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Geochemistry of the Los Chocoyos Ash, Quezaltenango Valley, Guatemala

W. I. Rose, N. K. Grant and J. Easter
Geochemistry of the Los Chocoyos Ash, Quezaltenango Valley, Guatemala (in Ash-flow tuffs, C. E. Chapin (editor) and W. E. Elston (editor))
Special Paper - Geological Society of America (1979) (180): 87-99


Analysis of 30 individual pumice blocks, together with bulk samples from the ash-flow member of the Los Chocoyos Ash within the Quezaltenango Valley, Guatemala, demonstrates that prior to its eruption, its associated magma-chamber was zoned. Eruption of a high-K (K (sub 2) O/Na (sub 2) O > 1), crystal-poor, biotite-bearing rhyolite with crystal equilibration temperatures of less than 800 degrees C produced the widespread H-tephra member and the initial phases of the ash-flow member. As the ash-flow eruption continued, a more-heterogeneous, low-K, crystal-rich, cummingtonite- and hornblende-bearing rhyolite became predominant; its phenocrysts had equilibrated at temperatures of about 950 degrees C. The water content of the high-K rhyolite was several percent, whereas the low-K rhyolite was much drier. Bulk samples of the ash-flow member are homogenized mixtures of matrix shards that represent either the high-K or low-K rhyolite magmas; the overall ratio for the ash-flow member is 60% high-K and 40% low-K type. The (super 87) Sr/ (super 86) Sr ratios for both high-K and low-K magma types are identical and average 0.70405 + or - 0.00003. This value is nearly the same as all basaltic, all andesitic, and most rhyolitic Quaternary volcanic rocks tested in Guatemala so far. The (super 87) Sr/ (super 86) Sr ratios for bulk samples of the ash are significantly higher and more variable (0.70426 + or - 0.00009), probably because of xenocrystic contamination. Detailed mixing and Rayleigh calculations using observed mineral phases in the ash show that the concentrations of 8 major and 17 minor elements in the ash are consistent with the derivation of high-K rhyolite from low-K magma by crystal fractionation at shallow depths. The time required for such fractionation is at least 10 (super 4) yr. The absence of a continuum of compositions from low-K to high-K rhyolite and the differences in p (sub H (sub 2) O) and temperature suggest that the two magmas were separated during fractionation. The Los Chocoyos Ash is the most silicic major Quaternary unit in the Guatemalan Highlands; the volume of magma from which it was derived is far greater than that of all other Quaternary volcanic rock units in the area.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Issue: 180
Title: Geochemistry of the Los Chocoyos Ash, Quezaltenango Valley, Guatemala
Title: Ash-flow tuffs
Author(s): Rose, W. I., Jr.Grant, N. K.Easter, J.
Author(s): Chapin, C. E.editor
Author(s): Elston, W. E.editor
Affiliation: Mich. Technol. Univ., Dep. Geol. and Geol. Eng., Houghton, MI, United States
Pages: 87-99
Published: 1979
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Ash-flow tuffs, 16 years after Smith (1960); The Geological Society of America, Rocky Mountain Section, annual meeting
Meeting location: Albuquerque, NM, USA, United States
Meeting date: 197605May 1976
References: 39
Accession Number: 1981-005950
Categories: Geochemistry
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. tables, sketch maps
N14°45'00" - N14°55'00", W91°40'00" - W91°25'00"
Secondary Affiliation: Miami Univ., USA, United StatesOberlin Coll., USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 1981
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