The dome lava of Santiaguito is a soda-rich dacite of the calc-alkalic suite. This lava has differentiated quite significantly from the overwhelmingly abundant pyroxene andesite magma which makes up Santa María and the older volcanic rocks in the area. Trace element analyses of the lavas, along with major element data suggest a differentiation by fractional crystallization under constant or increasing PO2. Sr isotope determinations could not detect contamination of the lavas by radiogenic crustal material; the Sr87/Sr86 ratio in Santiaguito rock averages 0.7043. The Santiaguito lavas are tridymite-bearing hypersthene dacites, characterized by strongly zoned plagioclase phenocrysts and oxyhornblende.