Towns, J., Green, N., Powell, J., Garcia, B., (2006), Complex Mafic Andesite Evolution at Mount Baker Volcano, Washington, Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract V23C-0635

Complex Mafic Andesite Evolution at Mount Baker Volcano, Washington

The Holocene Sulphur Creek basaltic andesite of Mount Baker exhibits textural and chemical evidence of a complex crystallization-mixing history. The flow is highly porphyritic, with 25-43% phenocrysts and microphenocrysts of plagioclase, olivine, clinopyroxene and orthopyroxene (average 35% crystals, comprising 66% plagioclase, 15% olivine, 18% pyroxenes and 1% oxides). Phenocrysts show somewhat variable distribution throughout the flow length, whereas microphenocryst proportions, except olivine, tend to be more regular. Most phenocryst phases show textures interpreted as reaction or resorption due to superheating, e.g. olivine (Fo66 ) overgrowth on corroded hypersthene (En71-68 ) xenocrysts. The lava exhibits a general change from mafic andesite to basaltic compositions towards its terminus. Major-element variations, which are compatible with fractional crystallization of observed phenocryst proportions from the basalt, are accompanied by increasing Cs, Rb, Ba, Sb, Tl, Th and U and decreasing Ni, Co, Zn, Cr, V, Y, Hf, Zr, and HREE contents with differentiation. Inter-element ratios (e.g., Ba/Nb, Ba/La, K/La, Ba/Th, U/La, Th/Nb, and Ba/Zr) typically associated with a subduction signature exhibit up to a two-fold variation within the lava. Magma evolution of the Sulphur Creek magma appears to have occurred within a chemically zoned system subjected to periodic injections of basaltic magma and its mixing with more evolved liquids.