Geologic mapping, petrologic and geochemical studies have confirmed the existence of a contemporaneous bimodal suite in the Mormon Mountain volcanic field, Arizona. This suite is composed of a transitional alkalic basalt series with high alumina affinities and a calc-alkalic silicic series. The basalts represent a complex interplay of partial melting of mantle pyrolite (.1 per cent water) at depths of 5 to 7 km and fractional crystallization. Two petrogenetic models are proposed for the calc-alkalic series, a co-genetic model and a non co-genetic model, least squares linear mixing calculations, phase chemistry and petrologic observations suggest that the calc-alkalic rocks may be related to the basalts by fractional crystallization of undersaturated amphibole from a hydrous alkali basalt (co-genetic relation). An alternative non co-genetic origin for the calc-alkalic series involves the melting of lower crust material (gabbro or norite). These petrogenetic models can be tectonically related to the Cenozoic activity of the East Pacific Rise and the probable sub-oceanic mantle characteristics of the Basin and Range province.