Igneous layering in a dacite: on the origin and significance of Layer CakeMountain, Kelowna, BC, Canada

A Tertiary, dacitic volcanic land-form in Kelowna, British Columbia, shows layering that has not been recognized elsewhere. Layering is expressed as t hin (0.5 m) layers separated by thick (4.5 m) layers exposed along a weathe red fault scarp. The major elements show that both thick and thin layers ar e dacitic and geochemically very similar. Trace element modelling indicates that thin layers formed from thick layers via crystal fractionation involv ing removal of plagioclase, biotite and magnetite in the proportions 75:20: 5, and with only 12% fractionation. The thin layers represent segregation v eins generated during crystallization of the dacite. They formed when the c rystal mush at the bottom of the upper crust successively, thermally contra cted, fractured and foundered, siphoning evolved interstitial liquid from t he mush into the horizontal crack. Cooling of the segregation veins led to further fracturing. Later, fluids following these fractures altered the thi n layers and precipitated secondary carbonate minerals. The altered thin la yers weather preferentially, thus visually accentuating the small primary c hemical differences between thick and thin layers. The scale of layering, m ode of formation and differentiation mechanisms appear different from those in felsic magma chambers and it is unclear how common this phenomenon is. However, similar layering is more easily identified and commonly developed in mafic lava flows.