Palaeogene and Neogene volcanic rocks are widespread on the Biga Peninsula of Northwest Turkey. These rocks were formed during the Eocene, Oligocene-Miocene, and late Miocene, and the early Miocene Sapc volcanic rocks in the Balkesir area consist of andesitic lava flows and associated pyroclastics. Temperatures, pressures, and oxygen fugacities calculated for the hornblendes in these andesitic rocks are 903-930 degrees C, 3.3-4.8 kbar, and -9.91 to -11.88, respectively, and for the biotites they are 755-788 degrees C, 1.30-1.74 kbar, and -14.88 to -13.98, respectively. SHRIMP U-Pb dating of zircons from three andesite samples gave ages of 22.72 +/- 0.19, 22.97 +/- 0.23, and 18.72 +/- 0.17Ma (early Miocene), and these are regarded as crystallization ages. Geochemical analyses show that the volcanic rocks are mainly high-K and calc-alkaline, and have high contents of large-ion lithophile elements and low contents of high-field strength elements, revealing that they evolved from parental magmas that were derived from an enriched subcontinental lithospheric mantle source. The chondrite-normalized rare earth element patterns of the rocks are concave upwards with La-CN/Lu-CN=11.9-21.2 and Eu-CN/Eu*=0.84-0.92, implying significant fractional crystallization of hornblende during their evolution. According to the petrological data with regional geology, Neogene magmatic activity on the Biga Peninsula has a post-collisional feature, and was closely related to slab break-off geodynamic model after collision of Tauride-Anatolide Block and Sakarya continent.