Petrology and petrogenesis of volcanic rocks in Iscehisar (Afyon) region

In this study, mineralogical-petrographical and geochemical characteristics of the Karakaya Volcanics of Iscehisar region, which is a part of the Anatolide-Tauride Block, is located at the Afyon zone were examined and the origin and processes which were effective in the formation of volcanism were tried to be clarified. In the investigation area, the basement is represented by Çobanlar schists and Iscehisar marble which is overlain by Seydiler Ignimbrites, occuring product of explosive volcanism. Karakaya Volcanics obduct uncomformably these units. Late Miocene-Early Pliocene aged conglomerate and sandstone which belongs to Gebeceler Formation deposited in stream-lacustrine environment and Quaternary aged alluvium unconformably overlie all former units. Volcanic products are composed of trachybasalt, basaltic-trachyandesite and rhyolitic pyroclastics in study area. Mineralogical and petrographical studies show that volcanic rocks consist predominantly of plagioclases, pyroxenes, biotites and oxide minerals. According to mineral chemistry studies Olivine is observed only in rocks with basaltic trachyandesite composition whereas quartz is observed in rhyolitic rocks. Plagioclases are observed as andesine, labradorite and bytownite while clinopyroxenes are seen as diopsite, augite in samples of trachyandesitic composition. The rocks show porphyritic, microlitic porphyritic, hypocrystalline porphyric, vitrophyric, glomeroporphyritic and trachytic textures. With the result of geochemical examinations, the volcanic rocks present in this region, are basaltic trachyandesite, trachyandesite and rhyolite and show alkaline and calcalkaline characteristics. The trends given by major and trace element versus SiO2 diagrams indicate the effect of fractional crystallisation processes in the formation of these volcanic rocks. Relatively high LIL elements (Ba, Rb, Sr and LRE) content relative to HFS elements (Nb, Hf, Ta, Zr,Ti), Nb/La<1.3 and high Th/Nb (0,33-2,74) ratio indicate that not only the fractional crystallization processes but also magma mixing, subduction and crustal contamination process was effective in the formation of Karakaya volcanic rocks in the region.