THE BASALTS OF BUTTE 5206: GEOCHEMICAL EVIDENCE FOR FRACTIONAL CRYSTALLIZATION AND MAGMA MIXING TO PRODUCE EASTERN SNAKE RIVER PLAIN OLIVINE THOLEIITE BASALTS

Butte 5206, a small monogenetic olivine tholeiite basalt (OTB) shield, erupted < 200 k.y. ago near the southern end of the Arco-Big Southern Butte Volcanic Rift Zone in the eastern Snake River Plain (ESRP). Geochemical and petrographic data from Butte 5206 basalts and the most primitive observed ESRP composition (MgO = 11.4 wt %) were compared to data from the Graveyard Point Intrusion in the western SRP. Graveyard Point was interpreted by White (2007) to be a shallow layered mafic intrusion composed of ferrogabbros and ferrodiorites with likely genetic relations to SRP lava flows. Mass balance and thermodynamic models demonstrate that Butte 5206 magmas evolved by low pressure fractional crystallization of relatively primitive olivine tholeiite magmas at multiple crustal levels and by magma mixing in a shallow reservoir, similar to the Graveyard Point Intrusion, prior to eruption. This interpretation is consistent with earlier ESRP petrogenetic models that invoke varying amounts of fractional crystallization and assimilation at P ~ 2 to 8 kbar. Comparison of the Butte 5206 petrogenetic model to other SRP basalt groups demonstrates that variations of the model can be used to interpret the petrogenesis of other OTB compositions observed on the Snake River Plain.