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,11 Department of Geology and Geophysics, University of Wisconsin–Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, USA
New 40Ar/39Ar dating coupled with detailed field mapping, stratigraphy, and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of Seguam Island in the Aleutian Island arc, Alaska. Sixty new 40Ar/39Ar ages from lavas, domes, and pyroclastic deposits were obtained using furnace incremental-heating techniques on replicate samples of whole-rock and ground-mass separates, and they constrain the duration of Pleistocene to Holocene subaerial volcanism to 318 k.y. The 40Ar/39Ar plateau ages indicate that over 85% of the complex, 
68 km3 of material, was erupted almost continuously between 318 ka and 9 ka. At ca. 9 ka, a stratocone on the eastern half of the island partially collapsed producing a 4-km-wide crater. Rhyolitic dome-forming eruptions followed from vents in the newly created crater, and were likely contemporaneous with 8.0 km3 of basaltic and basaltic andesitic effusions from Pyre Peak, and a 1.4 km3 basaltic eruption from a monogenetic cone on the far eastern end of the island. Geochemical changes over the last 318 k.y. are subtle. Most notably, the earliest eruptions from 318 to 142 ka produced no andesite, and basalt from this period has larger ranges in Zr/Rb and La/Yb than younger basalts. Small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal-liquid fractionation process that varied only slightly in successive eruptive phases over 318 k.y. We identified minor geochemical changes in magma composition during each of the three main stages of volcanism, but overall the monotonic variations in major- and trace-element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed-system fractional crystallization of basalt. Using the 40Ar/39Ar geochronology and estimates of individual flow volumes, we calculated a time-averaged eruptive rate at Seguam that is similar to growth rates of other well-dated arc volcanoes in the Cascades and Chilean Southern volcanic zone but less than that of Mount Katmai and Mount Mageik, which are located on the Alaska Peninsula. The eruptive flux at Seguam has been highly variable, fluctuating more than an order of magnitude, from 0.07 km3/k.y. during the early history of bimodal volcanism to 1.18 km3/k.y. over the past 9 k.y.
Key Words: Aleutian island arc • 40Ar/39Ar geochronology • magmatic evolution • Seguam Island • eruptive rates
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