Abstract

Mafic lavas of the Nicola Group are divided into three distinct petrographic and geochemical types: type 1 lavas are strongly augite-porphyritic picrites, basalts, and andesites that belong to a high-potassium to shoshonitic rock series; type 2 lavas are augite- and plagioclase-porphyritic basalts and andesites that belong to a low-potassium calc-alkaline series; and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites.Low concentrations of Ti, Zr, Y, and Nb and moderate to high concentrations of K, Rb, Ba, and Sr in type 1 and 2 lavas clearly indicate a subduction-related tectonic setting of eruption. Type 3 lavas show chemical affinities intermediate between modern-day island-arc and intraplate volcanics. Type 1 (shoshonitic) lavas generally lie east of and are younger than type 2 (calc-alkaline) lavas, a relationship that implies an east-dipping early Mesozoic subduction zone beneath the Nicola arc. These interpretations resolve previous uncertainties regarding the tectonic setting of eruption of the Nicola Group.Several major 205–220 Ma plutons that intrude the Nicola Group crystallized from type 1 and 2 magmas and represent the final stages of Late Triassic to Early Jurassic arc-related igneous activity in southern Quesnellia.

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