HR: 0800h
AN: V31C-0636    [Abstracts]
TI: Eruption of Alkaline Basalts Prior to the Calc-alkaline Lavas of Mt. Cleveland Volcano, Aleutian Arc, Alaska
AU: * Bridges, D L
EM: dbridges@ksu.edu
AF: Kansas State University, Dept of Geology 108 Thompsan Hall, Manhattan, KS 66506 United States
AU: Nicolaysen, K P
EM: knic@ksu.edu
AF: Kansas State University, Dept of Geology 108 Thompsan Hall, Manhattan, KS 66506 United States
AB: Mt. Cleveland is a 1,730 m stratovolcano, located on Chuginadak Island, that has erupted at least 23 times historically, with the latest occurring in August 2005. Major, trace, and REE analyses of 63 samples from Mt. Cleveland, including 8 from proximal cinder cones and 4 from andesitic domes on the lower flanks, identify two distinct lava suites. Modern Cleveland (MC) basalts to dacites (50.5-66.7 wt.% SiO₂) exhibit a calc-alkaline differentiation trend. Major element trends suggest crystal fractionation of plagioclase +/- ortho- and clinopyroxene in MC lavas and olivine in cinder cone deposits. Resorption textures on plagioclase and olivine phenocrysts and multiple populations of plagioclase predominate throughout the MC suite suggesting magma mixing is a major process at Cleveland. Frothy white xenoliths of plagioclase + quartz + biotite are encased in glass and erupted as small pumiceous fragments in 2001. The partial resorption of the xenocrysts indicates assimilation is also an active crustal process at Cleveland. MC trace element spider diagrams exhibit a typical arc pattern in which HFS elements including Nb are depleted, and Pb and LIL elements are enriched. Th/La, Sm/La, and Sr, Nd, Pb, and Hf isotopic ratios indicate both a North Pacific MORB and a sediment component in the source of modern Cleveland lavas, consistent with sediment flux estimates of 90 to 95 m³/m/yr and an updip sediment thickness of 1300 to 1400 meters. Average ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd values for the calc-alkaline suite are 18.93, 15.58, 0.70345, and 0.51303 respectively. The second suite consists of 3 olivine-rich, mildly alkaline basalts (48.5-49.4 wt.% SiO₂), of older stratigraphic position than MC lavas representing deposits from an older phase of activity (ancestral Cleveland, AC). La/Yb, Sr/Y, and Th/Nb ratios indicate lower degrees of partial melting, relative to MC lavas, and suggests presence of garnet in the source region. The AC lavas, however, are not similar to the eclogite melts that have created adakites in the western Aleutians. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd isotopic compositions for one of the alkali basalts are 18.60, 15.52, 0.70286, and 0.51315 respectively, which suggest, little sediment contributed to the source of AC lavas.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005