U-Pb-Hf zircon studies of the pre–Late Carboniferous basement rocks (Mladinovo Complex) of the Sakar Unit (Strandja zone, SE Bulgaria)

Abstract U-Pb zircon geochronology was used in this study to decipher the provenance and tectonic evolution of the Strandja zone (SE Bulgaria/NW Türkiye), which contains peri-Gondwanan fragments within the southeastern Alpine-Carpathian orogenic belt, subsequently affected by Variscan and Alpine reworking. The Paleozoic evolution of the Strandja zone remains poorly understood due to a lack of geochronological data. We present results from pre–Late Carboniferous meta-igneous and metasedimentary rocks of the Mladinovo Complex (Sakar Unit, Strandja zone). Zircon U-Pb protolith ages of 613 ± 11 Ma (amphibolite) and 457 ± 3 Ma and 459 ± 3 Ma (orthogneisses) were obtained. Seven metasedimentary rocks gave U-Pb detrital zircon maximum depositional ages ranging between 578 Ma and 455 Ma. These rocks were affected by a Variscan metamorphic event at 327 ± 2 Ma. Felsic dikes, which crosscut the Mladinovo Complex, intruded at 308 ± 8 Ma and 305 ± 2 Ma. This indicates that the metasedimentary rocks were deposited in two intervals (pre– and post–mid-Ordovician) and were sourced from the Trans-Saharan belt and Saharan metacraton of northern Africa, which was confirmed by Hf isotopic studies in zircon. These data suggest the Strandja zone together with the Sredna Gora zone, Balkan fold-and-thrust belt, and Serbo-Macedonian Complex represent remnant “Cadomia-type” terranes that were positioned off the northern African margin during the late Neoproterozoic to early Cambrian. Late Ordovician magmatism in the Strandja zone, Rhodope metamorphic complex, and Serbo-Macedonian Complex may have been associated with back-arc rifting along the peri-Gondwanan Cadomian belt. The provenance of post–mid-Ordovician metasedimentary rocks in the Strandja zone and Balkan fold-and-thrust belt suggests that these rocks were deposited along the northern Gondwanan margin. Therefore, it is possible that the peri-Gondwanan terranes of the Strandja zone, Balkan fold-and-thrust belt, Sredna Gora zone, and Serbo-Macedonian Complex remained attached to Gondwana until the Devonian.