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Date of Award
Master of Science (MS)
DeBari, Susan M., 1962-
Housen, Bernard Arthur
Babcock, R. Scott (Randall Scott)
The Early to Middle Jurassic Bonanza island arc on Vancouver Island, Canada, exposes the middle and upper crust of an ancient arc crustal section. The arc is exposed for a length of ~500 km along the west coast of Vancouver Island. The three components of the Bonanza arc represent different crustal levels of magmatism. The deepest level of magmatism is represented by the Westcoast Crystalline Complex, the intermediate level is represented by the Island Intrusions Suite, and the surficial level is the represented by Bonanza Group volcanics. Samples of the volcanic section were collected in the Pemberton Hills region of northern Vancouver Island, the Nootka Sound region in central Vancouver Island, and the Alberni region of southern Vancouver Island. These three localities represent ~400 km of strike length. The Bonanza volcanics are medium K and calc-alkaline. They range from high-alumina basalt (Al2O3 >15 wt.%) to dacite, with 48.5-72.5 wt.% SiO2 and 4.0-8.7 wt.% MgO. They have a moderate enrichment in the light rare earth elements (LREEs), with abundances 25-90x chondrite with chondrite-normalized (La/Yb)n values from 4.0 to 14. Geochemistry of the volcanics was analyzed to document along-strike variations that occur within the Bonanza arc. Examination of specific trace elements and trace element ratios suggests that the Bonanza Arc was influenced by the addition of a sediment component during magma generation, with the greatest influence in the Alberni and Pemberton Hills region. While an altered oceanic basaltic slab fluid signature is present in all portions of the arc, it is less pronounced than the sediment component. However, some of the slab fluid component signature may have been modified by alteration. Geochemical modeling of fractionation processes (least squares calculations for major elements and Rayleigh fractionation of the REE) and magma mixing were undertaken to evaluate processes that diversify parental melts in the arc. In the Alberni region, where this analysis was focused, the volcanic stratigraphy is divided into two distinct facies: The Red Bed Creek facies and the Klanawa facies. A model that combines both crystal fractionation and magma mixing has moderate success for reproducing compositions in the Klanawa facies, while compositions in the Red Bed Creek facies requires assimilation of country rock, crystal fractionation and magma mixing between primitive melts and silicic melts derived from the Westcoast Crystalline Complex.
Western Washington University
Vancouver Island (B.C.)
Copying of this thesis in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Paulson, Benjamin D. (Benjamin David), "Magmatic processes in the Jurassic Bonanza arc: insights from the Alberni region of Vancouver Island, Canada" (2010). WWU Graduate School Collection. 32.