BONANZA ARC CRUSTAL SECTION EXPOSED ON SOUTHERN VANCOUVER ISLAND, B.C.: TALKEETNA SOUTH?

The evolution of the Jurassic Bonanza island arc is recorded by igneous rocks exposed in oblique section on southern Vancouver Island, British Columbia. Tilting of the arc has exposed a crustal section of plutonic rocks ranging in composition from two-pyroxene gabbro to K-feldspar granite, with diorite dominating the suite. A detailed study of the section involving mapping (1:25,000), geochronology, thermobarometry and geochemistry is being undertaken to understand the evolution of arc crust. Intrusive rocks of the Bonanza arc are overlain by contemporaneous volcanics of basalt and basaltic andesite, with minor andesite and pyroclastics. Hornblende barometry on 22 intermediate plutonic rocks shows that the crustal section exposes rocks from ~ 15 - 18 km depth in the west to less than 5 km in the east, less than that inferred by Lithoprobe profiles. The U-Pb ages of intrusions throughout the arc section vary from 165 Ma to 182 Ma and show no regular pattern with depth (DeBari et al, 1999, CJES; this work). Peridotite and pyroxenite bodies within the mafic-intermediate plutons of mid-crust in the Bonanza arc appear to be the complement of the more evolved intrusive rocks. Although not magnesian enough to be considered fractionates of a primary mantle magma, the Bonanza arc cumulates document olivine-controlled fractionation of a hydrous magma in a mid-crustal magma chamber. The section is remarkably similar to that of the Jurassic Talkeetna arc section to the north in Alaska, but a significant volume of ultramafic rocks is missing, and is either structurally displaced by faulting, or was lost at depth by foundering/delamination of the lower arc crust. These possibilities and the overall mass balances involved to build continental crust within the Bonanza arc are currently under further study.