V31E-3060
Longevity of Yellowstone hotspot volcanism: Isotopic evidence linking the Siletzia LIP (56 Ma) and early Columbia River Basalt Group (17 Ma) mantle sources

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Douglas G Pyle1, Robert A Duncan2, Ray E Wells3, David W Graham2, Barry B Hanan4, Benjamin K Harrison5 and Bereket Haileab6, (1)University of Hawaii at Manoa, School of Ocean and Earth Science and Technology, Honolulu, HI, United States, (2)Oregon State University, College of Earth, Ocean, & Atmospheric Sciences, Corvallis, OR, United States, (3)U.S. Geological Survey, Menlo Park, CA, United States, (4)San Diego State University, Geological Sciences, San Diego, CA, United States, (5)University of Minnesota, Department of Earth Sciences, Minneapolis, MN, United States, (6)Carleton College, Department of Geology, Northfield, MN, United States
Abstract:
Siletzia is a Paleocene-Eocene accreted terrane of submarine and subaerially erupted mafic lavas exposed in the Cascadia forearc. This large igneous province [LIP] is exposed in multiple volcanic sections from Vancouver Island, B.C., to southern Oregon [~700 km]. We estimate Siletzia magmatism at ~2.3 x 106 km3 west of the Cascades and may reach 4.6 x 106 km3 if correlative with Alaskan Yakutat terrane and significant portions of the LIP filled the Oregon Embayment. 40Ar-39Ar ages show the bulk of Siletzia erupted over a 6-7 Myr interval beginning at 56 Ma, implying eruption rates of 0.3-0.7 km3/yr. In Oregon, Siletz River volcanism began in the south [56-53 Ma] and migrated northward [54-50 Ma]. Concurrent eruptions of Metchosin and Crescent basalts do not show a southerly age progression. Therefore, Siletzia likely erupted south of the Kula-Farallon spreading center with ridge collision at or north of the Metchosin igneous complex.

Isotopic data for 29 Siletzia lavas have initial 7/6Sr 0.7030-0.7037, ΕNd +4.9 - +7.7, 6/4Pb 18.70-19.94, 7/4Pb 15.49-15.63 and 8/4Pb 38.27-39.53. Olivine yield 3He/4He from 9.4 to 13.7 (R/Ra) and high MgO lavas display a narrow 187Os/188Os range (0.131-0.134) when age corrected. Both He and Os tracers are elevated above typical depleted MORB mantle and indicate plume involvement. Pb-Pb and Pb-Nd arrays suggest 3 mantle components for Siletzia volcanism: a depleted source with isotopic and trace element characteristics expected for spreading center lavas (i.e., Ku-Fa) influenced by a plume, a HIMU contaminant (i.e., high 6/4Pb; low 7/6Sr) confined to southern Siletzia, and a plume source (6/4Pb 19.00; 7/4Pb 15.55; 8/4Pb 38.60; 7/6Sr; 0.7033; ΕNd +6.4; γOs +5.0). Siletzia plume mantle is a close match to recent Yellowstone plume estimates based on early CRBG lavas. Mounting geophysical and geochemical evidence supports the contention that Siletzia is an early product of the Yellowstone hot spot in a sub-oceanic setting.

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