Deep canyons cut by the Snake, Salmon, Grande Ronde, and Imnaha Rivers near the point where Washington, Oregon, and Idaho join expose >800 m of Grande Ronde Basalt flows. The Grande Ronde Basalt, erupted 16.5 to 14.5 m.y. B.P., makes up 85 vol % of the Columbia River Basalt Group. An area enclosing ∼ 1,000 km2 of the deep canyon country dominated by this basalt was mapped, and samples from 16 measured sections in the Grande Ronde Basalt were analyzed for major, minor, and trace elements. Three magnetostratigraphic units, R1, N1, and R2, were helpful in mapping the area.

Using physically distinct flows, the magnetostratigraphic units, and certain chemical variations, a detailed flow-by-flow stratigraphy was developed, and the lateral extent of many individual flows was traced. The number of flows and thickness of each stratigraphic unit decrease from west to east.

These tholeiitic flows are fine-grained and aphyric. Clinopyroxene and plagioclase are the predominant minerals, but olivine, orthopyroxene, pigeonite, and accessory minerals are minor but important phases. The flows are primarily of the low-Mg chemical type, with lesser amounts of the high-Mg chemical type. A cyclical trend in the over-all major- and trace-element chemistry through the stratigraphic section is dominated by a Ca-rich clinopyroxene chemistry. Rare-earth elements show significant light-over-heavy enrichment, but overall patterns remain similar from flow to flow. A small negative Eu anomaly is present in many flows.

Three models employing fractional crystallization schemes and one combining fractional crystallization and partial melting were tested using least-squares modeling. A petrogenetic model is proposed wherein partial melting of a heterogeneous, clinopyroxene-dominated mantle supplied magma that ascended to the crust, was temporarily stored, and recurrently erupted. During storage, new basaltic melt with slightly different isotopic and trace-element abundances was added and mixed into the liquid that remained after previous eruptions. Orthopyroxene, plagioclase, and, to a lesser extent, olivine fractionated from these temporarily stored liquids. Upon eruption, clinopyroxene and plagioclase with accessory minerals crystallized at the surface.

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