Research Information System
INTERNATIONAL GEOLOGY REVIEW, vol.43, no.7, pp.640-660, 2001 (SCI-Expanded)
Miocene volcanism of the Oglakci region (Sivrihisar, Eskisehir) in northwestern Central Anatolia, Turkey, is represented by basaltic and trachytic groups of rocks. Samples of both groups have been investigated using mineral-chemical 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 rhyolite. Both groups are of alkaline character. The basaltic rocks contain plagioclase (An(29-63)), alkali feldspar (Or(12-74)) olivine, orthopyroxene (En(64-67)), clinopyroxene (Wo(43-48)), biotite (Mg#(82-88)). and Fe-Ti oxide 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 phenocrysts. 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 basaltic 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 products of post-collisional magmatism and originated from a complex interplay of crustal assimilation, magma mixing, and fractional crystallization processes following the demise of Neotethys. Trace-element characters also are consistent 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 during the neotectonic evolution of Turkey.