Postglacial eruptive history, geochemistry, and recent seismicity of Aniakchak Volcano, Alaska Peninsula

Bacon, Charles; Neal, Christina; Miller, Thomas; McGimsey, Robert; Nye, Christopher

Aniakchak is a Pleistocene to Holocene composite volcano of the Alaska-Aleutian arc that suffered at least one caldera-forming eruption in postglacial time and last erupted in 1931. The glacially sculpted Pleistocene edifice lies atop gently folded Jurassic to Tertiary sedimentary rocks. Aniakchak products have geochemical characteristics typical of magmatic arcs. Parental basaltic magmas with a range of incompatible-element concentrations are derived from the mantle wedge above the Wadati-Benioff zone, drawn from a region broader than the edifice footprint, and focused in the crust into a conduit and storage system beneath the volcano. Over time, a column of crystal-liquid mush and solidified intrusions has developed to as shallow as a few kilometers below sea level (bsl), allowing differentiated melts to evolve, segregate, and accumulate in crystal-poor bodies as large as several tens of cubic kilometers in volume. The dominant process of magma evolution at Aniakchak is considered to be crystallization differentiation. The oldest recognized postglacial explosive eruption, andesitic Aniakchak I, left nonwelded ignimbrite in valleys below the edifice and fines-poor welded ignimbrite or agglutinated fall deposits high on its flanks between ca. 9,500 and 7,000 years ago. A small caldera may have collapsed at the source of the ignimbrite, likely just west of the present Vent Mountain. Aniakchak I andesite is geochemically distinct among the postglacial Aniakchak suite for its high K2O, Zr, and other incompatible-element contents. Subsequent to Aniakchak I, Plinian eruptions ca. 7,000 14C yr B.P. from a vent northeast of the edifice summit produced the Black Nose Pumice, consisting of a lower unit of rhyodacite lava, pumice fall, and intraplinian welded ignimbrite and an upper unit of dacite pumice fall and northeast flank lava flow. At least 20 additional Holocene eruptions are thought to have occurred before the Aniakchak II caldera-forming event. The spectacular 10-km-diameter Aniakchak Crater is a caldera that collapsed during the voluminous Aniakchak II eruption ca. 3,590 cal yr B.P. (ca. 3,430 14C yr B.P.). This eruption produced rhyodacite Plinian pumice fall and ignimbrite, followed first by ignimbrite with both rhyodacite and andesite juvenile clasts, then by andesite ignimbrite. The mixed and andesite ignimbrite deposits commonly are partly welded. Ignimbrite is present in valleys surrounding the edifice and extends to the coast on both northwest and southeast. The crystal-poor rhyodacite is compositionally quite uniform (69.4-70.4 weight percent SiO2) and is the most evolved and only hornblende-phyric magma erupted from Aniakchak. The andesite is unusual for Aniakchak in having low V and high Na2O, Y, TiO2, and, especially, P2O5 concentrations. Because both Aniakchak I and II andesites have close analogs at Veniaminof volcano ~100 km down the Alaska Peninsula, these comparatively unusual compositions reflect common processes and are unlikely to be due to local effects of crustal assimilation. With its top as shallow as 1.6-3.7 km bsl, a rhyodacite melt lens at least 400 m thick underlay the approximately 6 km by 8 km area described by the foundered cauldron block. The andesite reservoir apparently was similarly large but not contiguous with that of the rhyodacite, and the two magmas only mingled during transit to the surface during the eruption. The rhyodacite evidently was unrelated to precaldera dacite magmas. It and the andesite represent a new cycle of recharge, storage, and differentiation...

Postglacial eruptive history, geochemistry, and recent seismicity of Aniakchak Volcano, Alaska Peninsula

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