Tectono-metamorphic and magmatic evolution of the Internal Dinarides (Kopaonik area, southern Serbia) and its significance for the geodynamic evolution of the Balkan Peninsula

The study is devoted to the tectono-metamorphic and magmatic evolution of the Internal Dinarides and it furthermore addresses the geodynamic evolution of the Balkan Peninsula. The investigated area is located in the internal-most part of the Dinarides and covers the contact zone between the Dinaridic orogen that essentially formed in Latest Cretaceous to Paleogene times and the “Serbo–Macedonian Massif“, that is a part of the Carpatho–Balkan orogen (Dacia Mega-Unit) which is characterised by older (pre-Turonian) deformations. The widespread occurrences of ophiolitic rocks, separated by different fragments of continental basement rocks led to a ,multi-ocean‘ concept whereby the oceans were separated by elongate continental terranes or micro-plates. By investigating the stratigraphic and tectonic evolution of the various continent-derived units and by studying their relation with the intervening ophiolitic belts this ,multi-terrane/multi-ocean‘ problem is critically addressed and a one-ocean model is preferred. Thereby the continental terranes simply represent the passive margin of Adria, exposed in windows below the ophiolites, which were obducted in Late Jurassic times. Strongly deformed and metamorphosed meta-sediments crop out in the Studenica valley and the Kopaonik area representing the easternmost occurrences of Triassic sediments within the Dinarides. Upper Paleozoic terrigeneous sediments are overlain by Lower Triassic siliciclastics and limestones, followed by Anisian shallow-water carbonates. A pronounced facies change to hemipelagic and distal turbiditic, cherty meta-limestones (Kopaonik Formation) testifies to a late Anisian drowning of the former shallow-water carbonate shelf. Sedimentation of the Kopaonik Formation was contemporaneous with shallow-water carbonate production on nearby and more proxi- mal carbonate platforms that were the source areas of diluted turbidity currents reaching the depositional area of this formation. The Kopaonik Formation was dated by conodont faunas as late Anisian to Norian and possibly extends into the Early Jurassic. It is therefore considered an equivalent of the grey Hallstatt facies of the Eastern Alps, the Western Carpathians and the Albanides–Hellenides. The coeval carbonate platforms were generally located in more proximal areas of the Adriatic margin, whereas the distal margin was dominated by hemipelagic/ pelagic and distal turbiditic sedimentation, facing the evolving Neotethys Ocean to the east. A similar arrangement of Triassic facies belts can be recognised all along the evolving Meliata–Maliac–Vardar branch of Neotethys, which is in line with a ‘one-ocean-hypothesis’ for the Dinarides: all ophiolites presently located southwest of the Drina–Ivanjica and Kopaonik thrust sheets are derived from an area to the east, and the Drina–Ivanjica and Kopaonik units emerge in tectonic windows from below this ophiolite nappe. On the base of the Triassic facies distribution neither arguments for an independent Dinaridic Ocean nor evidence for isolated terranes or blocks was seen. Two age groups for the Cenozoic granitoids in the Dinarides of southern Serbia were determined by high precision single grain U–Pb dating of thermally annealed and chemically abraded zircons: (i) Oligocene ages (Ko- paonik, Drenje, Željin) ranging from 31.7 to 30.6 Ma and (ii) Miocene ages (Golija and Polumir) at 20.58–20.17 and 18.06–17.74 Ma, respectively. Apatite fission-track central ages and modelling combined with zircon central ages, together with local structural observations, constrain the subsequent exhumation history of the magmatic rocks. They indicate rapid cooling from above 300 to ca. 80 °C between 16 and 10 Ma for the Oligocene and the Miocene age group, caused by extensional exhumation of the plutons that are located in the footwall of core-complexes. Miocene magmatism and core-complex formation thus not only affected the Pannonian basin but also a part of the mountainous areas of the internal Dinarides. Four different deformation phases (D1–D4) are distinguished in the study area. D1 to D3 are related to com- pression and metamorphism that pre-date the intrusion of I-type Oligocene plutons in Early Oligocene times, whereas the fourth deformation phase (D4) is related to extensional tectonics and exhumation that are contempo- raneous with the intrusion of Miocene S-type granitoids. The first event (D1) is probably linked to the obduction of the Western Vardar Ophiolitic Unit onto the distal Adriatic continental margin. It is associated with top-NW shear-senses observed in sigma-clasts in a ductilely deformed and slightly metamorphosed ophiolitic melange as well as with a penetrative foliation and a stretching lineation coupled to greenschist facies metamorphism in the Late Paleozoic to Early Jurassic sediments. During the Late Cretaceous (110–85 Ma) these sediments witnessed a metamorphic event that occurred under lowermost greenschist-facies conditions, associated with the ductile deformation phase (D2) represented by a well developed foliation and isoclinal folds overprinting D1. A higher greenschist- to amphibolite-facies overprint is observed during Middle to Late Eocene (45–35 Ma) due to nappe- stacking caused by out-of-sequence thrusting (D3). This event is associated with the E–W-oriented compression related to and following the closure of the Sava suture. During the Miocene the entire area of investigation un- derwent rapid exhumation, accompanied by intense N–S-oriented ductile stretching (D4). This extension is correlated with the Miocene extension in the Pannonian basin whose location is in the back-arc area of the W-directed subduction of the European lithosphere beneath the Carpathians.