New data on Mesozoic Radiolaria of Serbia and Bosnia, and implications for the age and evolution of oceanic volcanic rocks in the Central and Northern Balkans

The Dinaridic Ophiolite Belt (DOB) includes Middle Triassic cherty limestones, Upper Triassic radiolarian cherts associated with basaltic lavas and volcaniclastic sediments, and also Jurassic continental slope sequences. Sedimentary sequences grade upwards into Cretaceous red limestones with planktonic foraminifers of the Genus Hedbergella. Many of the Late Triassic to Late Jurassic cherts probably accumulated in an oceanic setting, and now crop out as blocks in olistostromal deposits. Jurassic radiolarite sequences of the margin of the Dinaride Ophiolite Belt are represented by Aalenian, Upper Bajocian–Lower Bathonian and Upper Bathonian–Lower Callovian, Middle Callovian–Oxfordian, Upper Callovian–Upper Oxfordian and Kimmeridgian–Tithonian-aged strata. These are overlain by Upper Tithonian–Valanginian sedimentary sequences with spongolite–radiolarites, volcaniclastic sediments and other continental slope deposits. The oldest units outcropping in the DOB are Mid-Triassic (Ladinian) red cherts and limestones. These are associated with alkaline and tholeiitic basalts in the Bistrica area, and with tholeiitic basalts in the Visoka area. In contrast, in the Vardar Zone Western Belt the oldest basalt–radiolarite formation is represented by cherts associated with tholeiitic basalt in the Ovčar Banja area. Upper Triassic red radiolarites of the Čačak area lie in direct stratigraphical contact with MORB-type pillow lavas and contain a Carnian–Norian radiolarian assemblage. The youngest radiolarian-bearing units are in the Upper Cretaceous part of the Vardar Zone Western Belt, in the Struganik area, where Coniacian–Santonian radiolarians occur in chert–clay–tuff sediments. In addition, cherty limestones from the northern slope of Kozara Mt. contain a Campanian radiolarian assemblage, together with Campanian–Maastrichtian planktonic foraminifers. Volcanic units associated with the siliceous sediments of Dinaride Ophiolite Belt and the Vardar Zone Western Belt are of MOR- and SSZ types, respectively. The ocean-related basalts of the Dinaride Ophiolite Belt and the Vardar Zone Western Belt show some geochemical differences, irrespective of age. Compared to those of the Dinaride Ophiolite Belt, the basalts of the Vardar Zone Western Belt are more variable chemically and exhibit much higher LREE/HREE and Th/REE ratios. Finally, possible paleo-reconstructions of the radiolarian sediments and the volcanic rocks are proposed.