but less evolved members display lower LILE/HFSE, with relatively high TiO₂ and Nb/Y. The Slavonia-Srijem Depression hosts A-type alkali-feldspar granites and quartz-alkali-feldspar syenite (83.7±0.6 to 78.8±0.7 Ma; εHf(i)= +9.7 to -0.8). They are characterized by a ferroan character, high alkali content and relatively low LILE/HFSE. While zircons from the Drava I-type granodiorite-monzodiorite contain numerous inherited cores, this is not the case for the Slavonia-Srijem A-type rocks. Both suites were generated by mixing of mantle and continental crust-derived melts, but distinctions arise from a lower crustal thickness in the Slavonia-Srijem area. High-temperature melting and efficient hybridization beneath a thinner lithosphere generated A-type magmas with minimal zircon inheritance. Thicker crustal domains enabled longer-lived magma-crust interaction, extensive assimilation of heterogeneous basement, and the production of lower-temperature I-type granitoids with abundant inherited zircon. This study points to an intracontinental origin of the Sava Zone magmatism, caused by asthenosphere uprise, and indicating complex (post-)collisional processes in the Late Cretaceous as a trigger for magmatism. The presented work is supported by the Croatian Science Foundation project SECret (HRZZ IPS-2023-02-2683).