Oligocene magmatism in the eastern Sakarya Zone, NE Turkey: Geodynamic implications

During the Cenozoic, the eastern Sakarya Zone, northeastern Turkey, has witnessed voluminous Eocene and subsidiary Miocene magmatism. By contrast, except in one previous study, Oligocene magmatism has not been reported. Here, we present new data about an Oligocene dyke swarm from the Artvin district in the easternmost part of the Sakarya Zone. Our study improves our understanding of the Oligocene magmatic and geodynamic evolution of the central Tethyan orogenic belt.

The Oligocene dykes consist of two distinct rock types: quartz-bearing trachytic and adakitic trachyandesite/andesite, dated at 31.0 ± 0.2 Ma and 31.1 ± 0.2 Ma to 29.2 ± 0.3 Ma by U-Pb zircon geochronology, respectively. Geochemical analysis reveals enrichment in LREE and MREE for quartz-bearing trachytic dykes, whereas adakitic trachyandesite/andesite dykes show depletion in MREE and HREE. Radiogenic isotopic ratios (⁸⁷Sr/⁸⁶Sr_(i) and ¹⁴³Nd/¹⁴⁴Nd_(i)) and trace element data suggest that the source of Oligocene magmatism was linked to low degree (≈10–15 %) partial melting of a metasomatised lithospheric mantle previously enriched by slab-derived melts and fluids. During the magma ascent in the crust, parental magmas of the trachyandesite/andesite dykes have assimilated crustal rocks and undergone fractional crystallization dominated by amphibole. By contrast, quartz-bearing trachytic dykes have recorded plagioclase fractionation.

Local extension, in conjunction with continuous foundering of delaminated thickened lower crust beneath the eastern Sakarya Zone, which likely triggered asthenospheric mantle upwelling and resulted in sporadic Oligocene magmatism.

Our study highlights the distinct tectonic and magmatic setting of the rare Oligocene magmatism in the eastern Sakarya Zone compared to Oligocene magmatism in the Tauride-Anatolide and South Armenian blocks.