Spatial Temporal Relationships and Rhyolite Geochemistry of the High Lava Plains, Oregon: Compositional Variations and Relationships to Other Volcanic Provinces

The High Lava Plains (HLP) are a Cenozoic bimodal basalt-rhyolite province located in southern and central Oregon, east of the Cascade Range and in conjunction with the Columbia River basalts, Steens basalts and Yellowstone - Snake River Plain (SRP) track, are part of the largest Cenozoic volcanic province in the world. New 40Ar-39Ar dates show two east to west time-transgressive patterns to the rhyolitic volcanism. Chemically, the <12 Ma rhyolites of the HLP separate into three distinct groups in major and trace elements. One age progression, which has been described by previous workers, runs sub parallel to the Brothers Fault Zone, a set of small, diffuse, northwest-striking normal faults with offsets generally less than 10 m. This trend spans from ~12 Ma to Recent eruptions at Newberry Volcano. Typically, these rhyolites are high silica (SiO2 > 75 wt. %) and follow a tholeiitic differentiation trend. The other age progression occurs in a north-south belt 60 to 110 km east of and parallel to the Cascade arc and ranges in age from is ~7.5 to ~4.5 Ma. These rhyolites are typically lower in silica and follow a calc-alkaline differentiation trend. These two age progressions overlap in the northwest corner of the HLP. Peralkaline rhyolites are also distributed throughout both age progressions and are high silica (SiO2 ~76.5 wt. %), high FeO/ast (> ~2 wt. %), high Zr, Nb, Ta and are significantly depleted in elements with a strong affinity for plagioclase (Al, Ba, Sr, Eu). The basalts of the province show no time- transgressive behavior and are primarily primitive HAOT (high-alumina olivine tholeiites) that have undergone variable amounts of fractionation. The tholeiitic differentiation trend rhyolites of the HLP are chemically similar to Icelandic rhyolites and both provinces are underlain by thin, mafic crust. The crust of the HLP is covered by extensive Cenozoic volcanism and sedimentary cover and likely consists of stitched Mesozoic accreted terranes. It is less mafic and slightly thicker (~35 km) than that of Iceland. These differences in crustal character may explain the differences in eruptive products between the two provinces, specifically the basalt to rhyolite ratio (HLP 2:1, Iceland 10:1) and higher iron at similar silica in Icelandic rhyolites. The tholeiitic trend rhyolites are also similar in composition to the Quaternary rhyolites of the SRP.