Geochemistry of the Grande Ronde Basalt of the Columbia River Basalt Group; A reevaluation of source control and assimilation effects

The Grande Ronde Basalt (GRB) is the most voluminous formation of the Columbia River Basalt Group of the northwestern U.S. The trace-element geochemistry of a portion of the GRB (26 flows) from a single section in southeastern Washington was evaluated to determine the relative contributions of source and assimilant chemical effects. High-Mg, low-Mg and transitional compositions occur within the section. Stratigraphic compositional variations within the section are not regular. No progressive variation is seen: the various chemical types recur throughout the section. Although the stratigraphic variations resemble reversals produced during replenishment, multiple evolution lines seen in major- and trace-element diagrams indicate multiple magma batches. These probably erupted contemporaneously through independent(?) plumbing systems and interlayered to form a composite section. The high-Mg and transitional compositions are similar in terms of their respective (La/Sm) nvalues. The more evolved transitional lavas, however, tend to possess lower values of (La/Yb) nand (Tb/Yb) nand higher incompatible-element (e.g., La, Th, etc.) content than the less evolved high-Mg varieties. This characteristic is inconsistent with the two types being derived from a common mantle source or, if the difference in major elements—particularly MgO and CaO—is considered, with their being related through assimilation processes. Rather, the variations are thought to reflect differences in the mantle source regions. Consistent with this are the preliminary isotopic data, which suggest that the high-Mg lavas have higher 87Sr/ 86Sr than the transitional flows. Although the nature of the mantle source cannot be well constrained with the present data, the high rare-earth-element/high-field-strength-element ratios [(La) n/Ta = 50–85] and low Ta/Ta* values (0.27 to 0.41) suggest that enriched subcontinental lithosphere was involved in the production of the basalts. The low-Mg compositions trend away from the transitional compositions in all diagrams and show a marked decoupling of the high-field-strength element (e.g., Hf) and large-ion-lithophile element (e.g., Th and La). The low-Mg compositions define linear arrays in variation diagrams that have trajectories similar to that predicted for combined assimilation-fractional crystallization involving parental transitional compositions and silicic crustal rocks. Assimilation of approximately 10 to 20 percent by weight of such crust can accommodate the variation observed in the low-Mg compositions. Preliminary Sr ratios indicate variations from 0.70454 in the least evolved low-Mg lavas to >0.707 for the more evolved flows. Trace-element variations indicate that, similar to the high-Mg and transitional groups, the low-Mg compositions evolved along more than a single evolution path—more than one parental magma was involved.