TWO-STAGE MODEL FOR YELLOWSTONE PLUME EMPLACEMENT AND EVOLUTION OF THE COLUMBIA RIVER FLOOD BASALT

Current thinking supports the notion of an offshore Yellowstone hotspot (YHS) that entered the Cascadia trench at ~45 Ma [1], thus trapping the YHS plume beneath the Farallon slab. Continued flux from the active plume tail resulted in the development of a relatively small plume head that grew until slab rupture allowed its rapid ascent, decompressional melting, and eruption of the main-phase Columbia River Basalt Group (CRBG) from ~17–16 Ma [2,3,4]. Several questions arise from this poorly understood event, not the least of which is the geographic misfit between the projected YHS in southwestern Idaho at ~17–16 Ma, well east of the coeval CRBG dike swarms. To help resolve such questions, we describe a two-stage conceptual model for the CRBG main-phase based on chronostratigraphic, petrochemical, and geophysical constraints. Stage I is defined by the eruption of Steens and Picture Gorge Basalts from ~17–16.6 Ma. Their source components are consistent with the broad westward spreading of a distal plume source strongly depleted by the entrainment of ambient mantle. The volume of Steens and Picture Gorge Basalt began to wane during Stage II, when fissure eruptions began along the Chief Joseph dike swarm (CJDS) to generate the Imnaha, Grande Ronde, and Wanapum Basalts from ~16.6 to 15.9 Ma. Imnaha Basalt was derived from a proximal plume source least diluted by ambient mantle, which we attribute to channelized plume flow beneath the Western Snake River Plain (WSRP), in close proximity to the coeval YHS. Gravity and magnetic data define the WSRP as a region of extreme mafic magmatism, with an elongated intrusive complex that cuts across the southern Idaho Batholith and terminates at the southernmost extent of the CJDS. It therefore appears to meet the best-fit isotopic and trace-element model for Grande Ronde Basalt genesis from the differentiation of Imnaha Basalt in a magma chamber(s) capable of assimilating rocks of the Idaho batholith [5]. A steady state of material and thermal flux promoted assimilation and provided the high magma overpressure necessary for northward dike propagation into the CJDS.

[1] Wells et al., Geosphere 10, 692–719 (2014) [2] Coble & Mahood, Geology 40, 655–658 (2012) [3] Camp & Wells, GSA Today 31, 4–10 (2021) [4] Obrebski et al., Geophys. Res. Let. 37, L14305 (2010) [5] Wolff and Ramos GSA Spec. Paper 497 (2013)