EVALUATING CRUSTAL CONTAMINATION IN CONTINENTAL BASALTS - THE ISOTOPIC COMPOSITION OF THE PICTURE GORGE BASALT OF THE COLUMBIA RIVER BASALTGROUP

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variat ion has been ascribed to multiple and heterogeneous mantle sources. Ba salts of the Miocene Columbia River Basalt Group in the Pacific Northw est have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effus ion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted t erranes in central Oregon, and earlier studies on these basalts provid ed no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation o f the Picture Gorge Basalt is very small, Sr-87/Sr-86 = 0.70307 - 0.70 371, epsilon(Nd) = +7.7 - +4.8. DeltaO-18 = +5.6 - +6.1, and Pb-206/Pb -204 = 18.80 - 18.91. Evaluation of the Picture Gorge compositional va riation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low Sr-87/Sr-86, high epsilon(Nd), and nonradiogenic Pb isotopic comp ositions. Basalts with C1 isotopic compositions have large MgO, Ni, an d Cr contents and mantle-like delta(O-18) = +5.6. C1 basalts have enri chments in Ba coupled with depletions in Nb and Ta. These characterist ics are best explained by derivation from a depleted mantle source whi ch has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific No rthwest. The second isotopic component has higher Sr-87/Sr-86 and delt aO-18, lower epsilon(Nd), and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd. and Pb isotopes with differentiation indicators such as decreasing Mg# , and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase e quilibrium and mineralogical constraints indicate that these compositi onal characteristics were inherited in the Picture Gorge magmas at cru stal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic com positions similar to accreted terrane crust found in the Pacific North west. Therefore, in spite of the disadvantages for crustal contaminati on and their narrow range in isotopic compositions, the process contro lling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basa ltic suites and careful consideration of these data must be made to te st for crustal contamination, before variation resulting from mantle h eterogeneity can be assessed.