Continentalization of an intraoceanic arc as exemplified by the Jurassic Bonanza arc of Vancouver Island, Canada

We present new U-Pb zircon ages, bulk rock chemistry, and field observations for volcanic and plutonic rocks of the Jurassic Bonanza arc of Vancouver Island, Canada, a proposed southern extension of the Talkeetna arc crustal section in Alaska. Rather than a synchronous and contiguous arc crustal section, the new field and geochronological data reveal three distinct periods in the arc development, wherein the oldest, “prima” (205−195 Ma), was built upon by the youngest, “dopo” (<180 Ma), with both preserving their volcanic and plutonic sections of the upper crust. In contrast, the middle “mezza” period of arc construction (195−180 Ma) has no volcanic expression and is represented almost exclusively as plutons in the middle crust emplaced below the brittle-ductile transition, and exhumed mainly along the western and southern coast of Vancouver Island. The new data and structural interpretation refute earlier models suggesting that the deeper, more mafic rocks of the middle crust (West Coast Complex) are either restite or the crystal cumulates that form the more silicic plutonic rocks (Island Plutonic Suite) in the upper crust. Both the middle and upper plutonic components are remarkably similar to average continental crust. The growth and development of the Bonanza arc in an oceanic realm—but on a thick, pre-existing marine plateau—promoted a “continentalization” of its bulk composition within ∼10 m.y., similar to what is proposed to have occurred in Costa Rica over the same timescale.