Synopsis of July 13, 2018 GSOC Friday Night Lecture by Emily Cahoon, PSU PhD Candidate
by Carol Hasenberg and Emily Cahoon
PSU PhD Candidate Emily Cahoon spoke to GSOC at the July Friday night meeting about her research into the origin of the Clarno and John Day magmatism. Her research is part of an ongoing push in the geoscience community to determine the origin of the magmas to erupt in Oregon. This is a tricky question when the magma originates somewhere in the earth’s mantle, is filtered by partial melting of intervening subducting plates and/or continental crust, erupts onto the earth’s surface, and is then is pushed and pulled, rotated away from its original location, covered up in some places and eroded away in other places.
A lot of evidential weight is put on the chemical composition of the basalt magma and its derivatives in more silicic forms as fractionation and assimilation occur. There are basically two types of basaltic magmas that erupt on land – tholeiitic basalt and calc-alkaline basalt. Of the two, tholeiitic basalt is richer in iron and more closely resembles the Mid-Oceanic Ridge Basalt (MORB), the basalt which is more purely derived from the earth’s mantle. Calc-alkaline basalts are generally associated with volcanic arcs and subduction zones.
The 800-pound gorilla in the room during a discussion of the source of volcanism in Oregon is the Yellowstone Hot Spot (YHS) and its assumed mantle plume origin. This gorilla has been rather hard to catch. That is, the proof that a mantle plume really exists and is causing the Yellowstone volcano and its track across time from Oregon to Wyoming. Nevertheless, it has left its signature in the basalts from the Columbia River Basalt Group (CRBG) in Oregon through the Snake River Plain in Idaho. An alternate explanation for at least some of the tholeiitic basalts may be delamination of the bottom layer of the continental crust, or interaction of the ancient Farallon slab below western North America — but for now the jury is still out. Newer techniques of mantle imaging may soon shed some light on this area.
But let’s get back to the history of the Oregon landmass and why we are concerned with the volcanism of the Eocene and Oligocene epochs. Ray Wells of the USGS (and a recent speaker for GSOC) presented a paper to the geoscience community on the origins and character of the Siletzia terrane which docked onto the Oregon coast about 50 million years ago and is composed of tholeiitic basalt. Wells makes a case for this large mass of land originating at the spreading center just off the coast of Oregon and its large mass pointing towards its production beginning at 55 million years ago as the extrapolated track of the YHS intersected with the spreading center.
Between the production of Siletzia (55-49 Ma) and what we normally mark as the initial outburst of the YHS, the eruption of the CRBG in eastern Oregon (17 Ma), there is an itsy-bitsy time gap of about 30 million years. If a long-lived mantle plume is responsible for all these other events, what was it doing during this gap of time? This is where Cahoon’s research begins.
Actually, there was quite a lot of volcanism in central and eastern Oregon during the Eocene and Oligocene. The Crooked River, Wildcat Mountain, and Tower Mountain Calderas have recently been mapped and identified as the eruptive sources that produced the Clarno and John Day Formations. However, these calderas are located somewhat to the north of where the hot spot track should have been. Were these calderas produced by the hot spot and were there other calderas to the south that have been eroded or covered by subsequent volcanism?
Cahoon’s methodology has been to target and collect a variety of samples of basaltic lavas of the age in question from John Day to Burns, Oregon. This is a region where there are no identified volcanic products of John Day or Clarno age. The objective is to get a better idea of the nature and extent of volcanism ~10 million years before initiation of the CRBG. Samples are analyzed for major and trace elements, radiogenic isotopes, and select samples are dated at an Ar-Ar geochronology lab. Through her own and her advisor Dr. Martin Streck’s research, they have identified a significant amount of lavas that are ~23-28 Ma. She is particularly interested in comparing these samples’ geochemistry in order to say something more definitive about the evolutionary path of the magma and if it is related to the YHS. Additionally, she is analyzing samples for their oxygen isotopic content to quantitatively say something about how much crustal material “contaminated” the basaltic magma before it erupted. Volcanism associated with the YHS tends to have anomalously low oxygen isotope ratios, ie, low values of the “heavy oxygen” isotope O18.
Cahoon also studies the Picture Gorge Basalt (a subunit of the CRBG), and has discovered that these lava flows cover a much larger area than previously recognized. The Picture Gorge Basalt, which has had its mantle source questioned, may also contain critical evidence to aid in the evaluation of the plume during the mid-Miocene.
Other samples include basalts associated with the Strawberry Volcanics, a suite of volcanic rocks that are coeval in time with the CRBG. Geochemically, many of these samples are similar to the older (Eocene/ Oligocene) basalts Cahoon has identified.
Wells R., David Bukry, Richard Friedman, Doug Pyle, Robert Duncan, Peter Haeussler and Joe Wooden, “Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot,” Geosphere (2014) 10 (4): 692-719.
McClaughry, J.D., Ferns, M.L., Streck, M.J., Patridge, K.A. and Gordon, C.L., “Paleogene calderas of central and eastern Oregon:eruptive sources of widespread tuffs in the John Day and Clarno Formations. Volcanoes to vineyards: geologic field trips through the dynamic landscape of the Pacific northwest,” Geological Society of America Field Guide 15, pp.407-434, 2009.
McClaughry J.D., Mark L. Ferns, Caroline L. Gordon, and Karyn A. Patridge, “Field trip guide to the Oligocene Crooked River caldera: Central Oregon’s Supervolcano, Crook, Deschutes, and Jefferson Counties, Oregon,” Oregon Geology, Volume 69, Number 1, Fall 2009.
Reidel, S.P., Camp, V.E., Tolan, T.L., Martin, B.S., Ross, M.E., Wolff, J.A. and Wells, R.E., “The Columbia River flood basalt province: Stratigraphy, areal extent, volume, and physical volcanology. The Columbia River flood basalt province,” Geological Society of America Special Paper 497, pp.1-43, 2013.
Bindeman, I., “Oxygen isotopes in mantle and crustal magmas as revealed by single crystal analysis,” Reviews in Mineralogy and Geochemistry, 69(1), pp.445-478, 2008.