MAGMA-CARBONATE INTERACTIONS WITHIN THE JURASSIC BONANZA ARC, VANCOUVER ISLAND

The Jurassic Bonanza arc on Vancouver Island, British Columbia, was built on an Upper Triassic carbonate platform, and is an ideal setting to quantify magma-carbonate interactions beneath an island arc. Here, we report preliminary results of magma-carbonate interactions within this arc at the Merry Widow Mountain area of northern Vancouver Island, where accessible and well-exposed contacts between magma and carbonate occur on a variety of scales from km-scale plutons to m-scale dikes. Detailed mapping and whole-rock chemistry is used to quantify the reaction extent, both physically and chemically. Mapping indicates the crustal section in stratigraphic order is ~1 km of limestone, ~0.25 km of stratified volcanic tuff, and ~0.5 km of gabbro. Mafic aphyric and plagioclase-phyric dikes cross-cut the limestone, tuff, and gabbro. In the lower section, garnet-diopside-magnetite ± wollastonite ± epidote skarn is prevalent along limestone-dike contacts. In the upper section, late-stage dikes rich in ocelli-like spherules cross-cut the gabbro and the earlier mafic dikes. In addition to their distinct texture, these dikes have a unique geochemical trend in comparison to other Bonanza arc rocks both locally and regionally, displaying an increase in Ca, decreases in Na, P, Ti and Fe, and markedly low REE abundances with differentiation. The results suggest the dikes are on a mixing trend with a Ca-rich, REE-poor lithology, identical in composition to the Upper Triassic limestone. Ongoing petrography, mineral chemistry, and O-isotope ratios (dikes and gabbros) are being used to further quantify how much the arc magmas assimilated d¹⁸O-enriched carbonate, and estimate the physical and chemical transfer of CO₂ into the magma.