ASSESSING SPACE AND TIME TRENDS IN MIOCENE CO-CRBG RHYOLITES AND RHYOLITES OF THE HIGH LAVA PLAINS TREND OF EASTERN OREGON

Miocene rhyolitic volcanism of eastern Oregon can be divided into two main episodes. Yellowstone plume upwelling produced Columbia River Basalt Group lavas and coeval >16.5–15 Ma silicic volcanism trending N-S from NE Oregon to northern Nevada. Rhyolites of the 12–0 Ma High Lava Plains (HLP) province are correlated either with buoyancy-driven westward plume spreading or slab rollback and mantle convection spanning from SE Oregon to Newberry volcano in the west. The apparent ~15–12 Ma eruptive hiatus suggested rhyolites of these provinces were a product of separate processes, yet this was based on an incomplete dataset with a lack of data on ~33 of the total ~50 rhyolite eruptive centers in the area where the two provinces overlap (117–119°W, 43–44°N), thus yielding only tenuous relationships between these two provinces.

We acquired ⁴⁰Ar/³⁹Ar ages and geochemical data for 26 previously unanalyzed rhyolite centers, with three in progress. Our new ages refine the first eruptive episode to be 16.8–14.4 Ma, starting with eruption of the 16.81 Ma Dome E of South Fork and waning after eruption of the 14.42 Ma McCain Creek rhyolite. We refine the ~15–12 Ma rhyolite eruptive hiatus to 14.4–12.1 Ma, recommencing with Beaty’s Butte at 12.1 Ma and with Sacramento Butte at 11.85 Ma. The 12.1–9.9 Ma pulse includes 24 rhyolite eruptions in the ~2.2 m.y. duration, waning with eruption of the 9.86 Ma newly defined Griffin Creek rhyolite. A hiatus of ~0.5 m.y. followed before the third 9.0–5.2 Ma eruptive episode.

Our new data suggest regional rhyolite eruptions are a series of episodic events related to arrival and storage of mafic mantle magmas. Paucity in rhyolite eruptions 14.4–12.1 Ma is likely related to decreased plume flux at ~15 Ma. Unlike the 16.8–14.4 and 9.0–5.2 Ma eruptive episodes, the apparent missing basalt magmas preceding the 12.1–9.9 Ma episode may be entirely due to a lack of ages on regional mafic lavas. The astounding trace element variability among 12.1–9.9 Ma rhyolites is likely due to a complex system of magmas generated by partial melting of terrane rocks, recycling of older rhyolites, and internal fractionation of silicic reservoirs. Periodicity of rhyolite volcanism along the HLP demands more punctuated basalt inputs rather than what continuous partial melting from west-spreading plume material would likely generate.