Age, construction, and exhumation of the midcrust of the Jurassic Bonanza arc, Vancouver Island, Canada

Abstract

We present new U-Pb zircon ages for heterogeneous mafic-felsic rocks of the West Coast complex, a midcrustal plutonic component of exposed Jurassic arc-crustal section on Vancouver Island. We examine the timing of juvenile plutonic crust production, and consanguinity of volcanism and plutonism in this arc. The midcrustal plutons were emplaced between 193 Ma and 174 Ma, contemporaneous with the upper-crustal (<10 km depth) plutonic component of the arc (Island plutonic suite). A 12 km thickness of plutonic arc crust was built as a series of sheets at a rate of ∼0.003 km3 yr−1. Deformation and emplacement were contemporaneous, and there are no correlations among age, differentiation (peridotite to granite), or structural level of plutons in the arc. The age range and a weak eastward younging age polarity of the Jurassic arc section on Vancouver Island match that of the Talkeetna arc-crustal section in Alaska, suggesting that the two arcs are correlative and evolved by either forearc erosion above of an east-dipping slab, or slab rollback during west-dipping subduction.