We compare two intraplate, Pliocene-Pleistocene volcanic fields in different tectonic settings—the central Basin and Range and the northwest Pacific Ocean. Both fields are characterized by widely scattered, small-volume, alkali basaltic volcanoes; within the fields, each volcano apparently originates from a separate, volatile-enriched parental melt from the upper mantle. There is no evidence at either field for locally anomalous heat flow or ongoing introduction of new fluids into the upper mantle such as might occur above a subducting slab. We conclude that the volcanic fields reflect deformation-driven collection of already existing partial melts in a heterogeneous upper mantle. Deformation-driven melt collection may be an important mechanism for other diffuse intraplate volcanic fields, and this is consistent with a tectonically controlled, low-flux end member for intraplate fields where magmatism is a passive response to regional deformation. Differences in the degree of fractionation and contamination between the two fields are inferred to be related to flexure-induced vertical variations in the orientation of principal stresses in the northwest Pacific Ocean, which cause stalling of ascending dikes in the lithosphere.

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