Petrology of the Grays River volcanics, southwest Washington: Plume-influenced slab window magmatism in the Cascadia forearc

Abstract

The Grays River volcanics are part of the Coast Range basalt province and consist of ~3500 m of tholeiitic basalt fl ows and volcani clastic rocks that erupted in the Cascadia forearc from 42 to 37 Ma. Chemical and isotopic data, combined with migration of the location of magmatism through time, indicate that Grays River volcanics magmatism was related to subduction of a plumeinfl uenced spreading ridge that produced a northward-migrating slab window. Involvement of a mantle plume source is indicated by ocean-island basalt (OIB)-like incompatible element enrichments and radiogenic Pb isotopic compositions ( 206 Pb/ 204 Pb > 19.3).These Pb isotope data are distinct from most Cascade arc rocks and from Cascadia sediment, but they overlap with compositions of other Coast Range basalts. A slab window setting accounts for the northward-younging age progression of the Grays River volcanics as well as geochemical traits, including low B/Be, that indicate the Grays River magmas ascended through the mantle wedge and subducting slab without acquiring an arc signature. Differentiation of Grays River magmas was dominated by clinopyroxene fractionation, which resulted in evolved compositions (Mg# = 59-32), low Sc contents, and Sr contents that increase with fractionation. Geochemical differences between the Grays River volcanics and other Cascadia forearc volcanic units that range from ca. 55 Ma (Crescent Basalts) to <3 Ma (Boring Lavas) were mainly caused by transient changes in tectonic setting (i.e., arrival of a mantle plume, ridge subduction) and do not record progressive chemical modifi cation of the mantle wedge.

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

“…Earlier K/Ar ages from the Tillamook area span an age range of 46-42.7 Ma, but are not suffi ciently precise to determine the duration of volcanism (Magill et al, 1981). Isotopic 40 Ar/ 39 Ar ages from the Yachats (36.9 Ma), Cascade Head (34.2 Ma), and Grays River (41-37 Ma) centers decrease to the north and south, away from the Tillamook center (this paper; ages summarized in Chan et al, 2012;Fig. 2), consistent with the geologic evidence of offl apping shields to the north. These centers are of mixed polarity (Simpson and Cox, 1977;Wells and Coe, 1985), and the Yachats and Cascade Head lavas are interbedded with mudstone containing nannoplankton referable to CP15 (38-34.5 Ma; Table DR1 in the Supplemental File [see footnote 1]).…”

“…Major element and trace element chemistry by Snavely and MacLeod (1974), Phillips et al (1989); Barnes and Barnes (1992), Davis et al (1995) Parker et al (2010, and Chan et al (2012) characterized the fl ows as enriched (E) MORB and ocean island basalt (OIB) lavas. Parker et al (2010) and Chan et al (2012) suggested that the Yachats, Cascade Head and Grays River volcanics may have been sourced in the asthenosphere beneath the Farallon plate, possibly from a plume or slab window source.…”

Initiation of the Wrangell Arc: A Record of Tectonic Changes in an Arc-Transform Junction Revealed by New Geochemistry and Geochronology of the ~29–18 Ma Sonya Creek Volcanic Field, Alaska

“…The Yakutat microplate has been correlated to the Siletzia terrane that underlies much of western Oregon, Washington, and British Columbia (Chan et al, 2012;Wells et al, 2014).…”

“…Mantle reservoirs are from Stracke (2012). Fields for reference data are from: Aleinikoff et al (1987;, Chan et al (2012), Chauvel andBlichert-Toft (2011), Clift et al (2005), Godfrey (2011), Greene et al (2008;), Hyeong et al (2011), Jicha et al (2004), Lassiter et al (1995, McLennan et al (1990), Mezger et al (2000), Parker et al (2010), Plank and Langmuir (1998), Preece (1997), Rioux et al (2007), Samson et al (1989;, Snyder and Hart (2007), Vervoort et al (2005), Von Drach et al (1986), White et al (1987), Yogodzinzki et al (2010;. See Figure 3.23 for a legend.…”

“…The current position of the Zodiac Fan is >1,000 km to the southwest of the WA (Stenvenson et al, 1983), and the size of the submarine fan and mineralogy implies a drainage area of >500,000 km 2 of a granitic-metamorphic terrane (Stevenson and Embley, 1987). Substantial sediment would have been delivered by major rivers through the and British Columbia (Chan et al, 2012;Wells et al, 2014). Given a lack of isotope date for the Yakutat microplate itself, a sample from southern Washington is used as an endmember (S) to represent possible Yakutat contribution of SCVF magmatism.…”