Origin of the metamorphic flysch sequence of the Strandja Massif (NW Turkey) in the Tethyan Realm: insights from new age and structural data

The Strandja Massif is a key location for understanding the Paleozoic and Mesozoic tectonic evolution of the Tethyan Realm in the NW Turkey. Some researchers have suggested that the Strandja Massif is a part of the Cimmerian continent, but others consider it as a section of the southern passive continental margin of the Eurasia. Traditionally the massif is divided into two tectono-stratigraphic units: 1) Pre-Permian crystalline basement and 2) Mesozoic sedimentary cover. However, the ages of the lithostratigraphic units have been significantly revised following the recent geochronological studies. Structural relations between these units are not simple and should be re-examined carefully. Our previous studies have shown that the crystallization time of the magmatic rocks and sedimentation ages of the rocks range from late Proterozoic to Permian especially at the east of the Strandja Massif. In this study, the Serves metagreywacke sporadically containing metabasic rocks and Kumlukoy quartz-rich metasandstones are investigated at the north of the Kıyıköy town, in order to check the first studies that assigned them to the Jurassic and Cretaceous cover deposits. These units stretch along the Black Sea coast and reveal significant differences with units that are exposed to the south. Particularly the Serves unit consists of alternation of lithic metasandstones, schists, and phyllites whereas metaconglomerate layers, marble and dolomite bodies are common among Jurassic rocks exposed in the south. Detrital zircon studies carried on the metasandstone reveal that the sedimentation should be younger than Visean-Serpukhovian, because the youngest U-Pb zircon age population obtained are between ~338 and 327 Ma. Considering widespread late Carboniferous magmatism (~312-306 Ma) in the Strandja Massif and bereft of such magmatics constrain deposition of this unit between ~327 and 312 Ma (early-middle Pennsylvanian). In contrast, the Kumlukoy Unit has quartz-rich metasandstones and it has lower metamorphic degree than the Serves Unit. The detrital zircons of these metasandstones, which were considered as Cretaceous in the previous studies, indicate that the sedimentation interval of the unit is younger than latest Permian (~256 Ma). According to the detrital ages obtained the Kumlukoy metasandstone represent a higher stratigraphical position than the Serves metagreywacke. The Kumlukoy metasandstone is most probably the equivalent of the Triassic metaclastics reported in the cover units of the NW Strandja Massif. Whereas the age and petrography of the Serves metagraywacke are similar to the Mahya Complex and Yavuzdere Arc which was interpreted as a paired magmatic arc-accretionary prism unit. Another interpretation is that the Serves Unit predates the Mahya Complex and Yavuzdere Arc and all of them represents a long-lasting subduction and accompanying accretion events in the late Paleozoic history of the Strandja Massif, namely the Silk-road Arc.