Abstract
In NW Anatolia, Eocene-Oligocene post-collisional volcanism is represented by K-rich volcanic rocks, including high K calc-alkaline products. Magmatic rocks in the Dededağ area (Biga Peninsula, NW Turkey) can be classified into 6 different groups with lithostratigraphic and chronostratigraphic data using 40Ar/39Ar methods (Eocene-early Oligocene, 42.08 ± 0.09 to 29.02 ± 0.29 Ma). From the Eocene period, there are calc-alkaline Laledağ granodiorite, Beyçayır volcanics with andesite and dacite composition, and also Kızıldam volcanics with alkaline and basic character (basaltic trachyandesite, trachybasalt). Furthermore, in the Oligocene period, calc-alkaline volcanism started with Dededağ volcanics with dacite and rhyolite intermediate-acidic composition, continued with Kırcalar volcanics consisting of basalt and basaltic andesite, and Harmancık volcanics ranging from basalt to rhyolite. The calc-alkaline Çamyayla pluton with quartz monzonite composition intruded into Oligocene Dededağ and Harmancık volcanics in the region. Alkaline lavas in the region are classified as high-K and shoshonitic, and calc-alkaline lavas are medium-high-K. On N-type mid-ocean ridge basalt and chondrite-normalized trace element diagrams, enrichment was observed for all volcanic units with regard to large ion lithophile elements and light rare earth elements. High field strength elements and heavy rare earth elements were depleted in all volcanic units. These compositions reveal the participation of a mantle source that was enriched by subduction components. Using geochemical data, all the volcanic rocks differentiated as a result of magma mixing/replenishment and contamination/assimilation processes. Consequently, the source of volcanism is thought to derive from the compressional regime associated with collision during the Eocene-Oligocene, and from a previously-enriched lithospheric mantle source with subsequent effects of the extensional regime.
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