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GSA Special Papers
Ash-Flow Tuffs
Author(s)
Geological Society of America

Volume
180
Copyright:
© 1979 Geological Society of America
Geological Society of America
ISBN print:
9780813721804
Publication date:
January 01, 1979
Book Chapter
Geochemistry of the Los Chocoyos Ash, Quezaltenango Valley, Guatemala
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Published:January 01, 1979
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 (K2O/Na2O > 1), crystal-poor, biotite-bearing rhyolite with crystal equilibration temperatures of less than 800 °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 °C. The water content of...
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