CENOZOIC BASALTIC VOLCANISM IN THE PACIFIC NORTHWEST

Subduction along the western margin of North America from the mid-Mesozoic on instigated several episodes of voluminous magmatism in the Pacific Northwest. The earliest phases include emplacement of the Idaho batholith in the late Cretaceous and the ignimbrite sweep that moved from northern Montana SW to central Nevada over the time period 50-35 Ma. By ~35 Ma, convergent margin volcanism had migrated west to near the current location of the Cascades, but intermediate to silicic composition volcanism was wide-spread across eastern Oregon, northern Nevada, and southwestern Idaho between 35-20 Ma. At 16.8 Ma, the volcanism switched to primarily bimodal basalt – rhyolite. The initial phase of this activity erupted huge volumes of flood basalt, most of which have quite evolved compositions, consistent with the establishment of large magma chambers in the mid- to lower-crust where primary magmas underwent crystal fractionation and crustal contamination before eruption. Few, if any, of the flood basalts have geochemical compositions suggestive of a plume source. The major volume of the flood basalt lavas has compositions indicative of shallow melting (e.g. high Al contents) of mantle metasomatized by the long history of subduction beneath this area (e.g. high Ba/Nb, high ²⁰⁷Pb/²⁰⁴Pb). As the volcanism waned by 14 Ma, the evolved flood basalts were gradually replaced by more primitive basalts erupted from scattered vents across most of eastern Oregon and northern-most Nevada. At the western margin, the trace element patterns of the basalts of the Cascades and Newberry show a large contribution from slab fluids released during subduction of the Juan de Fuca plate. Moving east, the imprint of subduction fluids decreases, but never vanishes. Some young lavas erupted around the Owyhee Plateau, along the Oregon-Idaho border, show elevated Ba/Nb, but the higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd of these lavas suggest either a contribution from the older lithosphere present in this area or that the subduction component is “fossil”, imposed on the lithospheric mantle sources of the lavas during earlier Cenozoic subduction beneath this region.