The Giant Crater Lava Field: Geology and geochemistry of a compositionally zoned, high‐alumina basalt to basaltic andesite eruption at Medicine Lake Volcano, California

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Abstract
The Giant Crater lava field consists of >4 km3 of basaltic lava, compositionally zoned from first‐erupted calc‐alkaline basaltic 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 enrichments 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 processes involving fractional 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.