Large volumes of trachyandesitic-dacitic lava flows and pyroclastics of Miocene age are associated with small volumes of alkali basalt lava flows in the Galatia volcanic province, northwest Central Anatolia, Turkey. The volcanism postdates continental collision, occurring in a transtensional tectonic setting associated with movement along the North Anatolian Fault zone. Major and trace element (including REE) and Sr-Nd isotope data and K-Ar ages for representative samples of mafic-intermediate volcanic rocks have been obtained from a series of localities within the province. The K-Ar age data indicate that alkali basalts were erupted during two distinct time periods in the Early Miocene (17-19 Ma) and Late Miocene (< 10 Ma). The two groups of basalts are inferred to have been derived from different mantle sources, based on their Sr-Nd isotope and geochemical characteristics. The Late Miocene basalts were derived from a more depleted mantle source than the Early Miocene basalts, which were generated by partial melting of an incompatible element enriched, subduction-modified, mantle source. The depleted source component is inferred to reside within the asthenosphere and has some affinities with the source of HIMU oceanic island basalts. On the basis of a comprehensive major and trace element and Nd-Sr isotope dataset for the intermediate-acid volcanics and the alkali basalts, it is possible to demonstrate a cogenetic relationship between the alkali basalts and the intermediate volcanics of Early Miocene age, involving fractional crystallisation and assimilation of a heterogeneous upper crustal component. (C) 1997 Elsevier Science B.V.