Pre-Eruptive Conditions of the Oligocene Wah Wah Springs Tuff, Southeastern Great Basin Ignimbrite Province

• Main Author: Woolf, Kurtus Steven
• Format: Others
• Published: BYU ScholarsArchive 2008
• Subjects: dacite tuff; Indian Peak Volcanic Field; monotonous intermediate; rejuvenation; Wah Wah Springs Tuff; Geology
• Online Access: https://scholarsarchive.byu.edu/etd/1568; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=2567&context=etd

The Wah Wah Springs Tuff (30.0 Ma) is one of several very large volume ash-flow tuffs (>3200 km³ of erupted magma) that were emplaced near the peak of the flare-up of activity in the Great Basin ignimbrite province of western North America. It can be characterized as a "monotonous intermediate" ignimbrite because of its intermediate concentrations of silica (~63 to ~70 wt. %), apparent uniform chemical and mineralogical characteristics, and crystal-rich nature (32 ± 10 % phenocrysts on a dense rock basis). The major phase assemblage found throughout deposit is similar to other monotonous intermediates with a few exceptions (pl > hnbl > bio, qtz >> cpx, opx > mt, ilm, ap, zcn, and po). Based on experiments on the monotonous intermediate Fish Canyon Tuff (Johnson & Rutherford, 1989a) and this phase assemblage, the Wah Wah Springs magma equilibrated between 775°C – 800°C. One hornblende-plagioclase thermometer with or without quartz (Holland & Blundy, 1994) and one Fe-Ti oxides thermometer (Anderson et al., 1993) most consistently yield temperatures within this range. The Fe-Ti oxides oxy barometer (Anderson et al., 1993) yield fO2 estimates 2 – 3 log units above the quartz-fayalite-magnetite buffer. The Al-in-hornblende geobarometer (Johnson & Rutherford 1989b) indicates pressures between 2.0 and 2.5 kb. Detailed compositional profiles across hornblende and plagioclase grains help constrain how intensive parameters changed during the evolution of the magma shortly before eruption. Plagioclase in the Wah Wah Springs displays oscillatory zonation with overall normal zonation (a maximum change of about An5 from core to rim). Hornblende is also zoned in Al2O3 and TiO2 which typically decrease as much as 2.5 wt. % and 1 wt. % respectively from core to rim. These zoning patterns are consistent with a decrease of temperature from core to rim that accompanied progressive crystallization of a large body of magma that closely approached equilibrium. These conditions in the parent magma for the Wah Wah Springs differ from interpretations of mineral compositions in the Fish Canyon Tuff which led Bachman et al., (2002) to propose that the near solidus magma body was "rejuvenated" or reheated immediately prior to eruption. This model cannot be applied to the Wah Wah Springs. Rather, the Wah Wah Springs magma appears to have been cooling and crystallizing prior to eruption.