Subhorizontal veins and lenses of coarsegrained rock composed of plagioclase, augite, and Fe-Ti oxides in a glassy and vesicular mesostasis occur within several thick subaerial basalt flows in the Columbia River Basalt province of Washington and in the eastern North American Mesozoic basalt province. The veins and lenses, referred to as pegmatitic segregation veins, are typically 1-10 cm thick and are enriched in Fe, Ti, K, P, Cu, Zr, Ba, and REE by a factor of about 1.1 to 3 over the host basalt; Al, Ca, Sr, Mg, Ni, and Cr are lower, and Si and Na are typically unchanged.

We interpret the pegmatitic segregation veins as the product of residual melt carried in vapor bubbles from the lower crystallization front of partially crystallized flows to the crystal mush within the upper crystallization front. Movement through the elevated temperatures of flow interiors fused any crystal nuclei in the segregation melt. Diminished nucleation density during crystallization may partially account for coarse grain sizes. In addition, diffusion of ions to large, rapidly growing skeletal crystals was probably enhanced by the viscosity-reduction effect of water enrichment. The water content of accumulated segregation melt may have increased whenever the escape of volatiles from partially crystallized flows was temporarily sealed off under a rigid barrier of solidified basalt. Rupture of the barrier by downward propagation of columnar joints would allow the resumption of effervescence and any remaining partially crystallized segregation melt-phase would be degassed and quenched into a glassy and vesicular mesostasis.

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