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Master of Science (MS)
Babcock, R. Scott (Randall Scott)
Burmester, Russell F.
Talbot, James L.
The Similkameen batholith is a 170 Ma plutonic complex that contains coeval mafic alkaline and granitic calc-alkaline assemblages. The marginal mafic alkaline suite is silica-undersaturated, characteristically potassic in composition and comprised of malignite, shonkinite, and mafic syenites (+/- nepheline) with biotite and amphibole-bearing pyroxenite cumulates. The granitic plutons of the core are calc-alkaline, I-type, ranging from monzonite to granodiorite to granite in composition.
Structural and petrofabric studies indicate that the mafic alkaline suite was intuded and deformed by the calc-alkaline granitoids. Magmatic fabrics within the mafic rocks are concentric to the lithologic zonations and have been overprinted by sub-magmatic flow deformation induced by diapiric emplacement of the core. There is no evidence within the batholith for regional deformation associated with the collision of Quesnellia and North America.
Major and trace element variations as well as Sr-isotopic data indicate that the alkaline and calc-alkaline suites are not related by crystal fractionation. However, crystal fractionation appears to have been the dominant process responsible for major element variations within the mafic alkaline suite. Fractionation of the hydrous assemblage clinopyroxene +/- biotite +/- amphibole + apatite + magnetite accounts for compositional variations within the alkaline suite. Calculated modal abundances of the fractionated mineral phases closely approximates observed modal abundances within the pyroxenites, which supports a crystal accumulation origin for the ultramafic rocks.
The alkaline and calc-alkaline suites exhibit a geochemical signature characteristic of subduction-related magmatism. Both suites are enriched in alkali and alkaline earth elements and depleted in high field strength elements. Major and trace element compositions of the mafic alkaline suite are consistent with partial melting of a garnet and phlogopite-bearing peridotite source. However, trace element variations within the alkaline suite demonstrate derivation from a chemically heterogeneous mantle-source. The malignite series contains incompatible element ratios consistent with a depleted MORB-type source, whereas the shonkinite series appears to be enriched, indicative of an OIB-type source. The Ba-enrichments of the malignites relative to the shonkinites probably reflect variations in source mineralogy, i.e., greater amounts of phlogopite in the source, not contamination by pelagic sediments. Furthermore, the low variability of initial-Sr ratios (0.7037-0.7042) indicates contamination of the Similkameen magmas by continental crust was relatively minor.
Variable degree partial melting of a lower crustal mafic granulite source could produce the incompatible element, REE, and isotopic abundances observed within the calc-alkaline granitoids.
Subduction and plate tectonic processes prior to 170 Ma variably enriched (metasomatized) portions of the lithospheric mantle wedge below the western edge of the North American craton. Derivation from this heterogeneous, metasomatized mantle- source resulted in the variably enriched nature of the mafic rocks. The granitoid magmas were the probable result of partial melting related to ponding of hot, hydrous mafic magmas of the alkaline suite at the lower crust-mantle boundary.
Alkaline suits, Calc-alkaline suites
Western Washington University
Subject – LCSH
Batholiths--British Columbia--Okanogan-Simikameen Region; Batholiths--Washington (State)--Okanogan County; Geology, Stratigraphic--Jurassic
Similkameen Batholith (Wash. and B.C.); Okanogan County (Wash.); Okanogan-Similkameen Region (B.C.)
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Buddington, Andrew M. (Andrew McNair), "The Similkameen Batholith of North-Central Washington and South-Central British Columbia: The Petrotectonic Significance of an Alkaline/Calc-Alkaline Magmatic Complex" (1990). WWU Graduate School Collection. 811.