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Reshuffling the Columbia River Basalt Chronology—Picture Gorge Basalt, the Earliest- and Longest-Erupting Formation Emily B

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Reshuffling the Columbia River Basalt Chronology—Picture Gorge Basalt, the Earliest- and Longest-Erupting Formation Emily B https://doi.org/10.1130/G47122.1 Manuscript received 17 April 2019 Revised manuscript received 3 December 2019 Manuscript accepted 5 December 2019 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Reshuffling the Columbia River Basalt chronology—Picture Gorge Basalt, the earliest- and longest-erupting formation Emily B. Cahoon1*, Martin J. Streck1*, Anthony A.P. Koppers2 and Daniel P. Miggins2 1 Department of Geology, Portland State University, P.O. Box 751, Portland, Oregon 97207-0751, USA 2 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA ABSTRACT divided into formations based on geographic The Columbia River Basalt Group (CRBG) is the world’s youngest continental flood basalt location of vents, geochemistry, and timing of province, presumably sourced from the deep-seated plume that currently resides underneath eruptions (Camp and Ross, 2004) and include Yellowstone National Park in the northwestern United States. The earliest-erupted basalts Steens, Imnaha, Grande Ronde, and Picture from this province aid in understanding and modeling plume impingement and the subsequent Gorge Basalts (e.g., Reidel et al., 2013). While evolution of basaltic volcanism. We explore the Picture Gorge Basalt (PGB) formation of the several models have been proposed for CRBG CRBG, and discuss the location and geochemical significance in a temporal context of early magmatism, many researchers support that the CRBG magmatism. We report new ARGUS-VI multicollector 40Ar/39Ar incremental heating flood basalts are sourced from the Yellowstone ages from known PGB localities and additional outcrops that we can geochemically classify mantle plume (e.g., Geist and Richards, 1993; as PGB. These 40Ar/39Ar ages range between 17.23 ± 0.04 Ma and 16.06 ± 0.14 Ma, indicat- Camp, 1995). Our study investigates plume im- ing that PGB erupted earlier and for longer than other CRBG main-phase units. These ages pingement both spatially and temporally through illustrate that volcanism initiated over a broad area in the center of the province, and the the lens of Picture Gorge Basalt (PGB) (Fig. 1). 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 PICTURE GORGE BASALT: PREVIOUS PGB among CRBG units indicates that the shallowest metasomatized backarc-like mantle WORK AND CONTEXT TO OTHER was tapped first and concurrently, with later units (Steens and Imnaha Basalts) showing CRBG UNITS increased influence of a plume-like source. Geochronological studies on the CRBG and resulting ages have a complicated past due to INTRODUCTION thought to represent the feeder systems to the accuracy and precision issues (e.g., Baksi, 2013; The Columbia River Basalt Group (CRBG) surficial eruption sites. However, large volumes Barry et al., 2013). CRBG main-phase eruptions of the Pacific Northwest of the United States is of these basaltic magmas can travel hundreds of were originally hypothesized to have occurred the world’s youngest flood basalt and has played kilometers subaerially and within dike and sill over ∼1–2 m.y., but this interval was later revised an important role in understanding the dynamics complexes, illustrating that even the location to ∼1.3 m.y., between ca. 16.9 and 15.6 Ma (Bar- of large igneous provinces (LIPs). Flood basalts of dike swarms is not a conclusive indication ry et al., 2013). Recent studies have reduced the are a type of LIP representing the most volumi- of where magmatism originated (Ernst and Bu- interval (Jarboe et al., 2010; Mahood and Ben- nous periods of volcanic activity on Earth, com- chan, 1997; Ernst et al., 2019). son, 2017) to ∼0.56 m.y. (Kasbohm and Schoene, monly coinciding with times of environmental As basaltic magmas traverse the crust, they 2018) and placed the end of main-phase volca- crisis. While flood basalt provinces can be ac- are prone to differentiation and contamination nism (i.e., end of Grande Ronde Basalt) at ca. tive for millions of years, the majority of lava processes which may modify the geochemical 16 Ma rather than at 15.6 Ma (cf. Wolff and Ra- erupts during the first million years, or “main signals of the mantle source in the eruptive prod- mos, 2013, and references therein). phase” of activity (Coffin and Eldholm, 1994). ucts. As flood basalt activity waxes and wanes, The only PGB geochronological study was This main-phase period is thought to represent these geochemical signals provide evidence for within the type section at Picture Gorge, Or- impingement of the mantle plume head on the temporal changes