PALEOZOIC EVOLUTION OF THE CENTRAL SAKARYA TERRANE BASEMENT: TIMING OF MAGMATISM AND METAMORPHISM

The Central Sakarya Terrane (CST) in NW Turkey includes the Söğüt Metamorphics and cross-cutting Sarıcakaya Granitoid as its basement. This preserved basement stands as one of the Variscan tectonic units within the Sakarya Composite Terrane (SCT). The metamorphics consist of para- and ortho-gneiss associated with bands, lenses, boudins, and tectonic slices of amphibolites and are intruded by arc-type granites-to-diorites. Here we present new zircon U-Pb and hornblende ⁴⁰Ar/³⁹Ar ages and geochemical data, constraining the timing and characterization of magmatic and metamorphic events throughout the Paleozoic evolution of this basement assemblage at the northern margin of Gondwana.

The Söğüt orthogneisses have yielded ca. 485-452 Ma zircon U-Pb ages for their protolith crystallization. They display signatures of typical arc-related magmas; however, their source melt characteristics indicate a transition from I- and S-type to A-type signatures, which is correlated with a shift from a continental arc to within-plate magmatism. This shift is interpreted to display the roll-back of the Iapetus slab beneath northern Gondwana. Two stages of mafic magmatism are defined with the crystallization of the amphibolite protoliths with zircon U-Pb ages at (1) ca. 465-460 Ma and at (2) ca. 335-327 Ma. The trace element systematics are suitable with an origin of continental rift magmatism for the former and intra-oceanic arc magmatism for the latter type of amphibolites. This two-stage magmatism is interpreted to (1) mark the opening of the Rheic Ocean triggered by the slab roll-back of the Iapetus slab and (2) occur within the Rheic Ocean, located to the north of the SCT. Hornblende ⁴⁰Ar/³⁹Ar ages distinguished two stages of metamorphism for the amphibolites to occur at (1) ca. 326-324 Ma for those with Ordovician protoliths and at (2) ca. 318 Ma for those with middle Carboniferous protoliths. These ages display a direct correlation with the emplacement of the Sarıcakaya granites at ca. 327-324 Ma and diorites at ca. 318 Ma and reveal the contribution of plutonism to the metamorphic evolution of the CST basement. This last stage is interpreted to mark the closure of the Rheic Ocean and is synchronous to the Variscan Orogeny as defined from Northern America to Europe to Iran.