The Littlefield Rhyolite, Eastern Oregon: Distinct Flow Units and Their Constraints on Age and Storage Sites of Grande Ronde Basalt Magmas

Abstract

The Littlefield Rhyolite consists of widespread, high-temperature, hotspot-related rhyolitic lavas that erupted in eastern Oregon contemporaneous to late-stage Grande Ronde Basalt lavas. The estimated total volume of erupted rhyolites is ~100 km 3 covering ~850 km 2 .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 often indistinguishable in the field, our data reveal that the Littlefield Rhyolite

The Littlefield Rhyolite and associated mafic lavas: Bimodal volcanism of the Columbia River magmatic province, with constraints on age and storage sites of Grande Ronde Basalt magmas

“…Without a thin section, the epiclastic tuff would likely be identified as being tuffaceous sandstone or tuffaceous siltstone (cf. Webb, 2017). Farther to the east, south of Cottonwood Mountain, deposits mapped by Brooks and O'Brien (1992b) form elongate, north-trending ridges marked by steeply dipping hydroclastic surge deposits interpreted by Ferns and McClaughry (2013) and Ferns et al (2017) to be proximal vent deposits of the Hunter Creek Basalt (Fig.…”

“…The Hunter Creek Basalt is made up of Fe-rich basaltic-andesite to andesite lavas and proximal vent deposits correlative with the Grande Ronde Basalt (cf. , representing the late-stage eruption of tholeiitic volcanism within the Malheur Gorge area (Ferns and McClaughry, 2013;Webb, 2017…”

“…It appears that this dacite was a continuous layer before it got disturbed by the emplacement of the overlying upper Littlefield Rhyolite. Analytical data and discussion of the epiclastic unit and dacitic tuff can be found in Webb (2017).…”

“…While numerous causes have been proposed, many researchers currently favor that impingement of a mantle plume associated with the Yellowstone hotspot generated this enormous magmatic pulse (e.g., Pierce and Morgan, 2009, and references therein). Although the basaltic component of the bimodal magmatic activity of the CRBG has received considerable attention, the silicic phase has until recently received considerably less attention (Cummings et al, 2000;Coble and Mahood, 2012;Streck et al, 2015;Mahood and Benson, 2016;Henry et al, 2017;Webb, 2017).…”

“…Throughout the Malheur Gorge, units that erupted in the north (Imnaha and Grande Ronde Basalts) and northwest (Picture Gorge Basalt) interfinger with lavas that erupted in the south (Steens Basalt), providing the stratigraphic basis for including the Steens Basalt into the CRBG as the oldest unit (V. Camp, 2016, personal commun.). The Malheur Gorge area is also one of the few places of the province that exemplarily illustrates the bimodal character of the "plume head" stage of the Columbia River magmatic province (e.g., Coble and Mahood, 2012;Ferns and McClaughry, 2013;Streck et al, 2015;Webb, 2017). In the Malheur Gorge, the regionally extensive Dinner Creek Tuff (e.g., Streck et al, 2015) and lava flow units of the Littlefield Rhyolite are intercalated with Grande Ronde Basalt lavas.…”