U-Pb ages of detrital zircons from synorogenic clastic sedimentary rocks in the Dinarides (Croatia)

In northwestern Croatia, synorogenic clastic formations record important events in the evolution of the Dinarides, including the Dinaride-Alpine transitional area, a region characterized by considerable geological complexity resulting from a severe and long-lasting Mesozoic and Cenozoic deformational history. Our study concentrates on the provenance of Cretaceous sandstones which occur on Mt. Ivanščica, Mt. Medvednica, and Žumberak Mts, with the intent of providing new insights into the evolution of the orogen and possible paleogeographic connections with neighboring tectonic units. Uranium-lead ages of detrital zircon populations place new constraints on the nature of source terrains being exhumed and eroded during the early stage of convergence of the Adria plate margin. Construction of the nascent mountain belt in this stage involved obduction of Neotethyan ophiolites and inclusion of basement units within propagating nappe structures. Zircon age spectra of Early Cretaceous sandstones from Mt. Ivanščica reveal Permo-Triassic, Variscan, Caledonian and Pan-African populations, and one grain with concordant Middle/Late Jurassic age. In Late Cretaceous sandstones from Mt. Medvednica and Žumberak Mts the spectra are dominated by Variscan, Caledonian and Pan-African ages, but in different proportions. Permo-Triassic zircons are completely absent in Mt. Medvednica sandstones, and reflected by only one grain in Late Cretaceous Žumberak Mts sandstone. Permo-Triassic zircons with a mode at ~240 Ma were probably derived from volcano-sedimentary sequence outcropping in northwestern Croatia, while Variscan and pre-Variscan zircons could have been derived both from reworked igneous/metaigneous or sedimentary/metasedimentary sources. The absence of Permo-Triassic zircons in Late Cretaceous clastic sequences precludes the resedimentation of Early Cretaceous sandstones. Significant variations in the age spectra of Late Cretaceous zircon populations point to diverse catchment areas. These results hint towards change in predominant source rocks from Early to Late Cretaceous, as well as local variability during the Late Cretaceous. The presented work is supported by the Croatian Science Foundation project SECret (HRZZ IPS-2023-02-2683).