Igneous Rock Associations 31. Evolution of a Flood Basalt Magma Chamber: Grande Ronde Basalt, Columbia River Basalt Group, Pacific Northwest USA

The Grande Ronde Basalt (GRB) is the main volcanic phase of the Columbia River Basalt Group. It erupted in less than half a million years yet makes up more than 72% (150,000 km3) of this famous Cenozoic flood-basalt sequence. The Teepee Butte Member is one of the most representative components of the Grande Ronde Basalt. It includes extensive lava flows, a well-preserved ~100 km-long system of dykes and a vent complex. The vent complex consists of tephra, “Pele’s tears” (frozen lava droplets), bombs, shelly pāhoehoe flows, a lava lake and a feeder dyke. The Teepee Butte Member includes two high-MgO basalt flows with two intercalated low-MgO flows, collectively representing two cycles of high-MgO to low-MgO eruptions. The first high-MgO flow (Joseph Creek flow) was followed by a hiatus during which the magma chamber fractionated olivine, plagioclase and clinopyroxene (i.e. gabbroic fractionation) and later erupted a low-MgO flow and remnant lava lake. The first low-MgO flow, the Joseph Creek Lava Lake flow, advanced over 350 km nearly reaching the Columbia River Gorge. The magma chamber was subsequently recharged with a high-MgO magma which then erupted as the Pruitt Draw flow, which advanced nearly 500 km to the Portland Basin in Oregon. After the Pruitt Draw eruption waned, the remaining magma again fractionated and then erupted as a separate low-MgO flow termed the Dog Mountain flow. This also reached the Columbia River Gorge. The flows of the Teepee Butte Member exemplify multiple cycles of high-MgO - low-MgO lava flows that persisted throughout the eruption of the Grande Ronde Basalt. Field relationships and geochemical data suggest that the composition of the Grande Ronde Basalt magma chamber was relatively consistent over time, remaining close to the composition of the highest-MgO GRB flow, but periodically fractionated to produce low-MgO flows. We document at least ten such cycles of high-MgO to low-MgO flows within the GRB. Each cycle is characterized by recharge of high-MgO magma into the magma chamber and eruption followed by fractionation in the chamber, producing a low-MgO magma that subsequently erupted, prior to the next recharge event. Poor correlations between pairs of compatible and incompatible elements support a model of periodic recharge, tapping and fractionation. The low-MgO magma compositions that mark the end of each fractionation and recharge cycle are tightly clustered, suggesting that the low-MgO magmas represent the steady-state composition of the GRB magma system. Variations in Zr/Nb and Zr/Y through the 10 cycles are broadly consistent with gabbroic fractionation; however, they also suggest the presence of subtle heterogeneities within the mantle source region supplying the recharging magma. The end of the Grande Ronde Basalt eruptions is marked by a hiatus and compositional change of the magma source, the latter resulted in the eruption of the younger Fe- and Ti-rich flows of the Wanapum Basalt.