Multiple <600 ka 1-10 km³ explosive deposits are exposed east of the South Sister volcanic range in central Oregon. A comprehensive chemical and petrologic study of the crystal-poor (<10 vol%) andesites (57-63 wt% SiO₂) to rhyolites (>70 wt% SiO₂) of the 5-6 named tuffs has not been completed. This study aims to physically and chemically characterize the Tumalo Tuff, Bend Pumice, and Shevlin Park Tuff to better understand the conditions under which they formed and how the volcanism may have shifted from an extensional to arc source.
In the Columbia Canal, 10 km northwest of Bend, all three units are exposed in a 7.5 m thick outcrop. The physically distinct units range in bulk chemistry: the basal Tumalo Tuff is 69.5-70.2 wt% SiO₂, the overlying Shevlin Park Tuff is 60-61.5 wt% SiO₂, and the Bend Pumice is 64.3 wt% SiO₂. With the exception of the Tumalo Tuff, all the units in the Columbia Canal share similar rare earth element trends with a shallow europium anomaly.
SEM quantitative analyses of ilmenites and magnetites were used in conjunction with the thermometer of Ghiorso and Evans (2008) to calculate the temperature and oxygen fugacity of the samples. The Shevlin Park Tuff (TT-12A and TT-13A) gave average temperatures of 959 ±33°C and 934 ±33°C, and fO₂ of 0.25 ±0.10 and 0.27 ±0.13 ∆NNO, respectively. Tumalo Tuff samples show multiple compositional populations that give various temperatures and fO₂. One particular subset in most of the Tumalo samples yields temperatures ~700°C and fO₂ of ≤-0.22 ±0.62 ∆NNO. The nearby 2-2.3 ka rhyolitic domes of the South Sister volcanic field record Fe-Ti oxide temperatures ~850˚C and fO₂ ~0.7 ∆NNO (Stelten and Cooper, 2012). The stark differences between the Tumalo Tuff and South Sister rhyolites suggest that the magmas don’t share the same source. Unlike the South Sister complex and the Shevlin Park Tuff, which have discrete mineral compositions and display the chemistry and fugacity of a classic subduction zone, the Tumalo Tuff contains oxides that span a broad compositional range. The different oxide subsets which yield lower temperatures and fugacities indicate reducing conditions, which suggests an extensional environment. However, the populations with higher temperatures and fugacities give an arc-like signature. It is possible that a mixing mechanism during a period of transition from extension to subduction is responsible for these younger explosive deposits in the Central Cascade Arc.