Stratigraphy and Geochemistry of a Large, Hotspot-related Rhyolite of the Pacific Northwest: The Littlefield Rhyolite, Eastern Oregon

Abstract:

During the middle Miocene, voluminous tholeiitic flood basalts erupted in eastern Oregon, followed closely by widespread eruptions of rhyolitic tuffs and lavas. While numerous causes have been proposed, many researchers are currently in agreement that the initiation of a mantle plume associated with the Yellowstone hotspot track is ultimately responsible for this magmatic activity. The mafic component of this magmatic activity has received considerable attention, contrary to the corresponding silicic volcanism.

The widespread Littlefield Rhyolite consists of rhyolitic lava flows that erupted in eastern Oregon contemporaneous to upper Grande Ronde Basalt lavas. The estimated total volume of erupted rhyolites is ~100 km³ covering ~850 km². While the areal extent and distribution of the Littlefield Rhyolite has been sufficiently delineated by geological mapping in the past, little is known about the internal stratigraphy and emplacement of this extensive rhyolitic unit. The focus of this study has been to investigate the stratigraphy and petrology of the Littlefield Rhyolite and whether field and geochemical relationships exist to help constrain the timing and storage sites of Grande Ronde Basalt magmas. Although indistinguishable in the field, our data reveal that the Littlefield Rhyolite consists of two geochemically distinct rhyolite flow packages that are designated here as lower and upper Littlefield Rhyolite according to stratigraphic relationships in Malheur River Gorge. There, several other units are sandwiched between the lower and upper rhyolites, including several lava flows and a one-meter thick agglutinated spatter deposit of local Grande Ronde Basalt units. The spatter deposit thickens to 10s of meters over a distance of 800 m where the deposit is strongly welded. We now recognize this as a venting site of local Grande Ronde Basalt. One of the local variants of upper Grande Ronde Basalt is icelanditic (~61 wt. % SiO₂) in composition and is found at a number of places including a location near the southern extent of the Littlefield Rhyolite. Geochemical modeling strongly suggests that icelandite lavas from Alder Creek resulted from mixing of Grande Ronde and Littlefield Rhyolite magmas, thereby tying a Grande Ronde magma storage site to within the greater Malheur River Gorge area.