Geochemical and isotopic perspectives on the origin and evolution of the Siletzia Terrane.

Abstract:

The Siletzia terrane, located in the Cascadia forearc region of Oregon, Washington and Vancouver Island, consists of a series of accreted basaltic pillow lavas, massive flows and intrusive sheets. It represents a late Paleocene–Eocene oceanic large igneous province (LIP), previously proposed to represent an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics formed by ridge subduction. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data on basaltic samples, has been used to assess the validity of the proposed tectonomagmatic models for Siletzia. The trace element data show REE patterns that are flat to LREE enriched with an absence of any arc signatures. These features are comparable to other oceanic plateaus such as the Ontong Java and the Caribbean and so therefore support a mantle plume origin. Initial isotope ratios range from ²⁰⁶Pb/²⁰⁴Pb = 18.869 – 19.673, ²⁰⁷Pb/²⁰⁴Pb = 15.527 – 15.609, ²⁰⁸Pb/²⁰⁴Pb = 38.551 – 39.220, ε_Hf = +9.0 – 14.8, ε_Nd = +5.0 – 8.0 and ⁸⁷Sr/⁸⁶Sr = 0.70304 – 0.70397. The isotope signatures become more varied southward across the terrane and reveal two trends: i) HIMU–DMM and ii) another extending from DMM towards the Imnaha component, thought to represent the mantle plume source of the Columbia River Basalts and Yellowstone ^1,2. The data may support the previously proposed idea that the volcanism of the Siletzia terrane represents initial melting of the mantle plume head of the Yellowstone hotspot ^3,4,5. Other evidence indicating a LIP origin includes the relatively rapid eruption/intrusion of an estimated magma volume of 2.6 x 10⁶ km^{3 6} between ~56-49 Ma ⁵, which, in conjunction with our new elemental and isotopic data, indicates that the Siletzia terrane most likely represents an accreted oceanic plateau.

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