Petrology of the zoned calcalkaline magma chamber of Mount Mazama,Crater Lake,Oregon |
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Authors: | Timothy H Druitt Charles R Bacon |
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Institution: | (1) U.S. Geological Survey, 345 Middlefield Road, 94025 Menlo Park, CA, USA;(2) Present address: Department of Geology, University of Wales College of Cardiff, PO Box 914, CF1 3YE Cardiff, UK |
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Abstract: | Evolution of the magma chamber at Mount Mazama involved repeated recharge by two types of andesite (high-Sr and low-Sr), crystal
fractionation, crystal accumulation, assimilation, and magma mixing (Bacon and Druitt 1988). This paper addresses the modal
compositions, textures, mineral chemistry and magmatic temperatures of (i) products of the 6845±50 BP climactic eruption,
(ii) blocks of partially fused granitoid wallrock found in the ejecta, and (iii) preclimactic rhyodacitic lavas leaked from
the chamber in late Pleistocene and early Holocene time. Immediately prior to the climactic eruption the chamber contained
≳ 40 km3 of rhyodacite (10 vol% plag + opx + aug + hb + mt + ilm, ∼880° C) overlying high-Sr andesite and cumulus-crystal mush (28–51
vol% plag + hb ± opx ± aug + mt ± ilm, 880° to ≥950° C), which in turn overlay low-Sr crystal mush (50–66 vol% plag + opx
+ aug ± hb ± ol + mt + ilm, 890° to ≥950† C). Despite the well known compositional gap in the ejecta, no thermal discontinuity
existed in the chamber. Pre-eruptive water contents of pore liquids in most high-Sr and low-Sr mushes were 4–6 wt%, but on
average the high-Sr mushes were slightly richer in water. Although parental magmas of the crystal mushes were andesitic, xenocrysts
of bytownite and Ni-rich magnesian olivine in some scoriae record the one-time injection of basalt into the chamber. Textures
in ol-bearing scoriae preserve evidence for the reactions ol + liq = opx and ol + aug + liq(+ plag?) = hb, which occurred
in andesitic liquids at Mount Mazama. Strontium abundances in plagioclase phenocrysts constrain the petrogenesis of preclimactic
and climactic rhyodacites. Phenocryst cores derived from high-Sr and low-Sr magmas have different Sr contents which can be
resolved by microprobe. Partition coefficients for plagioclase in andesitic to rhyolitic glasses range from 2 to 7, and increase
as glass %SiO2 increases. Evolved Pleistocene rhyodacites (∼30–25,000 BP) and rhyodacites of the Holocene Llao Rock center (7015±45 BP)
contain Sr-poor plagioclase and are derivatives from low-Sr magma. Rhyodacites of the Pleistocene Sharp Peak domes, Holocene
Cleetwood flow (∼6850 BP), and climactic ejecta contain discrete Sr-rich and Sr-poor plagioclase phenocryst populations and
are hybrids produced by mixing low-Sr rhyodacite (containing Sr-poor plag + opx + aug) with a more mafic high-Sr magma (with
Sr-rich plag + hb?]). The data reinforce the conclusions of crystal-liquid mixing calculations (Bacon and Druitt 1988),
and suggest some important refinements to the magma chamber model. |
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