UBC Theses and Dissertations
Physical and chemical evolution of Cracked Mountain glaciovolcano, SW British Columbia Harris, Martin Alexander
The focus of this thesis is Cracked Mountain Volcano, located in the Garibaldi volcanic belt (GVB), of southwestern British Columbia, Canada. This study investigates the physical and petrochemical evolution of Cracked Mountain. Field mapping, lithostratigraphy, petrography, geochemistry, and thermodynamics were used to model: i) the eruptive sequence, style, duration, and paleoenvironment, ii) the causes of petrochemical variation and the P-T-H2O contents for the Cracked Mountain magmas. The major findings show that Cracked Mountain has a glaciovolcanic origin. We find lapilli tuffs consistent with a phreatomagmatic (i.e. explosive) origin, followed by intrusions (dykes, peperites) and effusions (pillows, sheet-lavas), indicating a transitional eruptive style. Paleomagnetic directions record a single-pole direction (i.e. monogenetic eruption). The edifice morphology indicates syn-eruptive confinement by a paleo-ice sheet that was 850 m thick, capable of holding a ‘leaky’ paleolake system with a maximum of ~0.36 km3 of water. The glaciovolcanic (40Ar/39Ar) age of ~400 ka represents an important record of the Cordilleran Ice sheet (CIS) in southwestern BC during the mid-Pleistocene. Petrographically, Cracked Mountain volcanic rocks comprise two unique suites: i) an olivine-plagioclase phyric (OP) assemblage and ii) an olivine-plagioclase-augite phyric (OPA) assemblage. Thermodynamic modeling based on Rhyolite-MELTS (Gualda and Ghiorso, 2015) reveals that these differences in phenocryst assemblage require distinct storage conditions before the eruption. OP magma must have been stored at depths less than 6 km (<2 kbar) and temperatures of 1240-1155 ̊ C. In contrast, OPA magmas crystallized at depths between 7-9 km (~2-2.5 kbar) at 1250-1150 ̊ C. Both magmas contained ≤ 0.5 H2O wt.% in their respective systems. Incompatible elements show no significant chemical differences between the two petrographic suites, indicating a singular melt source. Detailed analysis of variations in major element compositions with Pearce Element Ratios (PER) suggests that the two distinct petrographic assemblages (OP and OPA) can be explained by crystal fractionation within different systems before the eruption. Subsequent mixing likely occurred during transport and eruption. Cracked Mountain provides clear evidence for a single eruption tapping multiple magma chambers stored at different crustal depths thereby indirectly informing on the connectivity of crustal stored magmas in the Garibaldi volcanic belt.
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