Spatio-temporal evolution of the main-phase Columbia River Basalt Group and its genetic link to the Yellowstone hotspot

Abstract The main-phase eruption of the Columbia River Basalt Group and related silicic rocks is described in a two-stage model. Stage I (ca. 17–16.6 Ma) generated Steens and Picture Gorge Basalts derived from melting of a distal plume source diluted by the entrainment of ambient depleted mantle. Ascent and decompressional melting of this mixed source occurred through one or more N-S ruptures in the Farallon slab, contemporaneous with rollback initiation and creation of the incipient Gorda plate. The eruption volume of Steens and Picture Gorge Basalt diminished during Stage II (ca. 16.6–15.9 Ma), when fissure eruptions began along the Chief Joseph dike swarm to generate Imnaha, Grande Ronde, and Wanapum Basalts. Imnaha Basalt is the product of fractional crystallization of an underplated primary magma derived from melting of a channelized plume source that was least diluted by ambient depleted mantle beneath the Western Snake River Plain (WSRP), lying in close proximity to the coeval Yellowstone hotspot. Gravity data defines the WSRP as a region of extreme mafic magmatism, i.e. an elongated intrusive complex that transects the southern Idaho Batholith and terminates at the southernmost end of the Chief Joseph dike swarm. This location conforms with the best-fit model for Grande Ronde Basalt genesis derived from the differentiation of Imnaha Basalt in a magma chamber(s) capable of assimilating rocks of the Idaho batholith. Focused, high magma flux into the WSRP intrusive complex promoted assimilation and provided the high magma overpressure necessary for northward dike propagation into the Chief Joseph dike swarm.