Abstract
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 tectonomagmatic models 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 206Pb/204 Pb: 18.751 to 19.668, 207Pb/204Pb: 15.507 to 15.661, 208Pb/204Pb: 38.294 to 39.2128, 176Hf/177Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), 143Nd/144Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and 87Sr/86Sr: 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.
Highlights
Accreted oceanic mafic terranes are an ubiquitous feature of most destructive plate margins (e.g., the northern Andes, (Kerr et al, 2002a); the coastal ranges of western Canada and Alaska (Greene et al, 2009a, 2009b), and Japan (e.g.,Ichiyama et al, 2014)), and of continental collision zones such as that represented by the closure of the Tethyan ocean (e.g., Turkey (Whattam and Stern, 2011); Tibet (Chen et al, 2001); and central Iran (Moghadam et al, 2010))
This study focuses on the Siletz terrane, a mostly Eocene large igneous province (LIP) located in the present Cascadia forearc region of Oregon, Washington and Vancouver Island (Fig. 1), consisting of a series of accreted basaltic pillow lavas, massive flows and intrusive sheets (Snavely et al, 1968; Wells et al, 2014)
The samples analysed in this study span a range of compositions from basalts to more evolved rocks there is no correlation between the degree of fractionation undergone by each sample and the radiogenic isotope ratios
Summary
Accreted oceanic mafic terranes are an ubiquitous feature of most destructive plate margins (e.g., the northern Andes, (Kerr et al, 2002a); the coastal ranges of western Canada and Alaska (Greene et al, 2009a, 2009b), and Japan (e.g.,Ichiyama et al, 2014)), and of continental collision zones such as that represented by the closure of the Tethyan ocean (e.g., Turkey (Whattam and Stern, 2011); Tibet (Chen et al, 2001); and central Iran (Moghadam et al, 2010)). The primary goals of this investigation are to use these new geochemical data to assess the nature of the mantle source region and the validity of the proposed tectonomagmatic models for the Siletz terrane
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
