Igneous Layering in a Dacite: On the Origin and Significance of Layer Cake Mountain, Kelowna, B.C., Canada

J. D. Greenough and J. V. Owen
Department of Earth and Ocean Sciences, University of British Columbia, Okanagan University College, 333 College Way, Kelowna, BC, V1V 1V7, Canada
Department of Geology, Saint Mary's University, Halifax, NS, B3H 3C3, Canada

Abstract: A Tertiary, dacitic volcanic land-form in Kelowna, British Columbia, shows layering that has not been recognized elsewhere. Layering is expressed as thin (0.5 m) layers separated by thick (4.5 m) layers exposed along a weathered fault scarp. The major elements show that both thick and thin layers are dacitic and geochemically very similar. Trace element modelling indicates that thin layers formed from thick layers via crystal fractionation involving removal of plagioclase, biotite and magnetite in the proportions 75:20:5, and with only 12% fractionation. The thin layers represent segregation veins generated during crystallization of the dacite. They formed when the crystal mush at the bottom of the upper crust successively, thermally contracted, fractured and foundered, siphoning evolved interstitial liquid from the mush into the horizontal crack. Cooling of the segregation veins led to further fracturing. Later, fluids following these fractures altered the thin layers and precipitated secondary carbonate minerals. The altered thin layers weather preferentially, thus visually accentuating the small primary chemical differences between thick and thin layers. The scale of layering, mode 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.

Keywords: igneous layering • dacite • fluids • differentiation • Kelowna • Canada

Mineralogical Magazine; December 1998 v. 62; no. 6; p. 731-742; DOI: 10.1180/002646198548061
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)