Magma Evolution and Eruptive History of Three Fingered Jack: A Mafic Composite Volcano in the Central High Cascades of Oregon

The central High Cascades graben in Oregon has been a center for mafic volcanism since its initiation. Three Fingered Jack is one of the larger mafic composite cones in the graben, located 20km south of Mount Jefferson. Three Fingered Jack erupted between 142.1 - 150.8ka, based on Ar40/Ar39 age dates. This study provides stratigraphic, compositional and petrographic evidence to understand the eruptive history of Three Fingered Jack and the processes involved in generating compositionally diverse basaltic andesites here. The basaltic andesites of Three Fingered Jack show three compositional trends of magma evolution: 1) enrichment, 2) depletion and 3) neutral trends. Based on compositional and petrographic data, we propose two different conceptual models for the petrogenesis of all Three Fingered Jack basaltic andesites. A single-source model proposes that a primitive LKT parent supplied magma that fractionated to produce the depleted basaltic andesites. This parent also supplied magma that underwent repeated fractionation and recharge to produce the enriched basaltic andesites. A two-source model suggests that a primitive LKT parent supplied magma that produced the depleted basaltic andesites. A second, primitive calc-alkaline basaltic parent supplied magma that fractionated to produce the enriched basaltic andesites. Petrographic similarities amongst compositional trends suggest that magma mixing occurred at near constant pressure and under similar H2O conditions. Three Fingered Jack has produced both enriched and depleted varieties of regionally occurring basaltic andesites. It shows compositional similarities and differences to other vents, acting as a piece in our overall understanding of basaltic andesite volcanism in central Oregon High Cascades.