The Lower Jurassic Rossland volcanic rocks of southern British Columbia form a succession of agglomerates, conglomerates, lavas, and fine-grained volcaniclastics and pyroclastics intercalated with marine shales and siltstones. They are variably metamorphosed up to greenschist facies, but some igneous phases are preserved. The lava types represented are strongly porphyritic and range from augite-rich ankaramites to plagioclase-phyric andesites. Previous work has shown these rocks to have evolved in an island-arc volcanic regime.
Despite the effects of secondary alteration, the suite is not strictly calc-alkaline but may be undersaturated with respect to SiO2 in part. Mineral and whole-rock compositional data indicate that most of the chemical variation of the suite is due to crystal control by the observed phenocrysts at crustal levels (2–10 kbar) and with a relatively low partial pressure of H2O. The origin of the parental magmas is thought to have been within the upper mantle, rather than within a subducted slab. Melting at about 20 kbar, followed by a degree of polybaric olivine plus pyroxene fractionation, can account for the observed ankaramitic compositions.