GSA Connects 2021 in Portland, Oregon

Paper No. 232-2
Presentation Time: 1:50 PM


CALVERT, Andrew, BACON, Charles and SISSON, Thomas, USGS, Volcano Science Center, 345 Middlefield Road, Menlo Park, CA 94025-0000

Well known in the record of arc magmatism are compositionally zoned single eruptions, as well as the long-term (1-10 Myr) evolution and zonation of single plutons, intrusive suites, and larger volcanic centers. Between these extremes, arcs rarely show simple, systematic compositional evolution over the activity span of a single volcano. Mt. Veniaminof, lower Alaska Peninsula, is an exception, consisting of a spectacularly exposed, 350 km3, 40 km diameter, 2500 m tall arc tholeiite series stratovolcano with an ice-filled summit caldera pierced by an active basaltic andesite cone. Adjacent voluminous centers are calc-alkaline intermediate (Black Peak) or tholeiitic mafic-intermediate (Kupreanof, Stepovak Bay). Geologic mapping, argon geochronology and major/trace element geochemistry inform its eruptive history, including a smooth progression from exclusively basalt to basalt+andesite and eventually dacite. Early eruptions (300-200 ka) were basalt to basaltic andesite (48-54 wt.% SiO2, 7-9 wt.% MgO) with high eruptive flux accumulating >220 km3 (>1500 m thickness) of variably porphyritic basalt between basal lavas (~300 ka) and the top of caldera rim (200 ka). Younger (200-130 ka) eruptives range to andesite (51-60 wt.% SiO2, 3-7 wt.% MgO) then (130-0 ka) to dacite (51-70 wt.% SiO2, 1-7 wt.% MgO). Eruptive volumes are challenging to quantify due to glaciation, but diminished markedly from the initial 300-200 ka mafic stage. Roughly equal volumes of <200 ka basaltic andesite and andesite (~6 km3 each) and ~13 km3 of dacite lavas are preserved, plus several 10s of km3 of estimated explosive andesite and dacite. Intermediate and silicic rocks are crystal poor with chemical trends dominated by shallow crystallization differentiation (Sisson et al., this meeting) in an intrusive complex revealed by Holocene-erupted gabbro through miarolytic granodiorite blocks. Causes are obscure for the eruption of progressively more evolved magmas approaching the present, but the correlation with diminished eruptive flux is clear, suggesting either declining supply of basalt from the mantle, or thickening of the hot, plastic intrusive complex retarding ascent of magmas from depth. Both possibilities allow progressively more evolved residual liquids to ooze from the shallow top of the intrusive complex.