Resnick, Jonah, 2003, Petrogenesis of the Paleogene Sloko Lake Volcanic Complex, northwestern British Columbia, and implications for early Tertiary magmatism in the Coast Plutonic Complex: Montreal, Quebec, Canada, McGill University, M.Sc. thesis.
The Sloko Lake Volcanic Complex (SLVC) is the largest erosional remnant (600 km2 in area) of a Paleogene volcanic complex in the northern Canadian Cordillera. It is located along a linear trend of volcanic and plutonic rocks that once formed part of a widespread continental volcanic arc that extended from west-central British Columbia into southeastern Alaska and southwestern Yukon Territory. The preserved stratigraphy in the SLVC is 1,500 m thick, consisting of flat to gently dipping volcanic and pyroclastic flows, with minor well-bedded epiclastic strata that were likely deposited in a westward-dipping half-graben. Four stratigraphic units are defined: (1) Lower Rhyolite Unit consisting of dominantly pyroclastic material, (2) Interbedded Unit of intermediate to mafic flow and tuffs, (3) Upper Rhyolite Unit comprising felsic pyroclastic flows and coarse-grained epiclastic lithologies, and (4) Upper Series Unit of agglomerate and intermediate to mafic flows. Dominant lithologies in the SLVC comprise plagioclase phyric to porphyritic basaltic andesite to andesite with subordinate basalt, rhyolite, dacite, quartz monzodiorite, and pyroclastic to epiclastic lithologies. SLVC rocks range from relatively primitive basalts (MgO ~8.00 wt. %) to highly evolved silicic rhyolites (SiO2 77 wt. %). Rocks vary from Low-K to High-K suites with increased differentiation, exhibit transitional calc-alkaline to tholeiitic differentiation trends lacking Fe enrichment, and exhibit typical arc-derived depletions in HFSE relative to LILE. Crystal fractionation modeling between primitive basalt to andesitic rock compositions using phenocryst compositions and experimental data best fit a model in which magmas fractionated under water-saturated conditions at upper crustal pressures (1-2.5 kb) and f O2 on the Q-F-M buffer. LILE concentrations rise significantly faster than crystal fractionation models with progressive differentiation, requiring open system processes, likely crustal assimilation/contamination. Intermediate dacitic magmas represent the least abundant volcanic rock type in the SLVC, yet the most abundant plutonic rock composition in the SLVC and regionally. The underrepresentation of dacitic volcanic rocks is interpreted to result from rapid increase in viscosity between the andesite and dacite fields due to flocculation of phenocrysts, creating shear strengths, at which flow becomes energetically unfavorable, causing ponding, and the formation of plutonic bodies. Rhyolite magmas at the base of the SLVC exhibit unique major, trace, and rare-earth-element differences from overlying stratigraphy, and are interpreted to have formed by partial melting of country rock surrounding an early SLVC magma chamber. Ignimbrites are compositionally very similar to shallowly emplaced granitic intrusions along the western margins of the Coast Plutonic Complex.
Theses and Dissertations