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Brophy, J.G., 1984

The chemistry and physics of Aleutian arc volcanism: The Cold Bay volcanic center, southwestern Alaska

Bibliographic Reference

Brophy, J.G., 1984, The chemistry and physics of Aleutian arc volcanism: The Cold Bay volcanic center, southwestern Alaska: Baltimore, Maryland, The Johns Hopkins University, Ph.D. dissertation, 422 p., illust., maps.

Abstract

The Cold Bay Volcanic Center is an extinct, calc-alkaline basaltic to andesitic complex located at the tip of the Alaska Peninsula. Late Pliocene extrusion of basalt (50-53 wt.% SiO2), basaltic andesite (53 to 56%) and andesite (56 to 59%) lavas was followed by Pleistocene extrusion of solely andesitic lava (56 to 62%). The basalts are characterized by high Al2O3 (18 to 22%), moderate Na2O (-3%) and K2O (-1%) and low MgO (4 to 6%) and TiO2 (< 1%) and are similar to basalts found at other Aleutian centers. With increasing silica, the variations in oxide abundances are typically calc-alkaline. Normalized LREE abundances range from 18 to 50 with La/Yb values of 1 to 5. Rb, Ba, and Zr abundances vary with silica from 9 to 42, 224 to 640, and 64 to 167 ppm respectively. Sr abundances average 400 ppm throughout, while V varies from 257 to 76 ppm. The majority of characteristics of the basalts, in particular the major element and REE abundances, are best satistifed by a quartz eclogite source with 2 to 5% pelagic and terrigenous sediment. Initial melting occurs at the Benioff zone (90 to 120 km) and, after about 20% melting, gravitational instability of the partially-molten source occurs, leading to diapirism. Further melting occurs during ascent and, after 40 to 50% melting, corresponding to a depth about 15 to 20 km above the Benioff zone, the melt is extracted and transported to the surface as basaltic magma. The major and trace element variations are satisfied by low-pressure fractionation of the observed phenocrysts (plag, oliv or opx, cpx, and mag). However, theoretical considerations of the mechanics of crystal settling in a magma suggest that such fractionation can only be responsible when fewer than 30% crystals are present. Fractionation is also viable only if the magma has Newtonian properties of non-Newtonian properties with very low (< 250 dynes/cm2) yield strengths.

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