>AbstractThe coastal northwestern part of the Sea of Okhotsk hosts local volcanic fields of olivine–two pyroxene andesites and basaltic andesites of the Kytyima volcanic complex, which were dated at about 48 ± 2 Ma (U–Pb, 40Ar/39Ar, and K-Ar methods). The age of the eruptions correlates with that of bottom sediments in the Sea of Okhotsk and may reflect local stages of extension on the continental margin of northeast Asia. The geochemistry of the lavas makes it possible to classify them as highly magnesian, calc-alkaline, moderately potassic volcanic series with clearly seen negative Nb, Ta, and Ti anomalies and with positive Sr and Pb ones. The isotopic composition of the rocks [εNd(T) = 2.4–5.5, 87Sr/86Sr(0) = 0.703415–0.704175] and the relatively “young” Cambrian Nd model ages of the Paleogene volcanic rocks indicate that the melts were derived from a mantle source depleted in radiogenic isotopes (presumably, fragments of the Cretaceous slab under the complexes of the Uda–Murgal ensimatic island arc), which later experienced fluid-assisted metasomatism and enrichment in trace incompatible elements. Variations in isotope composition and in concentrations of major and trace elements in the lavas were controlled by decompressional fractional crystallization without indications of any significant crustal contamination.