Age relations and assembly of an intraoceanic arc crustal section in Wrangellia: The Jurassic Bonanza arc, Vancouver Island, Canada
Rebecca Morris, University of Victoria
Dante Canil, University of Victoria
Abstract
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 rocks of the Bonanza arc. The new age dates, when integrated with field observations, reveal three periods of arc development: dopo (165 to 180 Ma) consisting of both volcanic and plutonic rocks; mezza (180 to 195 Ma) consisting of only plutonic rocks with no volcanic equivalent; and prima (>195 Ma) consisting of both volcanic and plutonic rocks. Dopo plutonic rocks intrude all older units (mezza and prima), where dopo volcanic stratigraphy commonly overlies prima volcanics. The mezza arc (195 to 180 Ma) has no volcanic equivalent, and is represented almost exclusively as mid-crustal plutons. Mezza plutonic rocks are mostly exposed along post-Jurassic thrust faults. Detrital U-Pb zircon age populations in Cretaceous Nanaimo Group sedimentary rocks on Vancouver Island suggests that the prima, mezza and dopo arc are exhumed and emergent between ~ 160 – 150 Ma. The new 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 always the restite or the crystal cumulate complement of more silicic plutonic or volcanic rocks observed in the upper crust.
Session
Cordilleran tectonics