MAGMA EVOLUTION AND ERUPTIVE HISTORY OF THREE FINGERED JACK: A DISSECTED MAFIC COMPOSITE VOLCANO IN THE CENTRAL OREGON HIGH CASCADES

The central High Cascades graben has been the center of mafic volcanism with ∼60 km³ of lava erupting from scoria cones and mafic composite cones in the past 15,000 years. One of these mafic composite cones is Three Fingered Jack, located south of Mount Jefferson, near the town of Sisters, Oregon. Three Fingered Jack is composed of porphyritic calc-alkaline basaltic andesite (53.2 – 56.3 wt% SiO₂) lava flows, with up to 3mm plagioclase and olivine phenocrysts. Geochemical analysis of 40 samples from lava flows, dikes and intrusive plugs, show at least 2 different trends of magmatic evolution: 1) an increase in incompatible element concentration with magma evolution, which is consistent with fractional crystallization. 2) a decrease in incompatible element concentration with evolution, which is expected with magma mixing or crustal contamination.

Prior studies have highlighted two types of basaltic andesites in the central High Cascades: 1) Mount Washington type, a medium-K basaltic andesite that is incompatible element richer. 2) North Sister type, a low-K basaltic andesite that is incompatible element poor. At Three Fingered Jack, compositionally contrasting basaltic andesites of the two trends are analogous to these regionally occurring High Cascades basaltic andesite types. However, based on incompatible element concentrations, the basaltic andesites at Three Fingered Jack follow more pronounced fractional crystallization and magma mixing trends than elsewhere.

Three Fingered Jack has been deeply dissected from glaciation, exposing much of its eruptive stratigraphy. Complete sequences of stacked, thin lava flows, are exposed on the southern and eastern flanks. The core of the edifice is highly stratified with layers of pyroclastic flows, scoria deposits and lava flows. The edifice core is cut by numerous dikes that are fed from intrusive plugs. Presence of hyaloclastite layers and highly vesicular scoria at lower stratigraphic levels suggest phreatomagmatic to magmatic eruptions early on. A goal of this study is to place occurrence of different basaltic andesites in a temporal stratigraphic framework, to better understand the eruptive history of Three Fingered Jack. This may in turn provide insights into the eruptive patterns of other intra-arc mafic composite cones in the central High Cascades graben.