Collision-related Otlakbasi volcanism to the east of Lake Van occurred by eruptions from extensional fissures. In this study, new Ar-Ar age data and major, trace and rare earth elements characteristics of the Otlakbasi basaltic volcanism are investigated. The Ar-Ar age data indicate that this basaltic volcanism erupted in Early Pliocene time in contrast to its previously known as Quaternary age. The volcanic products are basaltic composition and close to alkaline-subalkaline division line. Results of fractional crystallization accompanied by assimilation (AFC) modelling imply that fractional crystallisation can be negligible compared to crustal contamination in the evolution processes of the magma chamber and ratio of the crustal contamination to the fractional crystallisation (r values) varies from 0.1 to 0.35. Enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high strength field elements (HFSE) and behaviour of mobile elements (Ba and Th) reveal that the mantle source region of the Otlakbasi basaltic volcanism might have been enriched by melts that were derived from subducted sediments. Depletion in Rb and K elements of the fractionated-corrected samples indicate that amphibole and/or phlogopite could be presence in the mantle source. Results of partial melting model studies show that this basaltic volcanism can be predominantly produced by the melting of the spinel peridotite source. Metasomatized spinel bearing lithospheric mantle may be responsible for the eruption of the Otlakbasi basaltic volcanism.