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Romick, J.D., 1990

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Romick, J.D., 1990, Silicic volcanism and granulite xenoliths from the Aleutian Islands, Alaska: Petrologic constraints for the evolution of the Aleutian Arc crust: Ithaca, New York, Cornell University, Ph.D. dissertation, 320 p., illust., maps.

Abstract

Silicic tephra from Kanaga, Adak, and Great Sitkin islands, in the Andreanof segment of the Aleutian island arc, document the role of amphibole fractionation and magma mixing during the evolution of the Aleutian calc-alkaline suite. Amphibole is a liquidus phase in andesitic and dacitic tephra and indicates fractionation at depths >7 kilometers, with water contents in excess of 4.5 wt. %, and oxygen fugacities >Ni-NiO. Bimodal mineral compositions in andesitic tephra, and trace-element modeling, suggest that many central Aleutian andesites are products of mixing between a basaltic andesite (53% SiO2) and a hornblende-bearing dacite (61% SiO2). In contrast, the hornblende dacites appear to have been derived by closed-system fractionation from andesitic magmas. These data imply that the mid-crustal levels of the Aleutian arc are dominated by amphibole-bearing cumulates, which formed in open-system magma chambers. Hornblende-bearing dacites from Piip Volcano, north of the Komandorski Islands in the westernmost Aleutians, were derived from a mantle source depleted in incompatible elements by the earlier extraction of basalts that formed the Komandorski basin. Enrichment of LILE and small deviations of Sr and Nd isotopic ratios relative to MORB indicate storage of a continental crust component in the back-arc mantle for 5-10 million years. Trace-element modeling suggests that the dacites are crystal fractionates of a high-magnesium andesite, derived by hydrous partial melting of the back-arc mantle. Tertiary granulite xenoliths from Kanaga Island are texturally and compositionally distinct from Quaternary xenoliths found on Adak Island, and exhibit a complex recrystallization history that occurred in the lower Aleutian Arc crust. The xenolith mineralogy, trace element compositions, and isotopic ratios are consistent with crystallization from basaltic to andesitic arc magmas. Pyroxene geothermometry and geobarometry, variations in Nd isotopic ratios for xenoliths from different localities, and different recrystallization textures, suggest that these xenoliths were sampled from different levels of the arc crust. Anomalously high abundances of Ba and K, and oxidized magnetite, are associated with recrystallization in some xenoliths, suggesting that an alkali-bearing, oxidizing fluid was present during recrystallization.

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