The Biga Peninsula, in the north-western part of Western Anatolia, is part of the Sakarya Zone of the Western Pontides and the tectonically overlying Ezine group. The basement rocks are intruded by the early Miocene Kestanbol Pluton and early to middle Miocene calc-alkaline to shoshonitic-ultrapotassic volcanic successions related to postcollisional continental extension. The Kestanbol Pluton mainly comprises monzonite and granodiorite and is cut by shoshonitic-ultrapotassic tephriphonolite dykes. Ar-40-Ar-39 ages of biotite (21.22 +/- 0.09 Ma) and leucite (22.21 +/- 0.07 Ma) crystals indicate that tephriphonolite dyke emplacement was coeval with the intrusion of the Kestanbol Pluton during the early Miocene (21.5 +/- 1.6, 22.8 +/- 0.2 Ma). The geochemical features of the tephriphonolite dykes suggest a phlogopite-bearing mantle source which may originate from a previously metasomatised subcontinental lithospheric mantle source. This mantle source shows the imprints of carbonate-reach oceanic sediment recycling and crustal material contamination processes, which evolved during northward subduction and closure of the northern branch of the Neo-Tethys Ocean beneath the Sakarya zone during the late Cretaceous to Eocene.