O RHYOLITE PRODUCTION IN THE CENTRAL SNAKE RIVER PLAIN

Two models have been proposed for the origin of the low-δ¹⁸O rhyolites of the Snake River Plain – Yellowstone hotspot trend. We critically evaluate the plausibility of each and provide new thermal and volume constraints which must be included in any model for the petrogenesis of low-δ¹⁸O rhyolites of the central Snake River Plain (CSRP-OH). The first model, developed for the <6 Ma Yellowstone Volcanic Plateau and Heise systems (YVP-H), involves either single multicyclic caldera systems, or multiple nested or overlapping systems. In the YVP-H model, the early-erupted products become hydrothermally altered by later volcanic activity, and are subsequently melted to form low-δ¹⁸O rhyolites (Bindeman et al., 2007, Geology, v35, pp 1019-1022). The second model, developed for the ~6 – 14.5 Ma CSRP-OH rhyolites, appeals to a crustal magma source which was hydrothermally altered in a thermal event significantly prior to the onset of CSRP-OH magmatism (Boroughs et al., 2005, Geology, v33, pp 821-824).

Water-rock ratio calculations (0.5 – 2.5) and thermal modeling indicates that hydrothermal alteration of the CSRP-OH source requires an increased thermal input of 40% - 280% over that necessary to produce rhyolitic melts from ambient temperature crust. In the YVP-H sytem, the necessary water-rock ratios are considerably smaller (0.1 – 0.4) due to the lower degree of overall ¹⁸O-depletion, and therefore the additional thermal budget for alteration in the YVP-H is only 10-30% greater than that needed for rhyolite production. Although the models of Boroughs et al. and Bindeman et al. are not mutually exclusive, for the CSRP-OH specifically, we conclude that a component of pre-existing low-δ¹⁸O crust is required in order to shift the thermal burden for such extensive hydrothermal alteration to an earlier magmatic episode. We propose that silicic rocks associated with the Idaho Batholith, altered during Eocene magmatic episodes, are a suitable source, and have calculated their potential volume to be >10⁴ km³ which is of the order necessary for CSRP-OH rhyolite production.