Abstract

The Westcoast Crystalline Complex is a belt of 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).Although the protolith of some deformed enclaves may be Paleozoic, most of these rocks were generated in a magmatic-arc setting and intruded in Jurassic time. Major- and trace-element chemistry of the Westcoast Crystalline Complex shows a sub-alkaline tholeiitic to calc-alkaline trend.The exponential cooling curves derived for Westcoast diorites are not consistent with in situ crustal magma genesis but instead indicate that these rocks intruded 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 magmatic arc of Vancouver Island.Reconnaissance 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 rapid initial cooling also characterize a Catface Intrusion dated at 41 Ma. This is again compatible with arc magmatism, but its proximity to the coeval trench is enigmatic.

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