| Summary: | 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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