Chemical versus Temporal Controls on the Evolution of Tholeiitic and Calc-alkaline Magmas at Two Volcanoes in the Alaska-Aleutian Arc |
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Authors: | GEORGE, RHIANNON TURNER, SIMON HAWKESWORTH, CHRIS BACON, CHARLES R. NYE, CHRIS STELLING, PETE DREHER, SCOTT |
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Affiliation: | 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 |
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Abstract: | The AlaskaAleutian island arc is well known for eruptingboth tholeiitic and calc-alkaline magmas. To investigate therelative roles of chemical and temporal controls in generatingthese contrasting liquid lines of descent we have undertakena detailed study of tholeiitic lavas from Akutan volcano inthe oceanic Aleutian arc and calc-alkaline products from Aniakchakvolcano on the continental Alaskan Peninsula. The differencesdo not appear to be linked to parental magma composition. TheAkutan lavas can be explained by closed-system magmatic evolution,whereas curvilinear trace element trends and a large range in87Sr/86Sr isotope ratios in the Aniakchak data appear to requirethe combined effects of fractional crystallization, assimilationand magma mixing. Both magmatic suites preserve a similar rangein 226Ra230Th disequilibria, which suggests that thetime scale of crustal residence of magmas beneath both thesevolcanoes was similar, and of the order of several thousandyears. This is consistent with numerical estimates of the timescales for crystallization caused by cooling in convecting crustalmagma chambers. During that time interval the tholeiitic Akutanmagmas underwent restricted, closed-system, compositional evolution.In contrast, the calc-alkaline magmas beneath Aniakchak volcanounderwent significant open-system compositional evolution. Combiningthese results with data from other studies we suggest that differentiationis faster in calc-alkaline and potassic magma series than intholeiitic series, owing to a combination of greater extentsof assimilation, magma mixing and cooling. KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales |
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Keywords: | : uranium-series Aleutian arc magma differentiation time scales |
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