Cenozoic granitoids in the Dinarides of southern Serbia: age of intrusion, isotope geochemistry, exhumation history and significance for the geodynamic evolution of the Balkan Peninsula

The Dinarides represent a complex orogen consisting of thrust sheets that contain ophiolitic as well as Adria-derived continental material. These thrust sheets are situated in a lower plate position with respect to an upper plate formed by the Tisza and Dacia Mega-Units with European affinities. The area around the Kopaonik massif in southern Serbia exposes the two innermost Dinaridic composite nappes, namely the Drina–Ivanjica and the Jadar–Kopaonik–Studenica composite thrust sheets. In the latest Cretaceous to Early Paleogene these innermost Dinaridic thrust sheets collided with the already existing (pre-Turonian) Carpatho–Balkan orogen that is a part of the Dacia Mega-Unit and constitutes the upper plate of the complex collision zone. A separating suture zone (Sava Zone) runs along the eastern rim of the innermost Dinarides that is along the internal limit of the Jadar–Kopaonik–Studenica composite thrust sheet and separates the Dinarides from the Carpatho–Balkan orogen. The metasediments of the Kopaonik and Studenica Metamorphic Series and the overlying Western Vardar Ophiolitic Unit were intruded by Cenozoic granitoids. Available structural data indicate that the intrusion of these plutons post-dates three phases of compressive deformation (D1–D3), the latest associated with thrusting in the internal Dinarides and suturing with the adjacent Carpatho-Balkan orogen.
   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 (Kopaonik, 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 both age groups, caused by extensional exhumation of the plutons that are located in the footwall of core-complexes (D4). Miocene magmatism and core-complex formation thus affected not only the Pannonian basin but also a part of the mountainous areas of the internal Dinarides.
   For the geodynamical setting of the Balkan Peninsula we propose, based on new Hf isotope analyses and the discussion of an extensive set of age data from the literature, that Late Eocene to Oligocene magmatism, which affects the Adria-derived lower plate units of the internal Dinarides, was caused by delamination of the Adriatic mantle from the overlying crust, associated with post-collisional convergence that propagated outward into the external Dinarides. Miocene magmatism, on the other hand, is associated with core-complex formation along the southern margin of the Pannonian basin, probably associated with the Wdirected subduction of the European lithosphere beneath the Carpathians and interfering with ongoing Dinaridic–Hellenic back-arc extension.