Continental arcs and island arcs, eventually accreted to continental margins, are thought to have been the locus of continental growth since at least the Proterozoic Eon. The Jurassic Bonanza arc, part of the Wrangellia terrane on Vancouver Island, British Columbia, exposes the stratigraphy of an island arc emplaced between 203 and 164 Ma on a thick preexisting substrate of noncontinental origin. We measured the bulk major- and trace-element geochemistry, and Rb-Sr and Sm-Nd isotope compositions of 18 plutonic samples to establish if differentiation involved contamination of the Bonanza arc magmas by the pre-Jurassic basement rocks. The 87Sr/86Sr and 143Nd/144Nd isotope ratios of the plutonic rocks at 180 Ma vary from 0.70253 to 0.7066 and 0.512594 to 0.512717, respectively. Assimilation–fractional crystallization modeling using trace-element concentrations and Nd and Sr isotope ratios indicates that contamination by a Devonian island arc in the Wrangellia basement is less than 10%. Rare earth element modeling indicates that the observed geochemistry of Bonanza arc rocks represents two lineages, each defined by two stages of fractionation that implicate removal of garnet, varying in modal proportion up to 15%. Garnet-bearing cumulate rocks have not been reported from the Bonanza arc, but their inference is consistent with our crustal thickness estimates from geological mapping and geobarometry, indicating that the arc grew to at least 23 km total thickness. The inference of garnet-bearing cumulate rocks in the Bonanza arc is a previously unsuspected similarity with the coeval Talkeetna arc (Alaska), where garnet-bearing cumulate rocks have been described. Geochronological data from the Bonanza arc show a continuum in plutonic ages from 203 to 164 Ma, whereas the volcanic rocks show a bimodal age distribution over the same span of time with modes at 198 and 171 Ma. We argue that the bimodal volcanic age distribution is likely due to sampling or preservation bias. East-west separation of regions of young and old volcanism could be produced by rollback of a west-dipping slab, forearc erosion by an east-dipping slab, or juxtaposition of two arcs along arc-parallel strike-slip faults.