THE CENTRAL SNAKE RIVER PLAIN LOW δ18O RHYOLITE PROVINCE: CRUSTAL MELTING PROCESSES

The Snake River Plain (SRP) is a topographic low stretching across southern Idaho, recording time progressive Yellowstone hotspot volcanism to the northeast. The discovery of the central Snake River Plain (CSRP) low δ¹⁸O rhyolite province has exposed a fundamental gap in understanding crustal melting processes in the region (Boroughs et al., 2005; Boroughs et al., 2012; Ellis et al., 2013). A low δ¹⁸O signature in rhyolite is not unusual, however the large quantity (~1.4 x 10⁴ km³) in the CSRP is abnormal. Previous studies have proposed two formation models for the rhyolite: recycling of intracaldera hydrothermally-altered material (Bindeman and Valley, 2001) and melting of previously altered protolith (Boroughs et al., 2005). Here we characterize both the chemistry of potential protoliths and the melting conditions that produced rhyolite. This study focuses on where the CSRP cuts through the Idaho batholith (Cretaceous) and associated volcanic rocks (Eocene) that stretch across, and presumably beneath, the SRP. These rocks contain a significant proportion of hydrothermally altered material. Cretaceous granite and hydrothermally altered Eocene volcanics were sampled south of the CSRP. Initial geochemical results from these rocks and data from previous studies north of the CSRP are used as input for petrologic modeling in this study.

Petrologic modeling of protolith melting was conducted using the rhyolite-MELTS software package. Melting regimes between 2-4 Kbar, 600-1200^oC, and variable magma water contents (1-3 wt%) at f0₂ = QFM (i.e. a full range of plausible conditions) have been conducted in order to match bulk chemistry of melt produced by partial melting of protolith and the CSRP rhyolite. Initial modeling results suggest incomplete melting of either the Eocene or Cretaceous rock at pressures of 3-4 Kbar, 800-900^oC, and medium to high water contents (2-3 wt%) can reproduce the bulk chemistry of the CSRP rhyolite. These initial results favor the previously altered protolith model.

References:

Bindeman, I.N., and J.W. Valley. (2001). 42, 8:1491-1517., Boroughs et al. (2005) Geology 33, 10:821-824., Boroughs et al. (2012) EPSL 313-314, 45-55.