Issue 1: February 1996

Abstract

Isotope and trace element geochemistry of Augustine Volcano, Alaska: implications for magmatic evolution

KE. Johnson, RS. Harmon², JM. Richardson³, S. Moorbath⁴ and DF. Strong⁵ 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. P3E 6B5, Canada, ⁴Department of Earth Science, University of Oxford, Parks Road, Oxford OX1 3PR, UK, ⁵Centre for Earth and Ocean Research, University of Victoria, Victoria, B.C. V8W 3P6, Canada and To whom correspondence should be addressed. Present address: Department of Geology and Geophysics, University of New Orleans, New Orleans LA 70148, USA
ABSTRACT
Augustine Volcano, a Quaternary volcanic centre of the eastern Aleutian Arc, produces predominantly andesites and dacites of low- or medium-K calc-alkaline composition. Mineralogical and major element characteristics of representative lavas suggest that magmatic evolution has been influenced by both crystal fractionation and magma-mixing processes. However, incompatible trace element variations (e.g. K/Rb) indicate that these evolved lavas have been contaminated by the mafic arc crust of the underlying Talkeetna accreted terrane. The limited range of isotope compositions also supports the assimilation of non-radiogenic mafic crust (e.g. ⁸⁷Sr/⁸⁶Sr=0.7032-0.7034; ¹⁴³Nd/¹⁴⁴Nd=0.51301-0.5130). In addition, Pb-isotope compositions parallel the North Pacific mean oceanic trend (²⁰⁶Pb/²⁰⁴Pb=18.3-18.8; ²⁰⁷Pb/²⁰⁴Pb=15.5-15.6; ²⁰⁸Pb/²⁰⁴=38.2-38.3) and do not require a subducted sediment component in the source. Relatively high (Ba/La)N (0.79-18.0) and B/Be (14.5) ratios do, however, suggest a metasomatic fluid component derived from the dehydration of the subducting plate.

The thickened continental crust (~35km) of the eastern Aleutian Arc prevents the ascent of basaltic melts, which fractionate and assimilate at various depths to produce andesitic magmas. These andesites evolve towards more silicic compositions by fractional crystallization. The absence of evidence for a large high-level crustal magma chamber implies that the magmatic system beneath the volcano is young and at an immature stage of evolution.

KEY WORDS. Augustine Volcano; Aleutians; assimilation; metasomatism; geochemistry

Pages: 95 - 115

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