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Abstract: Butte 5206, a small monogenetic olivine tholeiite basalt (OTB) shield in the Eastern Snake River Plain (ESRP), erupted during late-Pleistocene time along the Arco–Big Southern Butte Volcanic Rift Zone. Geochemical data from Butte 5206 and other basalts that erupted in the vicinity, including correlated samples from coreholes, are evaluated for possible mechanisms of magma evolution in the ESRP. Thermodynamic and mass balance models demonstrate that the evolution of Butte 5206 magmas reflect a two-stage process: (1) fractional crystallization of relatively primitive magma, possibly at multiple crustal levels, and (2) mixing of primitive and evolved liquids in a shallow reservoir prior to eruption. Application of this model to other basalts and local eruptive centers in the region suggest that similar processes are active during the petrogenesis of many ESRP basalts, and that the process of intrusion and mixing is cyclic throughout Pleistocene to Holocene time. [Copyright &y& Elsevier]