Abstract

The Gambier Group of the southern Coast Belt was developed on the eastern edge of the Wrangellia terrane in Early Cretaceous time. A U–Pb zircon age determination from rhyolite produced a date of 112.0 ± 0.3 Ma, consistent with Albian fauna in sedimentary units. Major element geochemistry of volcanic rocks from separate east-northeast and southwest volcanic centres displays both tholeiitic and calc-alkalic affinities across the axis of the Coast Belt. However, assemblages in the east-northeast have higher FeOT/MgO ratios, highlighting a more pronounced tholeiitic trend, and are characterized by high-Al basalts. Volcanic rocks in the southwest are enriched in K. Trace element variations feature a southwesterly increase in Ba, Rb, and Zr/Ti towards more alkalic values, and a decrease in Sr. When compared with across-arc geochemical trends from numerous other arcs, the data may be interpreted to suggest that the Gambier arc was east facing above a west- to southwest-dipping subduction zone. Such an interpretation is consistent with the occurrence of a subduction-melange complex along the southeastern Coast Belt and with a structural style dominated by southwest-verging thrusts in mid-Cretaceous time.

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