Melting of the juvenile lower crust in a far-field response to roll-back of the southern Neotethyan oceanic lithosphere: the Oligocene adakitic dacites, NE Turkey

Late Cenozoic tectono-thermal events and associated magmatism in the Sakarya Zone (SZ) are still contested. Although documented in the western part of the SZ, thus far, no magmatic activity has been identified in the eastern part of the Oligocene SZ. Here, we report a newly identified Oligocene magmatism to interpret the genesis with tectonic setting and gain new insight into the geological evolution of the eastern SZ. We present extensive geochemical, bulk-rock SrNd and zircon Hf isotope, and zircon UPb chronological analyses for the Tepebaşı dacites in the Artvin area, NE Turkey. Zircon UPb dating analyses revealed a dacite formation age of ~29.8 ± 0.3 Ma. Geochemically, with a K2O/Na2O ratio of 0.5 to 0.6, they are composed of rocks of a medium-K calc-alkaline adakitic affinity. The samples are further characterized by low Y (6–7 ppm), and high Sr (362–588 ppm) and Sr/Y ratios (58–98), with low Mg# (41–45) values, demonstrating a close affinity with the crustal source of adakitic rocks. They have slightly radiogenic isotope concentrations (87Sr/86Sr(t) = 0.70460–0.70544, εNd(t) = +1.7 to +2.0), and single-stage Nd model ages of TDM1 = 0.61–0.63 Ga, as well as uniform and positive εHf (t) of 8.2–10.5, with young Hf depleted mantle ages (TDM1 = 0.31–0.41 Ga). These isotopic features, in combination with the geochemical signature, preclude a mantle origin. Instead, they most likely originated from a juvenile mafic lower crustal material by low degree partial melting (