Analyses of Oregon Cascades basalts reveal two groups of High Cascades (9-0 Ma) lavas: (1) "enriched" basalts with high Ba, Sr, Ba/Nb, and Ba/La, and (2) "depleted" basalts with flat rare earth element patterns and relatively low Sr and Ba. Fluid mobile elements, such as B, are depleted in High Cascades basalts. The decoupling of B and Ba presents an interesting problem for volcanic arc magma generation, since both elements are believed to be controlled through fluid input from the subducting slab. Petrogenetic modeling reveals that most Cascade magmas can be derived by uniform. partial melting of variably metasomatized primitive or NMORB-source mantle or by varying degrees of melting of a mineralogically inhomogeneous metasomatized mantle. The decoupling of B and Ba may result from shallow dehydration reactions in the slab which release B into the overlying mantle, while Ba is retained in the slab and released during subsequent dehydration reactions.