Upper Cretaceous volcano-sedimentary complex of the westernmost Sava Suture Zone: Evidence from the Kostajnica area

The Kostajnica area (Croatia-Bosnia and Herzegovina boundary) presents the westernmost known exposure of the Sava Suture Zone (SSZ), i.e., tectonic boundary between the European and the Adria plates formed by Late Cretaceous collision. The sedimentary succession records a transition from Scaglia-type pelagic limestones which progressed to mixed carbonate-siliciclastic deposits and turbidites, reflecting progressive and increasing terrigenous input. Sandstones are dominated by volcanic lithic fragments compositionally similar to the associated basalts, indicating local derivation and syn-volcanic sedimentation. Planktonic foraminifera associations assign the carbonate succession to the Dicarinella asymetrica Zone, indicating a middle Santonian to possibly earliest Campanian age. The volcanic rocks comprise hydrothermally altered spilitic basalts that preserve skeletal, dendritic, and intersertal igneous textures indicative of rapid (under)cooling. Subsequent alterations are reflected by extensive albitization, saussuritization, chloritization, and element mobility (e.g., Cs, Rb, Ba, and U). Despite these effects, geochemical data indicate an E-MORB affinity with tholeiitic characteristics. Trace element systematics suggests partial melting of an initially enriched mantle source near the spinel–garnet transition, predominantly occurring within the spinel stability field. The Kostajnica basalts closely resemble coeval E-MORB-type basalts from the western SSZ (e.g., North Kozara and Požeška Gora), while volcanic rocks from the central SSZ (e.g., Ripanj and Klepa) display more enriched, alkaline to transitional compositions. This regional geochemical variability indicates Santonian–Campanian tectono-magmatic activity which occurred within a broadly extensional regime but involved heterogeneous magma sources and lithospheric controls. The precise geodynamic setting, however, remains uncertain, with the Kostajnica magmatism potentially associated to the active convergent system or post-collisional transtension.

The presented work is supported by the Croatian Science Foundation project SECret (HRZZ IPS-2023-02-2683) and by Science Fund of the Republic of Serbia project Recon Tethys (7744807).