HR: 1330h
AN: V32D-1048    [PDF]
TI: History and Eruptive Style of Mount Veniaminof, a Huge Alaskan Basalt-to-Dacite Volcano With Pleistocene and Holocene Caldera-Forming Eruptions
AU: * Bacon, C R
EM: cbacon@usgs.gov
AF: U.S.Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Calvert, A T
EM: acalvert@usgs.gov
AF: U.S.Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Nye, C J
EM: cnye@giseis.alaska.edu
AF: Alaska Volcano Observatory/ADGGS, 903 Koyukuk Dr., Fairbanks, AK 99775 United States
AU: Sisson, T W
EM: tsisson@usgs.gov
AF: U.S.Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The eruptive history of Mount Veniaminof, one of the largest and most active volcanoes in the Aleutian arc, is being established through geologic mapping, geochemistry, and $^{40}$Ar/$^{39}$Ar and K-Ar geochronology. The Veniaminof edifice has a basal diameter of $\sim$40 km, a volume of $\sim$350 km$^{3}$, and an 8-km-diameter ice-filled caldera whose wall reaches an elevation of 2500 m. Glassy lava flow carapaces with polygonal and columnar joints are common and indicate widespread ice/lava interaction, which resulted in ice-marginal flows and tuyas. Exposures from deep glacial valleys to the caldera rim reveal a long history of dominantly basaltic and basaltic andesitic activity from at least 250 ka to 100 ka that produced lavas with compositions as primitive as 9.4% MgO and 130 ppm Ni at 50% SiO$_{2}$. Silicic magmas vented beginning $\sim$100 ka are represented by virtually aphyric dacitic lavas (to 68% SiO$_{2}$) on the south and west flanks. Similarly, voluminous nearly-aphyric andesite and dacite were produced as recently as 36.4$\pm$6.5 ka (andesite) from a northwest-trending set of flank vents. Abundant dikes follow this arc-normal trend. Other abundant dikes have arc-parallel orientations and were emplaced prior to ~100 ka pre-caldera lavas. Most lava compositions fall in the tholeiitic field on an SiO$_{2}$ versus FeO/MgO diagram. With the exception of a small volume hornblende dacite lava flow (79.0$\pm$1.4 ka), no hydrous phenocrysts are known in any Veniaminof juvenile eruptive products. Basalts and basaltic andesites carry olivine and plagioclase (to 1 cm) phenocrysts, joined by augite in some crystal-rich lavas. Andesites and dacites typically have few, small phenocrysts of plagioclase, augite, and low-Ca pyroxene $\pm$ Fe-Ti oxide. Caldera collapse may have initiated in the Pleistocene with eruption of dacitic magma locally preserved as pumiceous pyroclastic-flow and -fall deposits on the west side of Veniaminof (Miller et al. 2002) and capped by basaltic andesitic lava dated at 27.9$\pm$4.4 ka. Although the caldera may have collapsed further during two large Holocene andesitic explosive eruptions, eroded lava displaying ice-contact jointing drapes the caldera wall and indicates that a caldera was present in the late Pleistocene. The youngest dated Veniaminof lavas on the south, ice-blanketed side of the caldera are $\sim$50 ka; a few south flank cinder cones may be latest Pleistocene/early Holocene. Despite the absence of an exposed southern caldera wall, no debris-avalanche-deposit evidence has been found for an alternative, sector-collapse origin of the caldera structure. Two voluminous andesitic Holocene pyroclastic eruptions (3700 yr BP and $>$4700 yr BP; Miller et al., 2002) emplaced pyroclastic-flow deposits on Veniaminof's flanks and in surrounding valleys. The younger deposited lithic breccia at higher elevations, including plutonic clasts with textures ranging from coarse-grained cumulate gabbro to medium-to-fine-grained miarolytic hornblende tonalite compositionally like Veniaminof dacite. A still younger (subplinian?), widespread dacitic (63.5% SiO$_{2}$) fall deposit up to $\sim$30 cm thick suggests that this long-lived magmatic system may produce explosive silicic eruptions in addition to the historically-known basaltic andesitic strombolian and effusive eruptions from the intracaldera cone.\\Miller, T.P., Waythomas, C.F., and Gardner, J.E., 2002, Possible multiple late Quaternary caldera-forming eruptions at Mount Veniaminof volcano, Alaska: Eos Trans. AGU 83(47), V11A-1376.
DE: 1035 Geochronology
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 5480 Volcanism (8450)
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting