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Romick, J.D., 1982

The igneous petrology and geochemistry of northern Akutan Island, Alaska

Bibliographic Reference

Romick, J.D., 1982, The igneous petrology and geochemistry of northern Akutan Island, Alaska: University of Alaska Fairbanks, M.S. thesis, 151 p., illust.


Volcanism on Akutan Island, in the eastern Aleutian Arc, is characterized by tholeiitic lavas ranging between 46 and 63 wt percent SiO2 with FeO*/MgO ratios between 1.3 and 4.2. Olivine-augite-plagioclase basalts were found at Lava Peak, while the remaining flows and dikes on the island consist of hypersthene-augite-plagioclase basalts and andesites. The Lava Peak basalts had <23 ppm Rb and >400 ppm Sr, while the other lavas sampled had higher concentrations of Rb and lower concentrations of Sr. K-Ar whole-rock dates of lava flows and dikes indicate that they erupted between 1.5 and 1.1 m.y. ago. Clinopyroxene, orthopyroxene, and olivine compositions are uniform across the island, and average Ca45Mg42Fe17Si2O6, Mg66Fe34Si03, and Mg72Fe28Si204, respectively. Plagioclase compositions vary between An93 and An51 with the more calcic plagioclase being restricted to the Lava Peak basalts. The chemical trends seen in Akutan lavas are consistent with the shallow-level fractionation of olivine, plagioclase, and clinopyroxene from a parental liquid, and the subsequent eruption of magmas from different levels of a zoned magma chamber.

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