AGE RELATIONS AND ASSEMBLY OF AN INTRAOCEANIC ARC CRUSTAL SECTION IN WRANGELLIA: THE JURASSIC BONANZA ARC, VANCOUVER ISLAND, CANADA

The Jurassic Bonanza arc of Vancouver Island has been proposed as a southern extension of the Talkeetna arc (in Alaska) but their correlation within Wrangellia is hampered by a missing lower crustal section of the Bonanza arc, and inadequate age dating for the volcanic rocks of the Talkeetna arc. We present recent U-Pb zircon ages, bulk rock chemistry and field observations for both volcanic and plutonic rock sections of the Bonanza arc. The new age dates, when integrated with our field observations, reveal three periods of arc development: ‘prima’ (205 – 195 Ma) consisting of both volcanic and plutonic rocks; ‘mezza’ (195 – 180 Ma) consisting of only plutonic rocks with no volcanic equivalent; and ‘dopo’ (<180 Ma) consisting of both volcanic and plutonic rocks. Dopo plutonic rocks intrude older units (mezza and prima), and dopo volcanic stratigraphy commonly overlies prima volcanic rocks. The ‘mezza’ arc (195 – 180 Ma) has no volcanic equivalent, and is represented almost exclusively as mid-crustal plutons. Mezza plutonic rocks commonly display prevalent deformation and are mostly exposed along post-Jurassic thrust faults. Our updated U-Pb chronology suggests some diachroneity in arc crustal sections, and refutes generalized models that the deeper, mafic rocks of the middle crust of an arc are the restite or the crystal cumulate complement of more silicic plutonic or volcanic rocks observed in the upper crust.