Magmatic Differentiation in a Basaltic Dike, Northeastern Oregon
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.
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