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Journal of Petrology Advance Access published online on April 22, 2005
Journal of Petrology, doi:10.1093/petrology/egi032
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received January 22, 2004
Accepted March 8, 2005

Article

The Petrology and Geochemistry of the Aniakchak Caldera-forming Ignimbrite, Aleutian Arc, Alaska

S. T. DREHER 1*, J. C. EICHELBERGER 2, and J. F. LARSEN 2

1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF DURHAM, DURHAM DH1 3LE, UK
2 ALASKA VOLCANO OBSERVATORY, UNIVERSITY OF ALASKA FAIRBANKS, FAIRBANKS, AK 99775, USA

* To whom correspondence should be addressed.
S. T. DREHER, E-mail: s.t.dreher{at}durham.ac.uk


  Abstract

Aniakchak caldera, Alaska, produced a compositionally heterogeneous ignimbrite ~3400 years ago, which changes from rhyodacitic at the base to andesitic at the top of the eruptive sequence. Interpretations of compositionally heterogeneous ignimbrites typically include either in situ fractional crystallization of mafic magma and generation of a stratified magma body or replenishment of a silicic magma chamber by mafic inputs. Another possibility, silicic replenishment of a more mafic chamber, exists. Geochemical characteristics of the caldera-forming rhyodacite and several late pre-caldera rhyodacites indicate independent origins for each, within a maximum of ~5000 years prior to caldera formation. Isotopic considerations preclude derivation of the caldera-forming rhyodacite from the caldera-forming andesite. However, the caldera-forming rhyodacite can be explained as the residual liquid of a mostly crystallized basalt, with addition of crustal material. The Aniakchak andesite probably formed in a shallow chamber by successive mixing events involving small volumes of basalt and rhyodacite, together with contamination. The pre-caldera rhyodacites represent erupted portions of intruding silicic magma, whereas another portion homogenized with the resident mafic magma. The caldera-forming event reflects a large influx of rhyodacite, which erupted before significant mixing occurred and also triggered draining of much of the andesitic magma from the chamber.

Keywords: Aniakchak; caldera-forming eruption; geochemistry; ignimbrite; silicic replenishment.
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