The western Anahim Belt: root zone of a peralkaline magma system

The Anahim Volcanic Belt extends easterly across central British Columbia, where it includes three large shield volcanoes and numerous small monogenetic cones and lava flows. Potassium–argon dates on the lavas suggest that the onset of volcanic activity progressed from approximately 8 Ma in the west to less than 1 Ma in the east. A western extension of the belt into the strongly uplifted and deeply dissected Coast Mountains is defined by an east-trending zone of chemically distinctive, high-level plutons, dyke swarms, and remnants of eruptive breccia that yield K–Ar dates of 10–14 Ma. The lavas and dykes include both basic (alkali basalt and hawaiite) and salic (trachyte and sodic rhyolite) suites, which have remarkably similar major-element abundances. In addition, moderately to strongly peralkaline rocks (comendite and pantellerite) constitute a major part of the volcanic piles and a lesser proportion of the dykes. Epizonal plutons show an upward zonation from hypersolvus syenite to miarolitic, alkaline to mildly peralkaline soda granite. The plutonic, hypabyssal, and eruptive rocks of the western Anahim Belt are considered to represent different levels of exposure in the root zone of an alkaline magma system that gave rise to peralkaline end members. Uplift rates, based on fission track dates, suggest that fractionation leading to mildly peralkaline soda granite took place in crustal reservoirs emplaced at depths of 2.5–4.5 km below the surface and that additional enrichment in alkalis took place at still shallower depths.