Basaltic andesite lavas belonging to the Grande Ronde Formation constitute ∼87% of the Columbia River Basalt Group. Although these lavas are chemically evolved, they generally contain <5% phenocrysts. Two hypotheses have been proposed to explain this observation. The first hypothesis suggests that Grande Ronde lavas were near-primary melts generated by large-scale melting of eclogite from the inferred Yellowstone plume. The alternative hypothesis proposes that Grande Ronde magmas were extremely hydrous and rose rapidly from the mantle such that the dissolved water kept the magmas close to their liquidi. We present new information on textures and mineral chemistry of phenocrysts in two well-characterized field sections through the Grande Ronde Formation. The new data lead us to conclude that at least some of the Grande Ronde primary magmas were highly hydrous; however, they all underwent extensive fractionation, magma mixing, and degassing in a shallow intrusive network prior to eruption as lava.

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