GEOCHEMISTRY AND TECTONICS OF NEOGENE ALKALINE VOLCANISM IN SIVRIHISAR (ESKISEHIR) IN NW ANATOLIA, TURKEY

Late Oligocene-early Miocene (~25 Ma) phonolites and trachytes occur as domes along the the Izmir–Ankara–Erzincan suture zone in the Sivrihisar-Eskisehir region of NW Anatolia. Flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid. Clinopyroxenes show compositional zoning, with aegirine (Na0.82–0.96Fe+30.68–0.83) rims and aegirine–augite cores (containing calcium, magnesium, and Fe+2). Some aegirine–augites are replaced by sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. Fine-grained trachytes include feldspar (oligoclase and sanidine) phenocrysts and clinopyroxene microphenocrysts within a groundmass composed largely of alkali feldspar microlites. Although there are some differences in their trace element patterns, phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a subduction-metasomatized lithospheric mantle melt source. High 87Sr/86Sr (0.706358–0.708052) and low 143Nd/144Nd (0.512546–0.512646) isotope concentrations of the alkaline lavas also reflect a mantle source that underwent metasomatism by subduction-derived fluids. Petrogenetic modeling suggests that the 25 Ma Sivrihisar alkaline lavas formed from magmas that were derived from a subduction-modified lithospheric mantle and that they experienced assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent. This Neogene alkaline volcanism in the Sivrihisar-Eskisehir area was part of a regional extensional magmatism, which developed as a result of slab breakoff-induced magmatism in NW Anatolia following the collision between the Sakarya and Anatolide-Tauride continental blocks in the early Eocene.