Magmatic Differentiation in a Basaltic Dike, Northeastern Oregon

Della Mosier

Department of Geology, Whitman College, Walla Walla, WA 99362

     XRF analysis of samples taken from a compound dike contradict previous theories describing the "outward-in" cooling behavior of the Columbia River Basalt Group. Major, minor, and trace element concentrations plotted against increasing distance from the dike-bedrock contact indicate contamination of the original injected body. Nine chemical variation patterns are observed for the 22 elements that were analyzed. Two trends exhibited by eight elements may indicate contamination through assimilation of bedrock. The other seven trends indicate multiple injective events before cooling of the emplaced body. Further analysis of petrographic textures in thin section reveal unusually high percentages of opaques near the contact. Phenocryst concentrations can be related to variations in some elements due to fractionation. No one model however, can completely explain the contradictory nature of the linear and varying trends. As a result, the careful description of trends is essential to the accuracy of further study of these variations.


Return to the Geology Department homepage
Return to the Whitman College homepage