Evidence for magma recharge and assimilation in the Picture Gorge Basalt Subgroup, Columbia River Basalt Group

A plausible parental magma for many flows of Picture Gorge Basalt Subgroup (PGBS) is represented by two high-Mg dikes in the Monument dike swarm. The dikes bear strong resemblance to primitive aluminous Mid-ocean Ridge Basalts (MORB), which have been interpreted as near-primary mantle melts. However, enrichment of large-ion-lithophile (LIL) elements in the dikes relative to MORB and the occurrence of a mixed phenocryst assemblage imply magma mixing and/or contamination. Alternatively, the LIL enrichment may be explained by an enriched mantle source. Six members and chemical types of Dayville Basalt, consisting of nine flows, were selected for this study for the overall similarity of their spider diagram signatures to that of the high-Mg dike average and for their wide range in composition, which spans much of the variation seen in the PGBS. Other flows of PGBS may display slightly different spider diagram patterns, an observation that suggests a different evolutionary history for them. Major-element modeling of the compositional variation between the high-Mg dikes and evolved flows requires a phenocryst assemblage consisting of 55 percent plagioclase, 25 percent clinopyroxene, and 20 percent olivine. Clinopyroxene is not a common phenocryst phase, nor would it be expected as one in these magmas if crystallization were at low pressure. However, the local occurrence of high-Al clinopyroxene phenocrysts is consistent with crystallization at higher pressure and resorption of entrained phenocrysts during storage at lower pressure. The occurrence of plagioclase and olivine showing evidence of disequilibrium in the flows suggests that magma mixing also played a role. Variable enrichment of most trace elements, relative to the high-Mg dikes, is closely related to bulk distribution coefficient and is interpreted as due to open-system differentiation involving magma recharge. “Over-enrichment” of the LIL elements necessitates an additional process, probably assimilation of crustal or granitic rocks that are isotopically similar to PGBS magmas. Trace-element concentrations in the Johnny Cake Member can be achieved using a magma recharge model with the high-Mg dikes as parental magma and a composite sample from the Wallowa batholith as assimilant. The other members and chemical types presumably reflect intermediate stages in the magma recharge system. For the chosen parameters of the model, approximately 23 recharge cycles are required to arrive at trace-element concentrations in the Johnny Cake Member.