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

Abstract

The Columbia River Basalt Group (CRBG) is the world’s youngest continental flood basalt province, presumably sourced from the deep-seated plume that currently resides underneath Yellowstone National Park in the northwestern United States. The earliest-erupted basalts from this province aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. We explore the Picture Gorge Basalt (PGB) formation of the CRBG, and discuss the location and geochemical significance in a temporal context of early CRBG magmatism. We report new ARGUS-VI multicollector 40Ar/39Ar incremental heating ages from known PGB localities and additional outcrops that we can geochemically classify as PGB. These 40Ar/39Ar ages range between 17.23 ± 0.04 Ma and 16.06 ± 0.14 Ma, indicating that PGB erupted earlier and for longer than other CRBG main-phase units. These ages illustrate that volcanism initiated over a broad area in the center of the province, and the geochemistry of these early lavas reflects a mantle source that is distinct both spatially and temporally. Combining ages with the strongest arc-like (but depleted) geochemical signal of PGB among CRBG units indicates that the shallowest metasomatized backarc-like mantle was tapped first and concurrently, with later units (Steens and Imnaha Basalts) showing increased influence of a plume-like source.

Changing Mantle Sources and the Effects of Crustal Passage on the Steens Basalt, SE Oregon: Chemical and Isotopic Constraints

“…The Steens reversal (R0 to N0) is estimated at 16.6 Ma (Baksi, 2010; Jarboe et al, 2008), at 16.7 (Jarboe et al, 2010), and is revised to 16.5 Ma (Mahood & Benson, 2017). Range of Picture Gorge Basalt from Cahoon et al (2020); we note that there is not yet paleomagnetic cross correlation on these new ages. Blue transparent bars highlight the high and low MgO trends in the lower B Steens stage, and emphasize the early increasing magnesian character at consistent ΔMgO.…”

“…The arrow points to the direction where average mid‐ocean ridge basalt (MORB) from Sun and McDonough (1989) would plot. Data from Camp and Hanan (2008), Wolff et al (2008), Moore et al (2018), and Cahoon et al (2020).…”

“…The Steens Basalt flows cover an area of 53,000 km 2 and have a total estimated volume of 31,800 km 3 or ~15% of the 210,000 km 3 estimated volume of the entire CRBG (Camp et al, 2013; Reidel et al, 2013; Figure 1). The Steens Basalt erupted as early as 17.0 Ma ago (Moore et al, 2018) and together with the Picture Gorge Basalt of north‐central Oregon (17.2–16.1 Ma; Cahoon et al, 2020) represents the onset of Columbia River Flood Basalt volcanism, 95% of which erupted between 16.7 and 15.9 Ma (Kasbohm and Schoene, 2018). At Steens Mountain, locus of many Steens dikes, the stack of sequential Steens Basalt flows is the thickest and the compositional range is the widest (Figures 1 and 3; Johnson et al, 1998; Brueseke et al, 2007; Bondre & Hart, 2008; Camp et al, 2013; Moore et al, 2018).…”

“…Age of initiation of Steens Basalt, MgO data and Steens Basalt stage designations from Moore et al (2018). Other age estimates from Barry et al (2013), the compilation of Camp et al (2013), Kasbohm and Schoene (2018), and Cahoon et al (2020). Stratigraphic section modified from Camp and Ross (2004), Camp and Hanan (2008), and Wolff and Ramos (2013).…”

“…Additional components are required to account for differences and scatter in and among members. Wolff et al (2008) and Wolff and Ramos (2013) suggest that the Steens and Picture Gorge Basalts are contaminated by either arc derived fluids (Picture Gorge; see also Cahoon et al, 2020; Brandon et al, 1993) and/or mafic accreted terranes (Steens). The later and more voluminous Grande Ronde lavas were extensively contaminated by cratonic crust (ibid.).…”