NEWLY DISCOVERED PERIODICITY IN MIOCENE, HIGH LAVA PLAINS AND CO-CRBG RHYOLITES OF EASTERN OREGON TIED TO COEVAL BASALT MAGMATISM AND REGIONAL EXTENSION EVENTS

Miocene rhyolitic volcanism of eastern Oregon can be divided into two main episodes. Mantle 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 22 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. Strong recommencement of rhyolite volcanism ~12.1 Ma is likely related to continued extension and rotation associated with clockwise rotation of Siletzia and the Northwest Basin and Range in a region previously weakened by CRBG volcanism. 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.