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Title: The origin and evolution of the Siletz terrane in Oregon, Washington and Vancouver Island
Author: Phillips, Bethan
ISNI:       0000 0004 6425 6291
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2017
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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, back -arc 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, has been used to evaluate the validity of the proposed tectonomagmatic models for the Siletz terrane along with the subsequent evolution of the magmas. Fractional crystallisation of the primary magmas of the terrane appears to have occurred at relatively low pressures. The estimated initial mantle potential temperatures of the Siletz terrane range from 1400 - 1500 °C while the amount of partial melting undergone generally varies between ~ 25 – 33 %. The rocks of the terrane are geochemically similar, both in trace element (generally flat to Light Rare Earth Element (REE) enriched REE patterns) and radiogenic isotope composition to several well-characterised oceanic plateaus. The data produced in this study are consistent with a mantle source for 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 significant above that of ambient mantle, indicates derivation of the Siletz magmatism from a source influenced by a mantle plume, possibly the Yellowstone Hotspot. Overall, the terranes’ geochemistry suggests interaction between the Farallon – Kula/ Resurrection ridge and a hotter enriched mantle source region. It is therefore concluded that the Siletz terrane represents an accreted oceanic plateau and so is the youngest oceanic plateau thus far characterised.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology