The Giant Crater lava field erupted high-alumina basalt (HAB), basalt, and basaltic andesite (BA) lavas about 10,500 years ago on the south flank of Medicine Lake volcano. Lava flows from several adjacent vents consitute a compositionally zoned eruption of ≈4.4 km3. The earliest lavas are the most evolved (53.5 wt% SiO2 and 6.5 wt% MgO), and latest lavas are the most primitive (48 wt% SiO2 and 10.5 wt% MgO). The evidence from major and trace element chemistry, isotopes and minearl chemistry in lavas and inclusions indicates that fractional crystallization and assimilation of a granitic crustal component played important roles in producing the observed compositional variations. Models that involve fractionation of primitive HAB, assimilation of granitic crust, replenishment of the magma reservoir by primitive HAB and mixing of these components (FARM) are presented for the development of the compositionally zoned eruption. In these models, extensive fractionation of HAB and melting of silicic crust occurs with little or no chemical interaction between fractionated basalt and melted crust. The melted granitic crust is produced by heat suppleid through fractionation of large volumes of basaltic magma. In an initial event, melted crust and highly fractionated basalt (ferrobasalt) mix with primitive HAB magma. Subsequent injections of primitive HAB mix with the ferrobasalt and the magma produced by the previous mixing event, and this sequence of mixing, reinjection, and mixing leads to the compositional zoning of the Giant Crater lava field. ¿American Geophysical Union 1991