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1912 sample (a mediumwey pumice block In the top-
most ash-flow unit) faUs in the lcange 61.5 to 64.5
percent SL02 (fig. 221, despite concerted search for
such material. Analyses reported by Fenner (1913,
1950) within the range 66 to 78 peceent SOa were of
mixed lava and of bulk tephra that mechanically frar
tionated find (or) mixed durlng emplacement. Ample
data confirm the large compositional gap between the
rhyolitic and intermediate magmas. The selected
compositional parameters listed below Ulustrate the
contrast between the rhyolitlc and intermediate ejecta
erupted in 1912:
Rhyolite
Despite this compositional gap, 80 titanomape-
Ute-ilmenite temperatures (Buddington and tlnddey,
1964; CarmichaeL 1967) show a continuous ranger
rhyolite, 805~- Ob; dacite, 855'-955'; and andeslte,
955°-9900~. % fractionations of 3.9 to 4.2 perrnll
between quartz and tftanomagnetite mppart these
values, as & measurements of the homogenization
te mperatures of vaporglas Inclusions in 1912 quartz
phenmrysts by Clocchiatti (1972). Continuity in 2 and
for the entire sequence, and continuous varlation
kbhenecrystic comporitions in the orystal-rich mde-
sitic and dacitic magmas, support the concept of a
single zoned s yw; @ does the similarity of all the
1922 ejecta in Sr/ Sr ratio (0.70361, KIBa ratio
(about 251, Ce/Yb ratio (abfit S), La/Sm ratio (about
31, Th/U ratio (2.21, and 6 0 (6.1-6.4 perrnil In
rhyolite, 5,8-6.1 permil In dacltt%% andesite).
If the rhyoLitic liquid separated from dacitlo
magma, extraction was so efficient that no dacitic
phenocrysts were retained and na 66- to 76percent-
SIO compobitions were created; if the liquid were a
meltof roof rocks atop an intermedate rnegma
body, then such roaks had no Sr-isotopic contrast with
the andesitic4acitlc magma and clearly did not in-
clude the Jurassic arkosic or granltlc basement. The
resence of Holocene domes of pre-I912 dacite
PPdng Mountain and Mount Cerbsrus, fig. 22) )dja-
cent to the 1912 vent suggests that the 6 km (or
more) of high-silica rhyolitic magma (a composition
rare in the Aleutian arc) was generated within e few
thousand yem. The 1912 vent is semiencircled by a
cluster of andesitic stratocones and is es clme to
Mngeik, Trident, and Griggs volcanoes as it Is to Mount
Katmai. Although Quaternmy bMt Is not known to
have erupted here, the intrusion of basaltic rnagrna
probably sustains the greater-V'M'S magm atie system
(see Wdreth, 1981, fig. 15).
Acknowl@ments.-We are grateful to Dan
Kosco, Terry !3. C, Keith, Peter Shearer, and tbe late
David A. - Jobton for collaboration, debate, and
hauling rocks; and to I. S. E. Carmlchael for micro-
probe facilities and 1976 field expenses under National
Science Po'undatlon Grants EAR-74-12782 and BAG
78-03648.
REFERENCES CITED
Buddington, A. P., and Lindsley, D. H., 1964, Irontitanium
oxide minerals and synthetic equivalents:
Journal of Petrology, v. 5, no. 2, p. 310-
351.
Carrnichael, L S. E., 1967, The Iron-titanium oxldes of
salic volcanic rocks and their associated fernrnagnesian
silicates~ Contributions to Mineralogy
and Petrology, v. 14, no. 1,- p. 36-84.
Clocchiattl, R., 1972, Lea crlstaux de quartz des
onces de La V&e des Dix Milles PU&
P Katmal, Alaska) [Quartz crystals from pumlae
of the Valley of Ten Thousand Smokes (Ketmai,
Alaska)]! Paris, Corn tes Rendw Hebdomadaires
des Shces de llAca dP rrde des Sciences, ser. D, v.
274, no. 23, p. 3037-9040.
Curtis, G. H., 1968, The stratigraphy of the ejecta
from the 1912 eruption of Mount Katmai and
Novarupta, Alaska, Studies in voKcanology: A
memoir in honor of Howel Wiuams: Geological
Sodety of America Memoir 116, p. 153-210.
Pemer, C. N., 1923, The origin and mode of emplace
ment of the peat tuff deposit in the VaUey of
Ten Thousand Smokes: Washington, National
Geographic Soclety Contributed Technical
Papers, Katmai Series, no. 1, 74 p.
-1930, Mt Katmal and Mt Mageik: Zeltschrift fiir
Vulkanologie, V. 13, no. 1, p. 1-24.
-1950, The chemical kinettcs of the Ratmd eruption:
American Journal of Science, v. 248, no. 9,
p. 593-627 (pt. 1); no. 10, p. 697-725 (pt. 2).
Griggs, R. P., 1922, The Valley of Ten Thousand
Smokes: Washington, National Geographla Soaiety,
341 p.
Hlldreth, Wes, 1981, Gradtents In silicic magma
chambers: ImpUeations for Lithospheric magmatkm
r Journal of Oeophysjcal Research, v. 88,
no. Bll, p. 10153-10192.
(Figure 23 shows study areas d lscd
New eges of mdioiarinn chert horn the Barnpart
d i s t r i c t , e a a t ~ ~
R e ~ ~ n c e
geologlc studies In 1981 north
and south of the Yukon River in the Rampart district