Quenched mafic inclusions in ≤2200 years B.P. deposits at Augustine Volcano, Alaska

Abstract

This article describes results from morphological, textural, mineralogical, and compositional analyses of lava clasts and quenched mafic inclusions from 10 Holocene debris-avalanche deposits (2200–125 years B.P.) that form the lower flanks of Augustine Volcano, Alaska. Mafic inclusions collected from the Rocky Point pyroclastic-flow deposit emplaced during the 2006 eruption are included for comparison. All deposits contain evidence for mixing between basalt and high-silica andesite prior to and during eruption, including compositional banding, disequilibria phenocryst assemblages, linear variation in major- and trace-element concentrations, and quenched mafic inclusions (51.3–57.3 wt.% SiO2) hosted by andesite lavas (59.1–62.6 wt.% SiO2). Generally, inclusions from all deposits share many morphologic and petrologic characteristics. Inclusions range in diameter from <1 to >36 cm, although 87% of the 959 inclusions analysed are less than 5 cm in diameter. Mafic inclusions account for an average of 8 vol.% of host andesitic lava clasts included in the oldest debris-avalanche deposits compared with subsequently emplaced deposits that contain an average of only 1–3 vol.%. Inclusions contain phenocrysts of plagioclase, amphibole, clinopyroxene, olivine, and rare orthopyroxene as well as microphenocrysts of plagioclase, amphibole, clinopyroxene, olivine, magnetite, ilmenite, and apatite in a glassy, vesicular, and acicular groundmass. Plagioclase phenocrysts in inclusions typically have a 30–150 μm-thick, fine-grained ‘dusty sieved’ rim superimposed over oscillatory zoned texture, suggesting that they existed in the host andesite magma prior to basaltic intrusion, but were engulfed during the intrusion and inclusion formation process. Mafic inclusions are calc-alkaline, low-K (0.45–0.82 wt.% K2O) basalts to basaltic andesites that are altogether different in terms of mineralogy and composition from olivine basalt contained in late Pleistocene-age fragmental deposits described by Plank et al. (2006, The Augustine basalt: Eos (American Geophysical Union Transactions), v. 87, V42B-06) and Larsen et al. (2010, Petrology and geochemistry of the 2006 eruption of Augustine Volcano, in Power, J.A., Coombs, M.L., and Freymueller, J.T., eds., The 2006 eruption of Augustine Volcano, Alaska: US Geological Survey Professional Paper 1769). Thus, the most reliable approximation of the basaltic endmember composition involved in magma mixing processes at Augustine Volcano over the past ∼2200 years are the most mafic inclusions, which consistently possess the (1) highest abundance of vesicles, (2) fewest number of phenocryst-sized plagioclase, and (3) largest volume of microlites compared with more contaminated inclusions.