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Geochemistry of Tertiary basic-intermediate volcanic rocks from S Danesfahan and S Karaj indicates two distinct rock series. S Danesfahan basic rocks are low in Ti (LT series), bearing the calc-alkaline signature of island arcs, while S Karaj basic rocks are high in Ti (HT series), representing midalkaline signature of oceanic islands. Geochemical characteristics of LT series volcanic rocks are as follows; low abundances of high field strength elements, HFSE, such as Ti, Nb and Ta and high abundances of large ion lithophile elements, LILE, such as K, Ba and Sr. This is consistent with attributing LT series volcanic rocks to the subduction of Neotethyan oceanic slab toward north in Eocene. LT series rocks are, therefore, magmatic products of a metasomatised lithospheric mantle wedge. Fluids released by the subducting oceanic slab, caused this metasomatism. On the other hand, contemporaneous, rather basic volcanics obtained from S Karaj , HT series, bear the characteristics of a deeper mantle source. S Karaj rocks, compared with the S Danesfahan’s, are enriched in trace elements particularly Ta and Nb. Trace element abundances and patterns of S Karaj volcanics are similar to the oceanic island basalts and imply the involvement of an asthenospheric (plume) mantle.