Sanganguey Volcano, located in the northwestern segment of the Mexican Volcanic Belt, is a calc-alkaline stratovolcano that has erupted a chemically continuous suite of andesites and dacites during the past 200,000 yr. Alkalic volcanism, however, appears to have been predominant in the area since the Pliocene. Alkalic basalts and their derivatives erupted along extensional fractures trending northwest through Sanganguey, and continued to erupt through the flanks of the stratocone following cessation of calc-alkaline activity. The calc-alkaline andesite-dacite volcanism was focused beneath Sanganguey and appears to have been controlled by crystal fractionation in a shallow magma chamber. No basalts that erupted in the area have chemical compositions that could be parental to the Sanganguey calc-alkaline suite. The most basic calc-alkaline rocks (basaltic andesites) have incompatible trace-element concentrations nearly identical to the most basic alkaline rocks, implying that the two suites were derived from distinct parental magmas. Alternatively, it is suggested that stagnation of mantle-derived alkalic basalts provided heat input to mobilize lower crustal rocks, resulting in the generation of calc-alkaline magmas.