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Journal of Petrology | Volume 45 | Number 1 | Pages 203-219 | 2004
© Oxford University Press 2004; all rights reserved
Chemical versus Temporal Controls on the Evolution of Tholeiitic and Calc-alkaline Magmas at Two Volcanoes in the Alaska–Aleutian Arc
1 DEPARTMENT OF EARTH SCIENCES, WILLS MEMORIAL BUILDING, UNIVERSITY OF BRISTOL, BRISTOL BS8 1RJ, UK
2 US GEOLOGICAL SURVEY, MS 910, 345 MIDDLEFIELD ROAD, MENLO PARK, CA 94025, USA
3 ALASKA VOLCANO OBSERVATORY, ALASKA DIVISION OF GEOLOGY AND GEOPHYSICAL SURVEYS, 794 UNIVERSITY AVENUE SUITE 200, FAIRBANKS, AK 99709, USA
4 DEPARTMENT OF GEOGRAPHY, GEOLOGY AND ANTHROPOLOGY, INDIANA STATE UNIVERSITY, TERRE HAUTE, IN 47809, USA
* Corresponding author. E-mail: rgeorge{at}els.mq.edu.au
The Alaska–Aleutian island arc is well known for erupting both tholeiitic and calc-alkaline magmas. To investigate the relative roles of chemical and temporal controls in generating these contrasting liquid lines of descent we have undertaken a detailed study of tholeiitic lavas from Akutan volcano in the oceanic Aleutian arc and calc-alkaline products from Aniakchak volcano on the continental Alaskan Peninsula. The differences do not appear to be linked to parental magma composition. The Akutan lavas can be explained by closed-system magmatic evolution, whereas curvilinear trace element trends and a large range in 87Sr/86Sr isotope ratios in the Aniakchak data appear to require the combined effects of fractional crystallization, assimilation and magma mixing. Both magmatic suites preserve a similar range in 226Ra–230Th disequilibria, which suggests that the time scale of crustal residence of magmas beneath both these volcanoes was similar, and of the order of several thousand years. This is consistent with numerical estimates of the time scales for crystallization caused by cooling in convecting crustal magma chambers. During that time interval the tholeiitic Akutan magmas underwent restricted, closed-system, compositional evolution. In contrast, the calc-alkaline magmas beneath Aniakchak volcano underwent significant open-system compositional evolution. Combining these results with data from other studies we suggest that differentiation is faster in calc-alkaline and potassic magma series than in tholeiitic series, owing to a combination of greater extents of assimilation, magma mixing and cooling.
KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales
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