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Isotope and Trace Element Geochemistry of Augustine Volcano, Alaska: Implications for Magmatic Evolution |
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| Authors: | JOHNSON K E; HARMON R S; RICHARDSON J M; MOORBATH S; STRONG D F |
| Institution: | 1DEPARTMENT OF GEOLOGY, 1112 TURLINGTON HALL, UNIVERSITY OF FLORIDA GAINESVILLE, FL 32611, USA 2US ARMY RESEARCH OFFICE P.O. BOX 12211, RESEARCH TRIANGLE PARK, NC 27709, USA 3GEOSCIENCE BRANCH, ONTARIO GEOLOGICAL SURVEY, WILLET GREEN MILLER CENTRE SUDBURY, ONT PSE 6B5, CANADA 4DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF OXFORD PARKS ROAD, OXFORD OXI 3PR, UK 5CENTRE FOR EARTH AND OCEAN RESEARCH, UNIVERSITY OF VICTORIA VICTORIA, B C. VBW 3P6, CANADA |
| Abstract: | Augustine Volcano, a Quaternary volcanic centre of the easternAleutian Arc, produces predominantly andesites and dacites oflow- to medium-K calc-alkaline composition. Mineralogical andmajor element characteristics of representative lavas suggestthat magmatic evolution has been influenced by both crystalfractionation and magma-mixing processes. However, incompatibletrace element variations (e.g. K/Rb) indicate that these evolvedlavas have been contaminated by the mafic arc crust of the underlyingTalkeetna accreted terrane. The limited range of isotope compositionsalso supports the assimilation of non-radiogenic mafic crust(e.g. 87Sr/86Sr = 0.7032–0.7034; 143Nd/144 Nd = 0.51301–0.5130).In addition, Pb-isotope compositions parallel the North Pacificmean oceanic trend (206Pb/204 Pb = 18.3–18.8; 207Pb/204Pb= 15.5–15.6; 208Pb/204Pb = 38.2–38.3) and do notrequire a subducted sediment component in the source. Relativelyhigh (Ba/La) N (0.79–18.10) and B/Be (14.5) ratios do,however, suggest a metasomatic fluid component derived fromthe dehydration of the subducting plate. The thickened continental crust ( 35 km) of the eastern AleutianArc prevents the ascent of basaltic melts, which fractionateand assimilate at various depths to produce andesitic magmas.These andesites evolve towards more silicic compositions byfractional crystallization. The absence of evidence for a largehigh-level crustal magma chamber implies that the magmatic systembeneath the volcano is young and at an immature stage of evolution. KEY WORDS: Augustine Volcano; Aleutians; assimilation; melasomatism; geochemistry
*Corresponding author. Present address: Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA |
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