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Compositional zonation and cumulus processes in the Mount Mazama magma chamber, Crater Lake, Oregon

Published online by Cambridge University Press:  03 November 2011

Timothy H. Druitt  and
Charles R. Bacon
Timothy H. Druitt
Affiliation:
Department of Earth Sciences, University of Liverpool, P.O. Box 149, Liverpool L69 3BX, U.K.
Charles R. Bacon
Affiliation:
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A.
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Abstract

The 6845 ± 50 BP climactic eruption of Mount Mazama discharged 47 ± 9 km3 of vertically zoned calc-alkaline magma, affording a virtually complete section through the chamber. Evidence for two andesitic parents with different trace-element (particularly Sr) and water contents is preserved in the ejecta. Prior to eruption, a dominant volume of rhyodacite was underlain successively by high-Sr andesite, high-Sr crystal mushes, and low-Sr crystal mushes. Intergranular liquids in the high-Sr magmas were probably richer in water than those in the low-Sr magmas. Thermal continuity throughout the ejecta favours eruption from a single, zoned reservoir. Insight into chamber development is given by preclimactic rhyodacitic lavas and tephra erupted between about 30,000 BP and the climactic eruption. The oldest of these lavas, contaminated derivatives of low-Sr magma, contain crystal-poor magmatic inclusions of low-Sr andesite; the youngest has inclusions of high-Sr andesite and, like rhyodacitic pumice in the climactic ejecta, is hybrid magma containing an admixed high-Sr component. A model for steady-state growth of the chamber is inferred whereby repeated recharge, first by low-Sr then high-Sr andesite (± basalt), builds up a cumulate succession, while derivative liquid fractionates convectively, segregates, and mixes with an incrementally growing silicic volume. The magma chamber at Mount Mazama may provide insight into the evolution of some granitoid plutons.

Keywords

cumulatehybridismmagma mixingpetrologyrechargestrontium
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 79 , Issue 2-3: The Origin of Granites , 1988 , pp. 289 - 297
Copyright
Copyright © Royal Society of Edinburgh 1988

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