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The Nil&uuml / fer Unit of the Karakaya Complex at the pre-Liassic basement of the Sakarya Composite Terrane is composed mainly of metabasaltic lithologies with limestones, and minor cherts and mudstones. These metabasic assemblages show OIB- and E-MORB-type geochemical signatures with variable enrichment in the most incompatible elements relative to N-MORB. The Eymir Unit consists of variably deformed metaclastics, and constitutes the matrix in which the Nil&uuml / fer-type blocks are embedded. In the Ankara region, the Eymir Unit is intruded by metadiabase dikes that display intra-oceanic SSZ-type signatures with a marked negative Nb anomaly combined with a slightly depleted HFSE budget relative to N-MORB. The wide range in trace element ratios displayed by the Nil&uuml / fer metabasic rocks can be explained by melt-mixing processes that has taken place within the spinel-garnet transition zone. Pb-Nd-Hf radiogenic isotope systematics reveal that the Nil&uuml / fer samples has been derived from enriched mantle sources, and a multi-component source mixing is required to explain their genesis. The Eymir metadiabases, however, require contribution from a sediment component that mixes with a depleted mantle source. The geochemical data when combined with the geological and petrographical observations suggest that the Nil&uuml / fer metabasic rocks represent ancient oceanic islands that were created by a heterogeneous mantle plume rising beneath the Palaeotethyan oceanic lithosphere. During the latest Triassic, these oceanic islands were incorporated into a subduction/accretion prism, where they mixed with the continental-derived assemblages of diverse origin, creating the Karakaya Complex. The intrusion of the SSZ-type metadiabases postdates the formation and deformation of the Complex.