SILETZIA: AN OCEANIC LARGE IGNEOUS PROVINCE IN THE PACIFIC NORTHWEST

Siletzia is an accreted terrane of Paleocene-Eocene submarine and subaerially erupted basaltic lavas presently exposed in the Cascadia forearc. Collectively, this province includes basalts of the Metchosin Fm (Vancouver, B.C.), Crescent Fm (Washington), and Siletz River Volcanics (Oregon). Seismic imaging and regional potential field data suggests this province approached 2.6 million km³ in original volume [Trehu et al., 1994; Wells et al., 1998; Parsons et al., 2006]. ⁴⁰Ar-³⁹Ar age data corroborate coccolith and U-Pb ages and show that much of the magmatism was produced over a 6 m.y. interval beginning at 56 Ma.

We present a geochronological and geochemical study of Siletzia lavas from Oregon supplemented with reconnaissance coverage of Metchosin and Crescent Fm basalts. Our data show Siletzia volcanism initiated at the southern end of the province (56-53 Ma) and migrated north (54-50 Ma) erupting a range of tholeiitic to undersaturated alkalic lavas that include picrites, ankaramites, and nephelinites in Oregon. By comparison, Metchosin and Crescent Fm lavas at the northern end of the province are less diverse and their isotopic range is more restricted. Siletzia lava compositions vary along linear arrays on Pb-Pb and Nd-Pb isotope plots that indicate mixing between “C” and HIMU mantle components [²⁰⁶Pb/²⁰⁴Pb 18.564-19.941; ²⁰⁷Pb/²⁰⁴Pb 15.494-15.676; ²⁰⁸Pb/²⁰⁴Pb 38.133-39.534; ⁸⁷Sr/⁸⁶Sr 0.7031-0.7037; ε_Nd +5.0 to +7.7; ³He/⁴He 9.4-13.7 R/Ra; ¹⁸⁷Os/¹⁸⁸Os 0.130-0.142]. “C” compositions are found in lavas throughout the province and this signature dominates all tholeiitic and mildly alkalic lavas regardless of age or location. This group of Siletzia lavas are isotopically identical to early Columbia River Basalt magmas (Imnaha, Picture Gorge) which strongly suggests they are derived from the same mantle source albeit separated in time. Melt contribution from a HIMU source is required in the oldest and southernmost lavas (Oregon) and its contribution progressively decreases northward. The total volume of magmatism, relatively limited age range, and Sr-Nd-Pb-He-Os characteristics of Siletzia argue that this LIP originated from a mantle source with the thermal and compositional characteristics of a mantle plume. We contend that Siletzia represents the plume head phase of the Yellowstone hot spot.