in mantle source and/or evi- egon (Fig. 1). The K-Ar ages from that study lithosphere (Ernst et al., 2005). dence for interaction with the crust (Peate et al., range from 15.9 to 14.7 Ma with uncertainties Continental flood basalt provinces are com- 2008). Age distribution patterns of basaltic flows of as much as 0.8 m.y. (2σ) (Watkins and Baksi, posed of pyroclastic rocks, lava flows, dikes, and and dikes, along with changing chemical sig- 1974), although presumed PGB eruptive activ- sills, and cover extensive areas >100,000 km2 natures, provide key fingerprints of underlying ity spans from 16.4 Ma to 15.2 ± 0.4 Ma (Barry (Coffin and Eldholm, 1994; Ernst et al., 2005). mantle dynamics of flood basalt provinces and et al., 2013). Using these ages and the magnetic The location of the mantle plume and its tempo- the involvement of a deep mantle component. reversal observed in flows at Picture Gorge, it ral development are evaluated based on age and Main-phase volcanism of the CRBG oc- has been inferred that PGB erupted concurrent- distribution patterns of lavas and dike swarms curred between ca. 16.8 and 15.9 Ma, and rep- ly with the N1 and R2 flows of Grande Ronde resents an eruptive volume of >210,000 km3 Basalt (Nathan and Fruchter, 1974; Reidel et al., *E-mails: [email protected]; [email protected] (Reidel et al., 2013). The erupted basalts are 2013) (Fig. 1C). CITATION: Cahoon, E.B., Streck, M.J., Koppers, A.A.P., and Miggins, D.P., 2020, Reshuffling the Columbia River Basalt chronology—Picture Gorge Basalt, the earliest- and longest-erupting formation: Geology, v. 48, p. XXX–XXX, https://doi.org/10.1130/G47122.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47122.1/4937461/g47122.pdf by guest on 01 February 2020 Figure 1. (A) Current distribution of the Colum- bia River Basalt Group (CRBG, northwestern United States) (gray), the A original extent of the Pic- ture Gorge Basalt (PGB) (dark gray, black dashed outline), and our added C PGB extent (shaded white region). Locations of dated PGB samples: D—town of Dale; HC—Holmes Creek; PG—Picture Gorge; AM—Aldrich Mountains; IR—Inshallah Ranch; GR— Gilbert Ridge; SM—Snow Mountain; I—town of Izee; WMB—West Myrtle Butte; CR—Castle Rock, north of the Malheur Gorge; PC— Pole Creek, in Malheur Gorge; RR—Rattlesnake Road (undated). CRBG extent is after Reidel et al. (2013), and Monument dike swarm locations are after Brown and Thayer (1966). B (B) Overview map with CRBG extent (gray), current location of the Yellowstone plume, and presumed hotspot track (orange) along the Snake River Plain (SRP). Box shows extent of panel A. WA— Washington; ID—Idaho; OR—Oregon; CA—Cali- fornia; NV—Nevada; UT—Utah; WY—Wyo- ming. Extent of main map is outlined with dashed line. (C) Stratigraphy and geomagnetic polarity of main-phase CRBG. Recent data on mid-Miocene magnetic rever- 1984; Bailey, 1989; Wolff et al., 2008; Wolff We sampled along stratigraphic sections of the sals highlight inconsistencies with PGB ages. and Ramos, 2013). known outcrop area of PGB, age-equivalent Jarboe et al. (2010) proposed that the older N0- basalts that are adjacent to the known outcrop R1 magnetic reversal occurred at ca. 16.5 Ma. METHODS area (between the towns of John Day and Burns; This is also supported by constraining the R0- Samples selected for 40Ar/39Ar dating are Fig. 1), and sections that were previously cor- N0 transition to ca. 16.6 Ma by sanidine geo- from known and newly correlated PGB locali- related with other CRBG units such as Steens chronology on interbedded silicic tuffs (Mahood ties (Figs. 1 and 2). or Imnaha Basalts at Malheur Gorge (Hooper and Benson, 2017), and by high precision U-Pb Preferred eruptive ages (groundmass sepa- et al., 2002; Camp et al., 2003, 2013). dating work (Kasbohm and Schoene, 2018). rates) for all samples are summarized in Table 1. When compared to all main-phase CRBG Kasbohm and Schoene (2018) also constrained Detailed analytical procedures and age spectra units, PGB is geochemically and isotopically the later R2-N2 transition to slightly younger are provided in the GSA Data Repository1 (Fig. most similar to Steens Basalt (Carlson, 1984; than 16.210 ± 0.043/0.048 Ma. Consequently, DR1, Table DR3, and supplemental text). Wolff and Ramos, 2013). PGB samples show a the N1-R2 transition falls between 16.5 and comparable SiO2 range (48.5–53 wt%) as Steens 16.2 Ma, signifying that PGB flows must be RESULTS and Imnaha Basalts (with <51% SiO2 for Rock older than the published K-Ar ages. PGB Geochemical Characteristics Creek and >51% for American Bar chemical PGB lavas erupted from NNW-trending Samples selected for geochronology are a types of Imnaha Basalt; cf. Hooper, 1984), but feeder dikes of the Monument dike swarm in subset of all samples collected in this study.
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