CONTINENTALIZATION OF AN INTRAOCEANIC ARC: THE CASE FROM THE JURASSIC BONANZA ARC, 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, a proposed southern extension of the Talkeetna arc crustal section in Alaska. An accurate pseudostratigraphy for the Bonanza arc has been complicated by faulting and led us to revisit the relationship between all components and whether or how they represent one continuous arc section. The new U-Pb zircon ages, when integrated with new field observations, reveal three periods of arc development. The youngest ‘dopo’ Bonanza has a maximum age of 180 Ma, and is built on the oldest ‘prima’ (>197 Ma), with both exposing their volcanic and plutonic components. In contrast the ‘mezza’ arc (195 to 180 Ma) has no volcanic equivalent, and is represented almost exclusively as plutons in the middle crust and extruded against the dopo or prima arc along post-Jurassic thrust faults. The new chronology refutes earlier models that the deeper, more mafic rocks of the middle crust of the arc section are either restite or the crystal cumulates to form the more silicic plutonic rocks in the upper crust. Limited ⁴⁰Ar/³⁹Ar ages for plutonic rocks, and detrital UPb zircon age populations in Cretaceous Nanaimo Group sedimentary rocks on Vancouver Island show that the prima, mezza and dopo arc are all exhumed and emergent between ~ 160 - 150 Ma. Although from different crustal levels, the mezza and dopo arc both have overall average bulk compositions similar to bulk continental crust, and appear unlike the bulk mid- and upper crustal equivalents of the Jurassic Talkeetna arc section to the north. In this way, continental-like crust was created in an intraoceanic realm for the Bonanza arc, possibly due its emplacement on a thickened pre-Jurassic oceanic plateau, whereas the Talkeetna segment was built on thinner oceanic crust.