The Giant Crater lava field consists of >4 km3 of basaltic lava, compositionally zoned from first-erupted calc-alkaline andesite to last-erupted primitive high-alumina basalt. On the FeO¿/MgO (where FeO¿ is total Fe calculated as FeO) versus SiO2 discrimination diagram commonly used to distinguish tholeiitic from calc-alkaline series lavas the compositionally zoned eruption crosses from the tholeiitic field to the calc-alkaline field. The lavas erupted in a brief span of time about 10,500 years ago from several closely spaced vents on the south flank of Medicine Lake volcano in the southern Cascade Range. Six chemical-stratigraphic groups were mapped. Lower K2O, higher MgO groups always overlie higher K2O, lower MgO groups. Group 6 lavas erupted last and are aphyric, have high contents of MgO and Ni, and contain as little as 0.07% K2O. Group 1 lavas are porphyritic and have as much as 1.10% K2O. Major element contents of primitive group 6 Giant Crater basalt are very similar to a subset of primitive mid-ocean ridge basalts (MORB). Group 6 lava is more depleted in middle and heavy rare earth elements (REE) and Y than is primitive MORB, but it is enriched in large ion lithophile elements (LILE). These LILE enrichements may be a result of fluid from the subducting slab interacting with the mantle beneath Medicine Lake volcano. The group 6 REE pattern is parallel to the pattern of normal-type MORB, indicating a similar although perhaps more depleted mantle source. The location of Medicine Lake volcano in an extensional environment behind the volcanic front facilitates the rise of mantle-derived melts. Modification of the primitive group 6 basalt to more evolved compositions takes place in the upper crust by process involving fractinal crystallization and assimilation. The group 1 calc-alkaline Giant Crater basaltic andesite produced by these processes is similar to other Cascade basaltic andesites, implying that a similar high-alumina basalt may be parental. ¿American Geophysical Union 1991