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UBC Theses and Dissertations

Petrography, geochemistry and petrogenesis of the Iskut-Unuk rivers volcanic centres, northwestern British Columbia Hauksdóttir, Steinunn


The Iskut-Unuk rivers centres consist of eight Recent volcanic centres located within the Stikine volcanic belt, northwestern British Columbia. The centres include: Iskut River, Tom MacKay Creek, Snippaker Creek, Cone Glacier, Cinder Mountain, King Creek, Second Canyon and Lava Fork and comprise lava flows, pillow lava, cinder and ash. The volcanic rocks range in age from 70,000±30,000 to -150 years B.P. and are dominantly alkali olivine basalts; hawaiite is observed only at Cinder Mountain volcanic centre. The basalts are olivine and plagioclase porphyritic and contain rare resorbed clinopyroxene. The groundmass includes olivine, plagioclase, titanaugite, magnetite and locally ilmenite. Large plagioclase crystals with extremely diverse crystal habits and textures are abundant in lavas from Iskut River, Snippaker Creek, Cone Glacier and King Creek volcanic centres. Crustal xenoliths are most abundant at Lava Fork but also occur within the lavas from Iskut River, Snippaker Creek, Cone Glacier and King Creek. Olivine compositions within the basalts range from Fo₅₅ to Fo₈₈; Cinder Mountain hawaiites contain Fo₃₃₋₅₄. Clinopyroxene MGf s range from 46 to 76 and can contain up to 6 wt% TiO₂ . The compositions of resorbed clinopyroxene phenocrysts indicates crystallization at slightly different magmatic conditions than the groundmass clinopyroxene. Based on textures and habits, plagioclase crystals are divided into 4 groups including: megacrysts, phenocrysts, sieved phenocrysts and groundmass. Megacrysts and phenocrysts range in composition from An5 0 to An7 0. Groundmass laths of plagioclase range from An3 8 to An6 8 in composition. Sieved phenocrysts of plagioclase are either of magmatic origin (An₅₀₋₇₀) or they are xenocrysts as suggested by prominent dissolution surfaces (sieved), observed with Nomarski technique, and low An-content (An₆₋₄₈). Most crustal xenoliths derive from granitic basement rocks; partial melting of xenoliths gives rise to glasses with compositions close to alkali feldspar. The chemical diversity observed within the centres cannot be explained by closed system processes involving the observed magmatic mineral phases. Two different hypotheses can explain this variation: i) source region processes including heterogeneous mantle melt or many separate partial melts and/or ii) assimilation of crustal material. With mass balance calculations the chemical variations of samples within Iskut River, Snippaker Creek and Cone Glacier volcanic centres, can be explained by fractionation of olivine (3.2- 5.7%) and plagioclase (1.2-11.22%) and assimilation of granitic melt (3.5-6.2%). Cinder Mountain intermediate rocks are not derived from a basalt collected from the area, but the variation within the hawaiite flows is possibly related through coupled fractionation and assimilation processes.

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