In north-central Oregon numerous small flows of alkali-olivine basalt occur in the Oligocene to early Miocene John Day Formation. Chemically, these basalts are characterized by relatively low SiO2 and K2O and very high TiO2 and iron. Fifteen analysed specimens (44 to 48 percent SiO2) have an average of 3.6 percent TiO2 and 15 percent total iron. The average composition of the Oregon basalts compares closely with the average hawaiite of the Hawaiian Islands, differing only in having slightly higher iron and slightly lower SiO2 and total alkalis. Closely associated flows of trachyandesite and quartz latite are chemically related to the basalts and probably formed by differentiation of an alkali-olivine basalt magma. Typical basalt specimens have 10 to 15 percent of modal olivine, interstitial alkali feldspar, and abundant clay minerals and chlorophaeite. Textures are subophitic or intersertal and phenocrysts are rare. Plagioclase laths are slightly zoned and range in composition from An68 to An44. Purplish-brown titaniferous augite is the only pyroxene, and ilmenite is the dominant opaque mineral. Distinct differences in composition and age, and the lack of transitional varieties indicate that these basalts are unrelated to the younger Columbia River basalts. They presumably represent a separate parent magma of alkalic affinity that was generated independently within the mantle. © 1969 Springer-Verlag.
