Magmatic evolution of emi̇rdağ (Afyon) volcanism

Volcanism that extends along a N-S-trending zone from Eskişehir to Afyon regions has contrasting features to those exposed in central and western Anatolia in terms of regional setting and geochemical characteristics. Volcanic exposures along the N-S lineament have been suggested to display younging from north to south. They have been dominated by alkaline and ultrapotassic rocks that were interpreted to have an anorogenic affinity. Previously published geochemical and mantle tomography data point out the generation of volcanism as a consequence of a slab tearing mechanism. Furthermore, the origin of NW-SE trending volcanic edifices is poorly known in this geodynamic context. In this study, petrography and whole-rock and Sr-Nd isotope geochemistry of Emirdağ and İscehisar volcanic units have been investigated and preliminary results have been presented. Emirdağ and İscehisar volcanic units overlie the Seydiler ignimbrite. Emirdağ volcanic unit consists of trachyte and trachydacite massive lava flows and the İscehisar volcanic unit is characterized by trachyte, basaltic trachyandesite, and minor trachydacite massive lava flows. Emirdağ trachytes and trachydacites contain quartz, sanidine, plagioclase, biotite, augite and kaersutite. İscehisar trachyandesites consist of sanidine, plagioclase, iddingsitised olivine, biotite and kaersutite while basaltic trachyandesites were formed by sanidine, plagioclase, augite and iddingsitized olivine phenocrysts. The majority of Emirdağ trachyte and trachydacite samples have subalkaline/calc-alkaline character, but two samples display transition to alkaline field owing to their high Na2O+K2O contents. All İscehisar volcanic samples alkaline character. Both volcanic units have shoshonitic character due to high K2O content. İscehisar volcanic unit display lamprophyre characteristics defined by low SiO2 (50.12-55.08 wt %), high Al2O3 (11.13-15.56 wt %), CaO (5.40-8.79 wt %), TiO2 (1.29-1.96 wt %), Pb (3-285 ppm) and Co (47-136 ppm). Harker variation diagrams of the Emirdağ and İscehisar volcanic units show contrasting geochemical features to each other. Calc-alkaline and alkaline samples have increasing Fe2O3, CaO, MgO, TiO2, P2O5 ve Rb, Th, U, Nb and decreasing Al2O3, Zr, V trends against increasing SiO2 content. Na2O decreases in İscehisar trachyandesites while it increases in other units. Moreover, K2O increases in İscehisar volcanic samples while it decreases in Emirdağ trachyte and trachyandesite samples. Trachyandesites of the İscehisar volcanic unit display strikingly fluctuated behaviour of Rb, Sr, Ba, Zr and Nb elements. Rb/Sr values range between 0.09 and 0.44 in the Emirdağ and İscehisar units. In primitive mantle normalized spidergrams, Emirdağ and İscehisar volcanic units have similar patterns to each other. Distinctive positive U and negative Nb, Pb, P and Ti in the İscehisar unit is noteworthy. In the chondrite-normalized spidergrams show that both units indicate a fractionation pattern from light rare earth elements (LREE) to heavy rare earth elements (HREE). LREE are enriched hundred times than HREE. Flat-lying patterns of HREEs Ho, Er, Tm, Yb and Lu suggest the presence of mantle contribution. Slightly negative Eu anomaly also implies plagioclase fractionation during the evolution of volcanic units. 87Sr/86Sr and 143Nd/144Nd ratios have been measured from a total of seven samples belonging to the Emirdağ and İscehisar volcanic units. Emirdağ and İscehisar volcanic units have 87Sr/86Sr and 143Nd/144Nd ratios of 0.706284 – 0.706790, 0.512463 – 0.512472 and 0.706527 – 0.707650, 0.512424 – 0.512464, respectively. All these results suggest that the Middle Miocene calc-alkaline and alkaline volcanism could have been associated with upwelling of strongly metasomatized lithospheric mantle owing to the extensional regime. Emirdağ and İscehisar volcanism might have been generated by mixing of strongly metasomatized mantle and lower crustal rocks and the following fractionation processes.