Geodynamic evolution of the Northernmost Dinarides revealed by the petrogenesis of Mid-Triassic calc-alkaline volcanic rocks

This paper presents mineralogical, petrological, geochemical, and Sr–Nd isotopic data on volcanic rocks from the Northwestern Croatian Triassic Rift Basin, which is a locally aborted back-arc rift system of the Paleotethys. The dataset also includes their isotopic ages, along with the biostratigraphic ages of all interbedded sedimentary rocks. These volcano-sedimentary successions represent the northernmost parts of the Dinarides. The obtained 40Ar/39Ar isotopic age (243±1.8 Ma) indicates the Late Anisian age of the basaltic eruption and is somewhat consistent with the obtained Early Ladinian biostratigraphic ages of the interbedded sedimentary rocks. The initial εNd varies between −0.08 to +1.90, while the initial 87Sr/86Sr varies in a wide range from 0.704765 to 0.709248. The investigated Late Anisian to Early ­Ladinian calc-alkaline/shoshonitic basaltic lavas were formed through: (i) low-degree partial melting (5–14 %) of a shallow (spinel stability facies; max. depth ~33 km) magma source region in the subcontinental lithospheric mantle and (ii) fractional crystallisation in a partially open magmatic system. The magma generation also involved: (i) melts from the subducted slab, which is likely linked to the ancient, subducted oceanic crust of the Paleotethys, (ii) less abundant, previously subducted continental crustal material – sediments recycled in the mantle wedge, and (iii) a negligible contribution from depleted or an ocean-island, basalt-like mantle. These conditions suggest that the magma formed through a complex process during magma ascent through tectonically weakened crustal zones. Although the intense effusive volcanic activity during the Middle Triassic in the Northwestern Croatian Triassic Rift Basin lasted for a relatively short period (~3–4 Myr; Bithynian–Fassanian), its geodynamic evolution was complex and can be attributed to: (i) the subduction of the Paleotethyan lithosphere associated with an ensialic volcanic arc developed in an active continental margin setting, and (ii) the processes of continental rifting along the mid-Triassic margins of the Adria Plate, i.e., Greater Adria. These processes represent the beginning of rifting, which affected a greater area on the Balkan Peninsula as well.