Abstract

Volcanic rocks of the Nicola Group belong to an arc built on the western margin of the Quesnel terrane in the Late Triassic to Early Jurassic. Low-K – high-K compositional types define a Rb–Sr isochron of 222 ± 15 Ma with initial 87Sr/86Sr = 0.70367 ± 2. The corresponding Nd isotopic compositions of these samples (εNd(222 Ma) = +5.1 to +7.8) fall within the range for early Mesozoic island arcs. A comparable range of εNd(222 Ma) (+5.0 to +7.9) in picrite–shoshonite samples precludes generation of increasingly potassic magmas by progressive metasomatism of the mantle wedge alone. Source-region heterogeneity, possibly imparted by changes in the composition of subducted slab components or interaction with amphibole or phlogopite in the source remnant of an earlier (Permian) arc on the Quesnel terrane, is required to account for geochemical differences between these rock suites. Crustal contamination is severely limited from the high εNd values, such that continental basement now underlying the Quesnel terrane is likely an artifact of later terrane obduction.

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