SEGREGATION STRUCTURES IN DIKTYTAXITIC BASALT FLOWS OF NE OREGON

Reconnaissance mapping of the volcanic highlands east of the Grande Ronde Valley in northeastern Oregon identified two units of vesicular diktytaxitic basalt flows. The lava flows host segregation structures such as vesicle cylinders and pods that are assumed associated with emplacement of inflated vapor-rich lavas. Petrographic, geochemical and SEM analyses are being used to compare the characteristics of these basaltic rocks to previously described flows of similar texture with segregation structures.

The two units vary in age, petrology and regional extent. The oldest diktytaxitic flow is part of the plagioclase and olivine phyric tholeiitic Imnaha Formation (~16.1 my) of the Columbia River Basalt Group (CRBG). Thus far, mapping has revealed one flow in the Imnaha with the diktytaxitic texture and associated segregation vesicles. Segregation structures include vesicle cylinders and diapiric pods. In post-CRBG calc-alkaline lava flows of the Powder River Volcanic Field, the entire 14.4 Ma olivine phyric basalt unit, consisting of 3-6 flows, has been summarily described as diktytaxitic and is now recognized to contain vesicle cylinders, vesicle pods, and vesicle sheets.

Some olivine crystals with high-temperature Iddingsite alteration zones are mantled, in varying degrees, by fresh olivine rims in the host basalt and segregation structures in the two units. The texture of the olivine basalt is coarsely porphyritic with phenocrysts in a subophitic matrix of augite, plagioclase, olivine, Fe-Ti oxides, and glass. Degree of alteration of olivine crystals in the matrix is relatively inversely proportional to size. Imnaha samples contain less modal olivine, are less coarsely olivine phyric, and exhibit nearly complete alteration of crystals. Although both units show decreased silica from matrix to vesicle cylinder, incompatible elements suggest magmatic differentiation. There is an unexpected consistency in texture, modal proportion of phases and degree of olivine alteration between the matrix and vesicle cylinder in the Imnaha basalt.

While both units are considered enriched in water vapor, as shown by the diktytaxitic texture and the presence of segregation structures, they took different paths of post-emplacement differentiation, as noted by the mineralogy of the segregation structures.