Geochemical polarity of the Early Cretaceous Gambier Group, southern Coast Belt, British Columbia

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.