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

We present data that distinguishes the long-known Littlefield Rhyolite of eastern Oregon (northwestern United States) into two distinct, voluminous, Snake River–type, high-temperature rhyolite lava packages that erupted in short sequence over <100 k.y. , with minimum volumes of 100 and 150 km3 respectively, contemporaneous with flood basalt volcanism of the Grande Ronde Basalt phase of the Columbia River Basalt Group. Contemporaneity of rhyolites with flood basalts is exceptionally demonstrated within the Malheur Gorge by intercalated mafic units belonging to the Grande Ronde Basalt that are stratigraphically constrained by underlying and overlying Littlefield Rhyolite flows, and the underlying Dinner Creek Tuff (unit 1). Our new ages of 16.11 Ma and 16.02 Ma for the lower and upper Littlefield Rhyolite, respectively, provide a narrow age constraint on the controversial lower age of Grande Ronde Basalt volcanism. Petrological data on local, intercalated Fe-rich andesitic (icelanditic) lavas provide further evidence for coeval existence of rhyolitic and mafic magmas, and additionally provide location evidence for storage sites of Grande Ronde Basalt magmas. Based on these data in addition to similar data on the nearby Dinner Creek Tuff rhyolite center, as well as the locations of other rhyolite centers that fall within the same period of intense rhyolite volcanism of ca. 16.1 Ma, we infer that Grande Ronde Basalt crustal magma reservoirs were widespread in this area of eastern Oregon. We further infer that the main eruptions of stored flood basalt magmas followed the magmas’ lateral transport from these reservoirs to the well-known dike swarms located at the periphery of the rhyolite distribution area where local eruptions of rhyolites are notably absent. Our study highlights the interplay of mafic and crustally derived rhyolite magmas, with implications for other continental flood basalt provinces that are less well preserved than the Columbia River Basalt province. INTRODUCTION During the middle Miocene, voluminous tholeiitic flood basalts of the Columbia River Basalt Group (CRBG) erupted from fissures in eastern Oregon, eastern Washington, and western Idaho (northwestern United States; Reidel et al., 1989; Camp et al., 2003, 2017; Camp and Ross, 2009). The onset of flood basalt volcanism is currently placed at 16.8 Ma (Camp et al., 2013; Barry et al., 2013). Widespread eruptions of rhyolite tuffs and lavas from vents in eastern Oregon, northern Nevada, and southwestern Idaho accompanied the most voluminous phase of CRBG volcanism (e.g., Coble and Mahood, 2012; Streck et al., 2015; Ferns et al., 2017; Henry et al., 2017) (Fig. 1). 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). The greater Malheur Gorge area of eastern Oregon (Fig. 2) contains important intercalated stratigraphy of main-phase CRBG lavas that have been geochemically and petrographically correlated with the Steens, Imnaha, and Grande Ronde Basalts (Binger, 1997; Lees, 1994; Camp et al., 2003; Hooper et al., 2002), and recently with Picture Gorge Basalt lavas (Cahoon and Streck, 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 GEOSPHERE GEOSPHERE; v. 15, no. 1 https://doi.org/10.1130/GES01695.1 17 figures; 2 tables; 1 set of supplemental files CORRESPONDENCE: streckm@ pdx . edu; BrianMcCullochWebb@ gmail . com CITATION: Webb, B.M. , Streck, M.J., McIntosh, W.C. , and Ferns, M.L. , 2019, 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: Geosphere, v. 15, no. 1, p. 60–84, https:// doi . org /10 . 1130 /GES01695.1. Science Editor: Shanaka de Silva Guest Associate Editor: Ben S. Ellis Received 21 March 2018 Revision received 3 September 2018 Accepted 8 November 2018 Published online 20 December 2018