ERUPTIVE HISTORY AND MAGMATIC EVOLUTION PROCESSES OF LONG-LIVED AKUTAN VOLCANO, EASTERN ALEUTIAN ISLANDS
Earliest Akutan lavas are basaltic and have high and variable La/Yb ratios consistent with low degrees of partial melting in the mantle wedge. A subset of Pleistocene lavas are calc-alkaline andesites and likely formed by mixing of mafic and evolved magmas, not through crystallization-derived differentiation. In contrast, Holocene lavas, erupted mostly from Akutan proper, plot along low La/Yb, discrete tholeiitic, basalt-to-dacite evolutionary trends that differ only subtly in initial TiO2 contents. Holocene basalts and basaltic andesites have diverse phenocryst contents, whereas the tholeiitic andesite and dacites are phenocryst poor and lack pervasive evidence for magma mixing. Plutonic-textured blocks, spanning a wide range in compositions, are present in the deposits of the volcano’s 1.6-ka caldera-forming eruption. They are interpreted to represent (1) completely crystallized injections of basaltic andesitic liquid and (2) crystal cumulates from tholeiitic fractional crystallization of the active Akutan magma system. The mineralogy and chemical compositions of Pleistocene to recent Akutan lavas, coupled with existing geodetic and seismic interpretations from the island, collectively support a conceptual model for the active magmatic system wherein hot, relatively dry magmas stall and differentiate shallowly (5–10 km depth) prior to eruption.