engleski

Provenance of Cretaceous synorogenic clastics from the northwestern Dinarides (Croatia): Evidence from zircon fission track dating

Cretaceous synorogenic clastic formations from the northwestern Dinarides of Croatia record important events in the early tectonic history of the Dinaric orogen. This area is characterized by considerable geological complexity resulting from a severe and long-lasting Mesozoic and Cenozoic deformational history (Haas et al., 2000), and is largely covered by the sediments of the Neogene Pannonian basin. Cretaceous clastics occur as isolated exposures in the mountains and hills surrounding Zagreb (Mt. Ivanščica, Mt. Medvednica, Žumberak Mts. and Samobor Hills). We have studied the sandstones from these sediments using detrital zircon fission track (FT) thermochronology in order to better constrain the characteristics of, and assess changes in the source terrains being exhumed and eroded from the Early to the latest Cretaceous. Approximately 60 detrital zircon grains per sandstone sample were dated by the fission track method. Zircon FT age populations were identified from single grain age distributions using PopShare computer software (Dunkl and Szekely, 2002). In sandstones of Barremian to Cenomanian age the youngest FT age populations identified are: 134± 14 Ma and 145± 36 Ma (two samples from the Barremian-Albian Oštrc Fm., Mt. Ivanščica) ; 162± 43 Ma (Albian-Aptian Bistra Fm., Mt. Medvednica) ; and 159± 30 Ma (Albian-Cenomanian Kravljak Fm., Žumberak Mts.). The sources of these zircons were most likely fragments of the Adria basement which had undergone thermal overprint related to obduction of the Neotethys in the Late Jurassic, and were subsequently exhumed, and incorporated into thrust sheets together with ophiolites. Also present within these sandstones are zircon grains which belong to older, more diffuse FT age populations ranging from the Early Jurassic to the Carboniferous: 193± 65 Ma and 219± 64 Ma (Oštrc Fm.) ; 306± 35 Ma (Bistra Fm.) ; 275± 51 Ma (Kravljak Fm.). In sandstones of latest Cretaceous age, FT ages differ significantly from those in the Barremian to Cenomanian sandstones. In the Maastrichtian Vivodina Fm. (Žumberak Mts.) a well defined FT age population lies at 76± 13 Ma, and incorporates most of the measured grains. A minor part of the zircon ages spread very diffusely up to the Carboniferous. Furthermore, in the Maastrichtian Glog Fm. (Mt. Medvednica) distribution of the single-grain ages is unimodal (two samples ; 80± 18 Ma and 73± 23 Ma). This very clearly identifies a distinct Eo-Alpine overprint in the continental units supplying the area of the northwestern Dinarides at the end of the Cretaceous.

Dinarides; Cretaceous; sandstones; fission track; zircon; provenance

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