MINERALOGICAL, GEOCHEMICAL, AND PHYSICAL CHARACTERIZATION OF MID-MIOCENE RHYOLITES IN EASTERN OREGON

Our recent work has contributed to the recognition that rhyolite volcanism associated with flood basalts, as part of the Columbia River magmatic province, was widespread from NE Oregon to northern Nevada over approximately the same age period of ~16.5 to 15 Ma. The published estimated volume of 3900km³ of rhyolites erupting as domes, lava flows, and pyroclastic deposit is likely a minimum as we better delineate eruption volumes of rhyolite centers north of 43^oN.

Our study investigates the mineralogy, geochemical characteristics, and physical parameters of eastern Oregon rhyolites within the flood basalt province. Insight into silicic volcanism will aid in a more comprehensive understanding of the physical and temporal relationship between eastern Oregon rhyolites and the flood basalt volcanism associated with the movement of the Yellowstone Hotspot.

The rhyolites of eastern Oregon can be categorized as A-type or as ‘calc-alkaline’-type based on the relative abundance of specific trace elements. A-type rhyolites are described as “hot and dry” magmas and are associated with bimodal provinces whereas ‘calc-alkaline’-type are “cool and dry” and are typically associated with volcanic arcs. This study aims to correlate physical parameters (such as calculated eruption temperatures derived from geothermometers) with geochemical characteristics of each rhyolite. After determining the mineralogy of each rhyolite, an appropriate geothermometer will be applied according to the mineral phases present such as two-feldspar, two-pyroxene, Ti-in-quartz, Ti-in-Zircon, and other silicate-focused geothermometers. Additionally, the oxidation conditions and temperature will in part be estimated by the two Fe-Ti Oxide model in an effort to correlate the physical parameters of eastern Oregon rhyolites with their whole rock chemistry. The rhyolites we investigate range from metaluminous to weakly peralkaline and range from those with strong A-type affinities to those being strongly ‘calc-alkaline’. The variations in oxide compositions of titanomagnetite and ilmenite with the composition of the rock is a secondary focus. Preliminary investigations into oxide Fe²⁺/Fe³⁺ ratios suggest that A-type rhyolites in eastern Oregon form in a wider range of oxidation conditions than ‘calc-alkaline’ (I-type) rhyolites.