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Figure 1. Morphostructural map of the Western Atlas system. In red are the different transects against the strike of the orogen along which bedrocks and present-day river sands were selected for thermochronological and cosmogenic isotopic analyses. Dashed black line: profile illustrated in Figure 2. Figure 2. Topographic profile (see Fig. 1 for location) against the Atlas system in SW Morocco with associated U-Th/He ages on Apatite from the different domains. NSAZ, SSAZ, NAF, AMF, SAF, SSAF, EKF: Northern Sub-Atlas Zone, Southern Sub-Atlas Zone, North Atlas Fault, Al Medina Fault, South Atlas Fault, Southern Sub-Atlas Front, El Klea Fault. Symposium 1: Structural Geology, Tectonics and Geodynamics
60 Symposium 1: Structural Geology, Tectonics and Geodynamics 1. Metamorphic and igneous evolution of the innermost Dinarides in Serbia Schefer Senecio*, Egli Daniel*, Frank Wolfgang**, Fügenschuh Bernhard***, Ovtcharova Maria****, Schaltegger Urs****, Schoene Blair**** & Schmid Stefan* *Geologisch-Paläontologisches Institut, Bernoullistrasse 32, CH-4056 Basel (senecio.schefer@unibas.ch) ** Geol. Insitute, Slovak Academy of Science, Dubravska Cesta 9, 84005 Bratislava ***Geologisch-Paläontologisches Institut, Innrain 52, A-6020 Innsbruck ****Département de Minéralogie, Rue des Maraîchers 13, CH-1205 Genève During Early Paleogene collision the innermost Dinarides of Serbia, paleogeographically part of the Adriatic plate, occupy a lower plate position in respect to the Cretaceous-age Carpatho-Balkan orogen, now constituting the upper plate (Schmid et al., 2008). This study provides new timing constraints of the igneous and metamorphic events during and after collision that are crucial for a better understanding of the geodynamic evolution of the Balkan area. The Kopaonik area in Southern Serbia, a N-S trending mountain range, exposes a part of the Jadar-Kopaonik thrust sheet in a tectonic window. There, the Kopaonik-Studenica Metamorphic Series, derived from the innermost Adriatic margin and overlying Western Vardar Ophiolitic Unit, are intruded by the Kopaonik, Drenje and Željin granodiorites, as well as by the Polumir granite. At the eastern rim of the Jadar-Kopaonik thrust sheet, a Cenozoic suture zone (Sava Zone) separates the internal Dinarides from the Carpatho-Balkan orogen consisting of Serbomacedonian “Massif” and tectonically overlying Eastern Vardar Ophiolitic Unit. The Kopaonik-Studenica Metamorphic Series (Paleozoic to Early Jurassic) are overlain by a Mid- to Late Jurassic ophiolitic mélange below the obducted Western Vardar Ophiolitic Unit. These three units are unconformably overlain by Late Cretaceous (“Senonian”) flysch that contains large olistoliths, including ophiolitic blocks and reworked metamorphics. We interpret the weakly metamorphosed N-S-trending “Senonian” flysch as the southern prolongation of the Sava belt, suturing the internal Dinarides against the Carpatho-Balkan orogen. The intrusion of Oligocene granodiorite bodies led to contact metamorphism and skarn formation in the Kopaonik-Studenica Metamorphic Series. We dated the intrusions by U-Pb (ID-TIMS) analyses of single, thermally annealed and chemically abraded zircons. All the analyses are concordant within analytical error. Two different age groups could be determined: (i) Oligocene intrusions, ranging from 31.7 to 30.6 Ma, whereby the Kopaonik intrusion yielded the youngest ages (30.94 Ma – 30.70 Ma). The Drenje and Željin intrusions cluster around 31.5 Ma. Hf isotopes for Drenje and Željin show relatively uniform εHf values ranging from 3.6 ± 0.5 to 4.3 ± 0.5. The Kopaonik sample, however, shows distinctively lower values ranging from 1.1 ± 0.5 to 1.6 ± 0.5. This indicates that the Drenje and Željin magmas had a higher juvenile component compared to the Kopaonik intrusion. (ii) A Miocene age of ca. 18.0 Ma was obtained for the westerly outcropping Polumir granite;, Hf isotopes for this intrusion are not yet available. Fission-track analyses for these samples yielded ages of around 28 Ma for zircon and 18 – 23 Ma for apatite. This indicates rather fast post-emplacement cooling and exhumation, which is most probably tectonically assisted. An extensional tectonic setting is indicated by numerous normal faults, magmatic foliation in the Drenje locality and syn-intrusive boudinage of Polumir granitic dykes. West of the Kopaonik area (Studenica valley) another part of the Kopaonik-Studenica Metamorphic Series crops out in the form of second tectonic window (so-called Studenica-slice, (Dimitriević, 1997) below the obducted Western Vardar Ophiolitic Unit, again made up of Paleozoic to Early Jurassic meta-sediments. Most of these series only underwent lower greenschist facies metamorphism, but in the core of this window, along the Studenica River, amphibolite grade meta-sediments were found. Ar-Ar dating on sericite fractions (2-6 and 6-12 µ) as well as on amphibole and biotite separates evidence Cretaceous-age metamorphism, overprinted by a second Paleogene metamorphic overprint of the Studenica Metamorphic Series. While the Cretaceous event (110 – 85 Ma) reached lowest greenschist facies conditions only, Paleogene metamorphism (40.9 ± 4.9) reached amphibolite facies conditions and was probably followed by rapid exhumation assisted by normal faulting. Our data document Cenozoic metamorphism in the innermost Dinarides of Serbia for the first time. This metamorphism is related to suturing of the Dinarides with the easterly adjacent Carpatho-Balkan orogen (including the Eastern Vardar Ophiolitic Unit) across the Sava Zone. So far Cenozoic metamorphism was only documented for the Sava Zone in northern Bosnia (Pamic, 1993; Ustaszewski et al. 2007). Moreover, our high-precision intrusion ages indicate an Oligocene-age intrusive suite, immediately followed by rapid exhumation according to the fission track data, possibly in a back-arc setting. This exhumation was then followed by a second magmatic pulse in the Miocene, again followed by rapid exhumation. This second magmatic and extensional event has to be seen in the context of the formation of the Pannonian basin.
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