Multi-stage tectonic record of the Central Sakarya Terrane Basement (NW Turkey): Implications for the Paleozoic evolution of Rheic Ocean and northern Gondwanan margin

In northwestern Turkey, within the Central Sakarya Terrane, the Söğüt Metamorphics is one of the Variscan tectonic units preserved as a pre-Jurassic basement assemblage of the Sakarya Composite Terrane (SCT). It consists of para- and orthogneisses that host bands, lenses, boudins, and tectonic slices of amphibolites, and are altogether intruded by the Sarıcakaya Granitoid. The characterization and timing of the precursor magmatic events for the protoliths of this assemblage and the timing and conditions of metamorphism have been the topics of a long-lasting debate. Here we present new geochemical and geochronological data from the Söğüt orthogneisses, amphibolites, and the crosscutting Sarıcakaya Granitoid to provide further understanding of the geodynamic evolution of the SCT Basement and the larger-scale tectonic events that controlled its formation. Available geological and geochronological data indicate that the generation of the orthogneiss protoliths began in the Late Cambrian (ca. 485 Ma) and continued until the middle Silurian (ca. 430 Ma), accompanied by the subduction of the Iapetus Ocean and its progressive roll-back beneath northern Gondwana. The slab roll-back caused mantle-upwelling and Ordovician (ca. 465 Ma) continental rift-related mafic magmatism, eventually leading to the opening of the Rheic Ocean. Prior to the Variscan orogeny, a short-lived episode of middle Carboniferous (ca. 336−328 Ma) intra-oceanic arc magmatism occurred within the Rheic Ocean north of the SCT. Accretion of all this material and continuous subduction beneath the SCT led to a two-stage metamorphism during the late Carboniferous (ca. 326−324 Ma and ca. 318 Ma) and synchronous generation of granitic (ca. 327−324) and dioritic (ca. 319−317 Ma) rocks of the Sarıcakaya Granitoid during the Variscan orogeny, terminating the life cycle of the Rheic Ocean.