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Petrology of the andesitic volcanism within the Boyali flysch - nortern Turkey

Çakıroğlu, Remziye Ezgi
The Araç-Boyalı Flysch Basin, a foreland basin formed following the closure of the Intra-Pontide Ocean during the Late Cretaceous-Late Paleocene on the platform of the Sakarya Composite Terrane contains a large number of dykes and sills. Dykes that intrude the deformed flysch sediments and also the olistolits, are covered by the volcano-sedimentary rocks of Mid-Eocene age. Majority of the dykes contain chilled margins and flow structures characterized by large plagioclase phenocrystals and amigdules filled with calcite and zeolite. Among the flysch sediments, lava flows up to 50 m thickness include massive and pillow lavas, lava and pillow breccias. Pillow lavas within the olistostromes were formed by submarine volcanism and accumulated by gravitational sliding. The lavas are dominantly plagioclase and pyroxene phyric and a few include biotite. Lavas as well as the dykes are variably altered, most of them showing chloritization, epidotization and carbonatization. Dykes are dominantly andesitic, and range from sub alkaline to alkaline basalts. Majority of lava and dyke samples are of calc- alkaline character. Harker diagrams are in accordance with plagioclase, pyroxene and biotite fractionation, as well as fractionation of Fe-Ti oxides. Tectono-magmatic discrimination diagrams of lavas as well as the dykes are indicative for destructive plate margin volcanism. Regional geological constraints together with geochemical characteristics of the volcanic rocks are in favour of extension in continental back-arc setting within the Sakarya Composite Terrane above the N-ward subducting Izmir-Ankara oceanic lithosphere of Neotethys.