Post-collisional miocene alkaline volcanism in the Oglakci region, Turkey:Petrology and geochemistry

Miocene volcanism of the Oglakci region (Sivrihisar, Eskisehir) in northwes tern Central Anatolia, Turkey, is represented by basaltic and trachytic gro ups of rocks. Samples of both groups have been investigated using mineral-c hemical data together with whole-rock major-, trace-element. and radiogenic Sr-Nd isotopic data. The basaltic volcanic rocks consist of mugearites and shoshonites, whereas the trachytic rocks include trachytes, latite, and rh yolite. Both groups are of alkaline character. The basaltic rocks contain p lagioclase (An(29-63)), alkali feldspar (Or(12-74)) olivine, orthopyroxene (En(64-67)), clinopyroxene (Wo(43-48)), biotite (Mg#(82-88)). and Fe-Ti oxi de phenocrysts, whereas the trachytic rocks contain plagioclase (An(21-64)) , alkali feldspar (Or(10-53)), clinopyroxene (Wo(41-49)), amphibole (Mg#(64 -83)), biotite (Mg#(79-85)), Fe-Ti oxide, titanite, apatite, and quartz phe nocrysts. The measured Sr-87/Sr-86 ratios of basaltic samples range from 0. 7045 to 0.7048, and those of trachytic samples from 0.7054 to 0.7056. The b asaltic samples have Nd-143/Nd-144 ratios ranging from 0.512753 to 0.512737 , and those of trachytic samples are 0.512713 to 0.512674. Isotopic, major- , and trace-element data suggest that the Oglakci volcanic rocks are produc ts of post-collisional magmatism and originated from a complex interplay of crustal assimilation, magma mixing, and fractional crystallization process es following the demise of Neotethys. Trace-element characters also are con sistent with an OIB-like mantle source. These volcanic rocks probably were associated with extensional tectonics, which occurred within the Anatolian plate as a result of collision of the Eurasian and Afro-Arabian plates duri ng the neotectonic evolution of Turkey.