Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions. The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle ( 87Sr /86Sr ~ 0.7030 and 143Nd /144Nd ~ 0.51305 ) and old, chemically heterogeneous, isotopically enriched subcontinental mantle ( 87Sr /86Sr ~ 0.7078 and 143Nd /144Nd ~ 0.51233 ). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic 206Pb /204Pb ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.