INSIGHTS FROM ND AND SR ISOTOPES INTO THE CONSTRUCTION OF THE BONANZA ARC CRUSTAL SECTION, VANCOUVER ISLAND, CANADA

The Jurassic Bonanza Arc (Vancouver Island, Canada), comprises the West Coast Complex (WCC) and Island Plutonic Suite (IPS). The Bonanza arc has a bimodal U-Pb zircon magmatic age distribution with peaks at 170 and 190 Ma and a weak trend of younger ages to the east. Previously published Nd and Sr isotopic data indicates that the WCC shows evolved εNd (at 180 Ma) values of 0.7 to 2, whereas the IPS is more juvenile (4.6 to 6.7), also with a weak trend to more evolved values to the west. All plutonic Bonanza rocks show a restricted range of initial (180 Ma) Sr isotopic compositions of 0.70341 to 0.70639, with an extreme outlier at 0.72649. The present study seeks to address the reasons and implications of these observations.

It is possible that what is currently called the Bonanza Arc is in reality two arcs active ~20 Myr apart, either accreted to each other or emplaced within one another. One reason for the difference in εNd between the WCC and IPS suites is that the former sampled an enriched mantle source whereas the latter sampled depleted mantle. Another possibility is that the WCC represents arc material that is contaminated with older crustal material. The pre-Jurassic basement to the Bonanza rocks are not of sufficient age or isotopic composition to produce the observed εNd values, except at high degrees of contamination. Analysis of a single sample of WCC indicates the presence of Precambrian inherited zircon, thus also allowing for the possibility of contamination by crust much older than currently known on Vancouver Island. We have investigated these possibilities by measuring Nd and Sr isotopes of 19 further samples of Bonanza arc rocks and report the results. These samples represent the complete ~15 km of structural thickness and straddle a tentative line separating younger (< 180 Ma) from older (> 190 Ma) plutons, allowing us to examine for lateral, vertical and/or temporal heterogeneity in contamination or source enrichment.