BASALTIC ANDESITE AND PALAGONITIC TUFF AT NORTH SISTER VOLCANO, OREGON HIGH CASCADES

Pleistocene North Sister Volcano of the Oregon High Cascades consists mainly of basaltic andesite and is the northernmost, oldest, and most mafic of the three major arc volcanoes comprising the Three Sisters. The cone of North Sister is composed of thin interlayered lava flows and scoria deposits cross cut by adundant dikes. Away from the cone, pyroclastic deposits give way to thick sequences of agglutinated lava flows. Lavas bear 10 to 40% phenocrysts dominated by plagioclase, which is followed by olivine (Fo 73.9 to 81.5), magnetite, orthopyroxene, rarely clinopyroxene and, in more silicic samples, rare amphibole. XRF analyses yield a restricted compositional range of 53 to 55 wt% SiO₂, and Mg# of 57.8 to 39.8. Ba/Nb coincides with regional calc-alkaline primitive basalts, rather than other primitive magma types, such as enriched OI-like basalts and depleted MOR-like basalts. A positive Eu anomaly, characteristic of North Sister basaltic andesite indicates that the magma type is not the direct result of fractionating calc-alkaline basalts. Low rare earth elements and Y distinguish North Sister from other basalt and basaltic andesite magma compositions in the region, which is consistent with Conrey and others’ (2001) interpretation of a deep crustal contamination history. A striking, 120m thick, yellow and red bedded palagonitic tuff unit on the eastern face of North Sister Volcano is intercalated with unaltered lavas and red scoria beds. Based on XRD and SEM analysis, the tuff’s glassy component is altered to nontronitic smectite, zeolites (chabazite and phillipsite), and a residual hydrated glass, concentrated in Fe, Ti, and Si (palagonite). Primary plagioclase and olivine crystals were unaffected by the alteration. The zeolite clay mineralogy restricts the temperature of palagonization to 60 to 75°C and neutral pH.