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A field, petrologic, and geochemical study of the Callahan Lava Flow, a basaltic andesite for Medicine Lake Shield Volcano, California

The basaltic andesite Callahan flow at Medicine Lake volcano, California, exhibits variations in major elements, trace elements and radiogenic isotopic ratios. Two compositionally distinct plagioclase phenocryst populations occur in the flow, and disequilibrium textures are common. This study investigates the chemical variations and textural relations in terms of their petrologic origin. Four models are applied to the data; 1) partial melting of some combination of the subducted oceanic plate and the hydrated mantle above, 2) fractional crystallization from a basaltic parent, 3) combined fractional crystallization and assimilation, and 4) mixing between basaltic and rhyolitic magmas. The major element variations are best explained by a combined assimilation and fractional crystallization model (AFC), in which the ratio of rate of assimilation to rate of fractional crystallization is 1.5:1 (r). The incompatible trace element variations (Rb, Ba, and Zr) are consistent with r values greater than 1:1. The lack of distinct variation in 87/86 Sr ratios suggests that the assimilant closely resembled the parental liquid in terms of 87/86 Sr ratios. The (r) value suggested by the model is higher than would be predicted by simple thermal models of the AFC process at upper crustal conditions. Modeling the AFC process in the compositional, thermal boundary layer of a basaltic magma chamber permits a higher rate of assimilation.