Volcanic activity at Mt. Karacadag, SE Turkey, developed between similar to 11 and similar to 0.01 Ma. In this paper we investigate the oldest products (older than 2.6 Ma) that created a large volcanic plateau and a N-S aligned volcanic edifice in the form of a shield volcano. These igneous rocks are mildly alkaline to transitional olivine-clinopyroxene phyric basalts with minor hawaiites, basanites and very rare differentiated lithologies (mugearites and benmoreites). The poor correlation of major elements with MgO in these lavas is qualitatively consistent with polybaric depths of magma production, variable degrees of partial melting (from similar to 2 to similar to 10 %), heterogeneous mantle sources and differences in the fractionating crystal assemblage. Primitive mantle-normalized patterns resemble typical anorogenic magma compositions, with peaks at HFSE (Nb, Ta, Hf, Zr) and high HFSE/LILE ratios. REE contents are compatible with derivation of the basanites from a mixed garnet-spinel facies peridotite after similar to 2% partial melting. Alkali basalts are compatible with higher degrees of melting (between 5 and 10 %) from the same type of source. Initial (87)Sr/(86)Sr ratios range from 0.70349 to 0.70522 while those of (143)Nd/(144)Nd range from 0.512853 to 0.512659. Early-stage lavas show higher (87)Sr/(86)Sr and lower (143)Nd/(144)Nd compared to plateau-stage lavas. The Sr-Nd isotopic variations and their relation with major and trace elements cannot be explained by AFC-like (Assimilation and Fractional Crystallization) processes involving average crustal lithologies. More likely, the Sr-Nd isotopic ratios are related to the existence of heterogeneous mantle sources with only minor involvement of AFC-like processes.