The andesite and porphyritic dacite dykes in the Sakarya Zone of the Eastern Black Sea Region (Pontides), an ancient magmatic arc, cut through the Late Cretaceous volcanic rocks. Andesite and porphyritic dacite dykes show porphyritic texture. Andesites consist mainly of plagioclase and augite minerals and plagioclases show sieve texture and polysynthetic twinning. Opaque mineral formations are common on the marginal parts of augite minerals. Porphyritic dacites consist of quartz, plagioclase, amphibole and biotite minerals and the edges of quartz crystals are partially corroded. Plagioclases are commonly sericitized and less calcitized. As a result of hydrothermal alteration, amphibole decomposed to chlorite, carbonate (calcite and ankerite) and opaque minerals, and biotite to chlorite. The uniform orientations in the main oxide and trace element exchange diagrams and the states in the mineral separation diagrams show that fractional crystallization is effective in the development of dykes. It was determined that plagioclase and augite differentiation was effective in the development of andesite dykes, and hornblende and plagioclase differentiation in the development of porphyritic dacites. In the normalized trace element diagram according to enriched mid-ocean ridge basalts (E-MORB), there are negative Nb, P2O5 and TiO2 anomalies, and large ion-diameter elements (LILE) are more enriched than high-gravity elements (HFSE). LowNb/U and high La/Nb and Th/Nb ratios indicate that andesite and porphyritic dacite dykes are affected by continental crust contamination. As a result, it is thought that the dikes are derived from a metasomatized mantle source, a similar origin and related to subduction, towards the end of Late Cretaceous in the arc environment