Oceanic mafic magmatism in the Siletz terrane, NW North America: Fragments of an Eocene oceanic plateau?

Article history: Received 11 October 2016 Accepted 5 January 2017 Available online 14 January 2017 The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene–Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmaticmodels for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from Pb/ Pb: 18.751 to 19.668, Pb/Pb: 15.507 to 15.661, Pb/Pb: 38.294 to 39.2128, Hf/Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), Nd/Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and Sr/Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau.