The composition and origin of continental crust is in part constrained by evidence preserved in exhumed arc crustal sections. The Bonanza arc on Vancouver Island, Canada, is a Jurassic-aged arc crust section exhumed in the Eocene. We use Al-in-hornblende geobarometry of felsic plutons in the Bonanza arc to assess the depths of crystallization of its exposed arc crust. Plutons of the unfoliated Island Plutonic Suite are emplaced as sheets at depths of 2–10 km within a stratigraphy of Devonian to Triassic supracrustal rocks. The West Coast Complex is more generally strained, and intrudes below the pre-Jurassic supracrustals at depths of 10–18 km. Previous work shows that rare ultramafic units within the West Coast Complex crystallized at depths of 14–26 km.
The Bonanza arc crustal section has a present structural thickness of at least 15 km. The average composition for each arc component (volcanic and plutonics) when apportioned into the crustal section derived by stratigraphy and hornblende barometry, shows the bulk composition of the entire arc is basaltic andesite (56% SiO2, Mg/Mg + Fe (Mg#) = 50), not unlike some estimates for bulk continental crust. Although only vestiges of ultramafic rocks occur in the Bonanza arc today, a significant component (6–20 km) of this rock type must have been present in the section for the bulk arc to have been a melt in equilibrium with mantle olivine, and may have been lost by foundering or tectonic thinning.