JOSEPH CREEK FLOW, GRANDE RONDE BASALT – AN EXAMPLE OF A COMPLEX COMPOSITIONAL EVOLUTION THROUGH TIME

The Joseph Creek flow of the Teepee Butte Member, one of the oldest flows of the Grande Ronde Basalt, is well exposed in SE Washington, NE Oregon, and W Idaho, and can be traced westward across the Columbia Basin. A vent-dike complex for this flow is exposed in Joseph Canyon in SE WA. The vent is about 1 kilometer in cross section and 30 meters high. Fountaining preserved pristine spatter, Pele's tears, and bombs totaling over 20 meters thick. Fountaining was coeval with an eruption of over 3,500 km³ of tholeiitic basalt. Magma withdrawal at the end of the eruption is evidenced by flow-back breccia and extensional faults in the spatter near the dike margin. A lava lake is preserved in the widened dike below the vent. Although the majority of the flow is characterized by relatively high MgO (>5.0 wt.%), Cr (100 ppm), Ni (40 ppm)and TiO₂ (>2.3wt.%), and lower SiO₂ (~52 wt.%), the flow preserved a complex record of the evolution of its composition through time. The initial phase of the eruption is record in the selvage along the margins of the dike and is highly evolved compared to the flow. Following nearly 3,500 km³ of lava constituting the main phase, the final eruptive phase is also compositionally evolved compared to the main phase (MgO-3.5 wt.%, SiO² -55wt.%, Cr-5 ppm, Ni-1 ppm),. The final event of the eruption is recorded in the lava pond underlying the vent. The composition of the lava pond is one of the most evolved compositions observed in the entire Grande Ronde Basalt and similar to the lava pond. The change in the composition through the course of the eruption suggests that the magma chamber that fed the flow underwent a complex evolution after each major eruption, probably consisting of fractional crystallization (up to 35% in this case) and assimilation as most studies have suggested (e.g. Wolff et al., GSA SP 497).