COMPOSITIONAL PROGRESSION FROM BASALT-ONLY TO BASALT+ANDESITE+DACITE DURING THE 300 KA ERUPTIVE HISTORY OF MT. VENIAMINOF, ALASKA (Invited Presentation)

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.