Abstract: In this study, it is aimed to determine petrographical and petrological and sources characteristics of theŞapçı volcanics. The Biga Peninsula (NW Turkey) located in Sakarya Zone is an important belt where collisional magmatism is common and magmatic-tectonic events are seen together. The geology of theBiga Peninsula is so complex that there are various metamorphic, sedimentary, and magmatic rocks fromPalaezoic to Pliocene. The oldest unit in the study area is Late Cretaceous aged Bornova Flysch. LateCretaceous aged Yayla Melange overlies tectonically this unit. These units are cut by Hallaçlar volcanics.Hallaçlar volcanics are also cut by Early Miocene aged Şapçı volcanics.The Şapçı volcanics are composed of andesite and pyroclastics with microlitic, microlitic porphyric,hyalomicrolitic, fluidal, and sieve textures. The volcanics contain mainly plagioclase, amphibole,biotite, and opaque oxides, and accessory apatite and zircon. Petrochemically, the Şapçı volcanics havemedium-K, calc-alkaline in character, and show large ion lithophile elements (LILE) enrichment onN-MORB normalized spider diagrams. On chondrite-normalized rare earth element plots, the volcanicrocks show a concave upward shape with LaCN/LuCN=7.16-21.84. These patterns and Harker variationplots suggest that amphibole and plagioclase fractional crystallization were played a signicant role inthe evolution of the studied volcanics. The studied Şapçı volcanics represent post-collisional setting, andtheir parent magma has derived from enriched mantle.