Geochemistry of Neogene-Quaternary alkaline volcanism in western Anatolia, Turkey, and implications for the Aegean mantle

The NS-trending alkaline volcanic province in western Turkey shows an age progression from early Miocene in the north to Quaternary in the south, becoming progressively more potassic-ultrapotassic southward. In three distinct volcanic fields, Seyitgazi-Krka (SKV), Afyon-Suhut-Sandkl (ASSV), and Isparta-Golcuk-Bucak (IGBV), potassic (shoshonitic) and ultrapotassic rocks show close relationships in space and time. Basaltic trachyandesites-trachyandesites and coeval rhyolites-ignimbrites (21-17Ma) in the SKV represent the oldest phase of alkaline volcanism in the area. The younger potassic rocks (14-8Ma) of the ASSV to the south are represented by trachyte and trachyandesite, whereas the youngest potassic rocks (4.7-4 and 200-24Ka) of the IGBV yet farther south are trachytic, trachyandesitic, and rhyolitic in composition. Ultrapotassic rocks of all three fields are transitional between lamproitic and Roman-type, although lamproitic composition is dominant in the IGBV. Potassic and ultrapotassic rocks of the SKV and ASSV and the potassic rocks of the IGBV generally display similar trace element characteristics. They are all enriched in large ion lithophile elements and light rare earth elements with respect to high field strength elements and show negative Nb, Ta, and Ti anomalies. The first two groups are characterized by isotopic ratios of ⁸⁷Sr/ ⁸⁶Sr=0.705219-0.707450, ^εNd=6.3-0.5, ²⁰⁶Pb/²⁰⁴Pb=18.90-19.07, ²⁰⁷Pb/ ²⁰⁴Pb=15.65-15.85, and ²⁰⁸Pb/²⁰⁴Pb=39.14-39.63, resembling the isotopic compositions of high-K, calc-alkaline lavas in western Anatolia and the shoshonitic and evolved lamproitic rocks in Serbia. In contrast, ultrapotassic rocks of the IGBV are characterized by higher contents of CaO, MgO, Cr, and Ni, with a narrow range of SiO₂ (47-52wt%). These rocks are represented by nearly primitive lamproites and resemble the more primary ultrapotassic rocks of Serbia. Ultrapotassic rocks of the IGBV are characterized by more restricted ⁸⁷Sr/⁸⁶Sr=0.7035-0.7036, positive ^εNd=1.7-2.5, ²⁰⁶Pb/²⁰⁴Pb=18.7-19. 1, ²⁰⁷Pb/²⁰⁴Pb=15.69-15.75, and ²⁰⁸Pb/ ²⁰⁴Pb=39.03-39.29. These geochemical and isotopic features are consistent with subduction-driven crustal-sediment recycling within the upper mantle. Observed variations in the Sr-Nd-Pb isotopic signatures of the contemporaneous potassic and ultrapotassic rocks suggest melting of a heterogeneous lithospheric mantle source veined through metasomatism by previous subduction events. The within-plate geochemical and isotopic components appear late in the evolution of the IGBV ultrapotassic rocks. Isotopic differences from north to south are consistent with decreasing amounts of subduction-derived crustal components in the mantle and an increasing role of asthenospheric input through time. Combined with the depleted mantle model (TDM) age data, these observations suggest the occurrence of a vertically zoned mantle beneath southwest Anatolia and the Aegean region.