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

Abstract

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 km 3 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.

Reshuffling the Columbia River Basalt chronology—Picture Gorge Basalt, the earliest- and longest-erupting formation

“…While mantle plume models illustrate that volcanism is most intense above plume tails (e.g., Morgan, 1981;Hill et al, 1992), the outward spreading of the plume at the base of the lithosphere can result in volcanism over a much wider extent, i.e., several hundred kilometers (e.g., Ernst et al, 2019, and references therein). In conjunction with an emerging new age distribution pattern for cogenetic CRBG rhyolites (Streck et al, 2017;Webb et al, 2018), our PGB ages and locations suggest that the earliest volcanism due to plume impingement occurred over a broad region, from Steens Mountain at the southern portion of the province north to the Monument dike swarm (Fig. 1).…”

The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA: Adjacent venting sites of two Dinner Creek Tuff units—the most widespread tuffs associated with Columbia River flood basalt volcanism

“…Ignimbrites of the mid-Miocene Dinner Creek Tuff of eastern Oregon, USA, are the most widespread silicic volcanic units of the mid-Miocene rhyolite flare-up that is associated with the nearly contemporaneous main-phase members of the Columbia River Basalt Group (Webb et al, 2018). Eruption of the Columbia River Basalt Group tholeiitic lavas is thought to have begun in southeastern Oregon and migrated northwards into northeastern Oregon and southeast Washington (Fig.…”

“…1) (Wolff and Ramos, 2013), although this has been recently called into question (Cahoon et al, 2020). Centers of silicic volcanism seemingly parallel this northward migrating trend, beginning with the High Rock and McDermitt caldera complexes in northern Nevada and southeastern Oregon, respectively, and migrating northwards to the Lake Owyhee Volcanic Field (Marcy, 2014;Streck et al, 2015;Coble and Mahood, 2012;Benson and Mahood, 2016;Benson et al, 2017;Henry et al, 2017), although new age data from undated centers and the redating of centers with prior age information in eastern Oregon have also called the northward migration of silicic centers into question (Streck et al, 2017;Webb et al, 2018) (Fig. 1).…”

“…The Hunter Creek Basalt is defined as a regionally extensive aphyric icelanditic basaltic andesite to andesitic lava that overlies the Dinner Creek Tuff in the greater area of this study mainly to the southeast; based on geochemical data, it is the youngest unit that has been correlated with the Columbia River Basalt Group and specifically the upper Grande Ronde Basalt. Samples of this unit have yielded 40 Ar/ 39 Ar ages ranging from 16.1 Ma to 15 Ma (Lees, 1994;Evans and Binger, 1997;Cummings et al, 2000), but recently it was more tightly constrained by Webb et al (2018) to range in age from 16.1 Ma to 16.05 Ma. Webb et al (2018) also showed that it is not a single lava flow but rather a series of lavas that range in composition from ∼55-63 wt% SiO 2 and overlie Dinner Creek Tuff unit 1.…”

“…Samples of this unit have yielded 40 Ar/ 39 Ar ages ranging from 16.1 Ma to 15 Ma (Lees, 1994;Evans and Binger, 1997;Cummings et al, 2000), but recently it was more tightly constrained by Webb et al (2018) to range in age from 16.1 Ma to 16.05 Ma. Webb et al (2018) also showed that it is not a single lava flow but rather a series of lavas that range in composition from ∼55-63 wt% SiO 2 and overlie Dinner Creek Tuff unit 1. This study will reemphasize this aspect and use the designation Hunter Creek Basalt for tholeiitic to icelanditic lavas that are younger than those of Dinner Creek Tuff unit 1.…”