SEARCHING FOR YELLOWSTONE PLUME VS. CASCADE ARC SIGNATURES IN 49-30 MA BASALTIC AND RHYOLITIC ROCKS OF WESTERN OREGON

Accretion of the Siletzia terrain to the N. American margin at ~49 Ma (Wells et al. 2014 Geosphere 10:696) caused subduction to shift to west and reinitiated volcanism in its modern locus; but, the timing of renewed arc magmatism in the Ancestral Cascades is debated. Further, the exact position of the Yellowstone plume, parental to Siletia, and its influence on Cascadia margin magmatism are also uncertain. Seligman et al (2014 Frontiers ErthS 2:34) argued that 32-28-Ma Smith Rock rhyolites of Crooked River caldera are “hot and dry” with low and diverse δ¹⁸O in zircon, which suggest a Yellowstone plume origin. Modern plate reconstructions put the Yellowstone plume position in western Oregon during the mid-Eocene, implying that plume melts may have leaked through or around the subducting Farallon slab and/or Cascadia forearc. In this study, we have studied selected Mid-Eocene (49–39 Ma) and younger Cascade volcanic rocks by ⁴⁰Ar/³⁹Ar laser heating ages and whole rock geochemistry to distinguish between Yellowstone plume and potential arc sources. Samples include landmark basaltic sills and lavas prominent rhyolitic tuff layers in Eugene, Oregon and westward in the Coos Bay area, coastal Oregon. These are: Spencer Butte (32.24±0.19 Ma), matching neighboring East Butte (32.38±0.31), but older than Pre-Rock lavas at UO Campus (31.86±0.2), voluminous Mohawk River ignimbrite of Springfield (30.9, U-Pb, Seligman et al. 2014), but younger than Mt. Pisgah lavas (33.85±0.17) and SW Hills Eugene Lavas (38.96±0.47); the Creswell Butte is 27.07±0.35Ma. Furthermore, Spencer Butte rocks are lowered in δ¹⁸OWR (3.8-5.5‰, while other lavas are mantle-like δ¹⁸O) suggesting that it was likely a subaerial volcanic edifice with meteoric hydrothermal system, likely a magmatic feeder system (e.g. Bindeman et al. 2020 Sci Rep 10:1). This 10-Myr duration of "Little Butte" volcanism in the Willamette Valley of modest volume and nearly identical IAB trace element chemistry suggest that they represent typical island-arc ancestral Cascade tholeiitic volcanism during deposition of the Eugene and Fisher formations. Likewise, detrital and igneous zircon U-Pb ages from ash beds in the Coos Bay area sediments constrain them to the Middle Coaledo and younger formations at ~40, 39, 32, and 8.2 Ma (Darin et al., 2021, GSA, this volume), all of which have island-arc geochemical features. The oldest ash has mixed arc/Yellowstone chemical features. The presentation will provide an update on these developments as well as potential correlation with 40-30 Ma tuffs in Eugene area.