John Fournelle, Bruce D. Marsh; Shishaldin Volcano: Aleutian high-alumina basalts and the question of plagioclase accumulation. Geology ; 19 (3): 234–237. doi: https://doi.org/10.1130/0091-7613(1991)019<0234:SVAHAB>2.3.CO;2
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High-alumina basalts (HABs) from volcanic arcs commonly contain 30%-50% (modal) plagioclase. It has been suggested that they reflect plagioclase addition and are not primary compositions. In rocks from the Aleutian volcano Shishaldin, we search for evidence of plagioclase accumulation: A12O3, CaO, and Na2O vs. modal plagioclase; europium anomalies in HABs; plagioclase-liquid equilibrium; and the HAB groundmass. The HABs do not appear to be results of plagioclase addition to liquids of dacitic, Fe-Ti-enriched, or high-Mg basaltic compositions. Plagioclase loss from HABs does appear to yield the Fe-Ti-enriched basalts. Shishaldin HABs may reflect near-primary compositions, and HAB phase equilibria may thus be useful in evaluating the origin of such are basalts.