XENOLITH-BEARING DIKES OF THE GRANDE RONDE BASALT, COLUMBIA RIVER BASALT GROUP, SOUTHEAST WASHINGTON STATE

Two sets of xenolith-bearing dikes of Grande Ronde Basalt, Columbia River Basalt Group are exposed along the Snake River near Lower Granite Dam in southeastern Washington State. The dikes are aphyric and vary in width from 0.3 to >10 m; the larger dikes appear to represent multiple injections. Geochemical data indicate that the dikes are correlative with the Sentinel Bluffs and possibly the Umtanum Members.

Xenoliths occur sporadically in these dikes, but are particularly abundant in a narrow dike with the same chemical composition as an adjacent, and larger, Sentinel Bluffs dike. Contacts between the xenoliths and host basalts varied from sharp to diffuse suggesting a range of lava-xenolith interactions. Xenolith textures ranged from predominantly crystalline to glassy. Quartz, feldspar (plagioclase ± alkali feldspar), pyroxene, and glass were present in all xenoliths. Plagioclase feldspars varied from largely in tact to those displaying well-developed sieve- or spongy-textures. Alkali feldspar was identified in two xenoliths. Fine-grained, euhedral ortho- or clinopyroxene and/or pigeonite were common in the xenolith glasses adjacent to rounded quartz grains. Glasses in the xenoliths varied from colorless to brown; they were SiO₂-rich and largely peraluminous; Fe was present in most glasses (<2.2 at%); Mg was present in approx. half of the analyzed glasses (<2 at%); Ca was consistently subordinate to both K and Na. Normative feldspar compositions suggest that the glasses are largely granitic and trondhjemitic.

Variations in the proportions of normative quartz, albite, and orthoclase in the glasses mimic the shift seen in the ternary minimum in the Ab-Or-Q system, possibly suggesting that the glasses may have formed by the partial melting of granitic crustal material over a range of pressures. This, combined with the dominance of plagioclase and quartz in the xenoliths, suggest that the xenoliths represent samples of the crust beneath this part of the Columbia Plateau that were incorporated as the basaltic magma ascended towards the surface. It is noteworthy that the granodiorite of Granite Point, an outlier of the Idaho Batholith, is exposed 6-1/2 km southeast of the dikes. Feldspars from Granite Point show a compositional range that is similar to that seen in some of the xenoliths.