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Geology, geochemistry, and geochronology of the Westcoast Crystalline Complex and related rocks, Vancouver Island, British Columbia Isachsen, Clark


The Westcoast Crystalline Complex occurs as a belt of complexly mixed plutonic rocks along the west coast of Vancouver Island. It is composed mainly of heterogeneous amphibolitic country rock (Westcoast Amphibolite), granitoids of trondhjemitic to gabbroic composition (Westcoast Diorite), and variable mixtures of these two components (Westcoast Migmatite). Various lines of evidence suggest that these rocks were generated in a magmatic arc setting and intruded the crust in Jurassic time. Major and trace element chemistry of the Westcoast Crystalline Complex shows a subalkaline tholeiitic to calc-alkaline trend compatible with petrogenesis in a magmatic arc. U-Pb isotopic dates of zircon separates are consistently concordant between 176 and 189 Ma and show no appreciable evidence for an older crustal component. Initial Sr ratios obtained for the Westcoast Crystalline Complex are between 0.70329 and 0.70360, compatible with a depleted mantle source region. The concave cooling curves derived for Westcoast Diorites are not consistent with in situ crustal magmagenesis but, instead, indicate that these rocks were intruded into relatively cool country rock. Based on age and chemistry, the Westcoast Crystalline Complex can be interpreted as the deeper crustal equivalent of the more differentiated Island Intrusions and Bonanza Volcanics. Taken together, these rocks provide a disrupted and perhaps incomplete cross-section of the Jurassic magmatic arc of Vancouver Island. Reconnaisance of the Wark/Colquitz Complex of southern Vancouver Island shows it to be essentially indistinguishable in petrography, chemistry and age from the Westcoast Crystalline Complex, and a similar history is inferred. A calc-alkaline chemistry and low initial Sr ratio were also determined for a Catface Intrusion dated at 41 Ma. This is again compatible with mantle-derived arc magmatism, but its proximity to the coeval trench is enigmatic.

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