Magmatic composition and tectonic setting of altered, volcanic rocks of the Fennell Formation, British Columbia

Abstract

The upper Paleozoic Fennell Formation in south-central British Columbia consists of basalts and associated marine sediments metamorphosed at low greenschist facies conditions. Although the microphenocrysts of plagioclase and Fe–Ti oxides are almost completely altered, those of augite and amphibole have survived this metamorphism. In the vicinity of the Chu Chua mineral deposit, relict augite microphenocrysts, which constitute a major proportion of the microphenocryst assemblage, are enriched in Al and Ti and are similar in composition to those from alkalic and transitional basalts. Relict amphiboles are also enriched in Ti (4.5–5.9% TiO2) and are classified as kaersutites. The occurrence of kaersutite and the chemistry of relict augites indicate that in this area the Fennell Formation basalts were originally alkalic and transitional in composition. On conventional Ti–(Zr/P2O5) and (Nb/Y)–(Zr/P2O5) immobile-element discrimination diagrams, both the kaersutite-bearing and kaersutite-free rocks plot in the tholeiitic basalt field. Accordingly, it is suggested that these diagrams may not provide clear evidence for the magmatic composition of altered volcanic rocks.Based on the lead isotopic compositions, petrographic features, and alkalic character of the Fennell Formation basalts, it is interpreted that these basalts were formed in a tectonic setting similar to that of present-day oceanic islands or seamounts.