The Itcha Volcanic Complex (IVC) is the easternmost shield of a series of bimodal central volcanic complexes which comprise the Anahim Volcanic Belt (AVB) in central British Columbia. The IVC lavas are distinctly bimodal, with a compositional gap between 52 and 58 wt% SiO$ sb2$ comprising late basaltic and hawaiitic lavas and early felsic lavas of Si-undersaturated to Si-oversaturated character. The hawaiite lavas dominate the late capping stage of the IVC. Their high Sr/Zr ratios indicate that they were derived by crystal fractionation of a clinopyroxene-dominated assemblage at high pressures ($ sim$10 kbar) from parental magmas with compositions similar to the most primitive alkali olivine basalts in the IVC. The majority of later evolved basalts have low Sr/Zr ratios indicating they formed by crystal fractionation at lower pressures. / The similarity of isotopic and the incompatible element ratios suggests that the felsic and mafic magmas of the IVC are co-genetic. The early basal trachytes were derived by an AFC process in crustal magma chambers and appear to have assimilated a significant (15-20%) crustal component. Later, more evolved felsic lavas exhibit a wider range of Si-saturation, which appears to require parental basaltic magmas with a range of silica saturations with less, and more selective, crustal contamination. The compositional gap between the mafic and the felsic lavas of the Itcha shield appears to be related to the difficulty of erupting crystal-rich viscous lavas of intermediate composition. Viscosity models indicate that the rise in viscosity with decreasing Mg is slower at high pressures, which would favour the eruption of lavas of more evolved composition. / Late balsanites have distinct Nb/Zr and isotopic signatures (low $ rm sp{87}Sr/ sp{86}$Sr), requiring a mantle source distinct from that of the alkali olivine basalts.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.28433 |
| Date | January 1994 |
| Creators | Charland, Anne |
| Contributors | Francis, Don (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Earth and Planetary Sciences.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001425300, proquestno: NN00084, Theses scanned by UMI/ProQuest. |
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