The Wah Wah Springs Tuff and the Wah Wah Springs Intrusive Granodiorite Porphyry(Wah Wah Springs Intrusion) both originated from the Indian Peak caldera complex, which wasa major focus of explosive silicic activity in the middle Cenozoic Great Basin ignimbrite flareup. This caldera formed 30.0 Ma when an estimated 5,900 km3 of crystal-rich dacitic magma erupted to create the Wah Wah Springs Tuff. The Wah Wah Springs Intrusion later intruded the tuff, causing resurgence of the caldera. Field, modal, and geochemical evidence suggest the tuff and intrusion are cogenetic. The mineral assemblages of the two rocks are similar: both include similar proportions of plagioclase, quartz, hornblende, biotite, clinopyroxene, and Fe-Ti oxides, with trace amounts of titanite, apatite, and zircon. Whole rock geochemistry also matches, and both rocks have distinctively high Cr concentrations. Plagioclase, hornblende, and clinopyroxene have similar compositions but biotite and Fe-Ti oxides have been hydrothermally altered in the intrusion. Both hornblende and quartz provide clues to the magmatic evolution of the Wah Wah Springs Intrusion. Hornblende grains are either euhedral, have reaction rims, or are completely replaced by anhydrous minerals. Deterioration of hornblende was caused by decompression as the magma ascended and then stalled and solidified at shallow depths. Two stages of quartz growth are shown in cathodoluminescence (CL) imagery. Quartz first grew then was resorbed during eruption, then grew again at lower pressures indicated by CL-bright quartz rims and groundmass grains. The geochemical and mineralogical similarities, together with the distinctive hornblende and quartz characteristics suggest that after the Wah Wah Springs Tuff erupted, the unerupted mush rose to a shallow level where it crystallized at low pressure to form the Wah Wah Springs Intrusion. This indicates that the both rocks formed in the same chamber, and that tuffs and associated intrusions can be intimately related.

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.