Rocky Mountain Section - 68th Annual Meeting - 2016

Paper No. 10-11
Presentation Time: 8:00 AM-6:00 PM

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


BIRCHARD, Caleb E., School of the Environment, Washington State University, PO Box 642812, Pullman, WA 99164, WOLFF, John A., School of the Environment, Washington State University, PO Box 642812, Pullman, WA 99164-2812 and BOROUGHS, Scott, School of the Environment, Washington State University, Pullman, WA 99164, caleb.birchard@wsu.edu

The Snake River Plain 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 assimilation of previously altered protolith (Boroughs et al., 2005). Here we characterize both the chemistry of the potential protolith and the melting/assimilation conditions that took place in the region.

This study focuses on where the CSRP cuts through the Idaho batholith (Cretaceous) and associated volcanic rocks (Eocene) that stretch across the Snake River Plain. These rocks are known to contain a significant proportion of hydrothermally altered material, but the extent of outcrop in the CSRP region is imperfectly known. Chemistry of the Idaho batholith and Eocene volcanics from north of the CSRP are used as constraints for petrologic modeling in this study.

Initial petrologic modeling of protolith melting and assimilation has been conducted using the rhyolite-MELTS software package. The median geochemistry of regional basalts has been used as a proxy for presumed basalt injection material, which supplied heat to the area. Melting regimes between 2-10 Kbar, 800-1200oC, and variable magma water contents (1-3 wt%) at f02 of Quartz-Magnetite-Fayalite (i.e. a full range of plausible conditions) have been conducted in order to match bulk chemistry between protolith and the CSRP rhyolite. Initial modeling results suggest incomplete melting of either the Eocene or Cretaceous rock at pressures of 3-4 Kbar, 850-950 oC, and low water contents (1 wt%) can reproduce the bulk chemistry of the CSRP rhyolite. These initial results favor the previously altered protolith model.

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.