Geochemistry of the Los Chocoyos Ash, Quezaltenango Valley, Guatemala
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 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 87Sr/86Sr ratios for both high-K and low-K magma types are identical and average 0.70405 ± 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 87Sr/86Sr ratios for bulk samples of the ash are significantly higher and more variable (0.70426 ± 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 104 yr. The absence of a continuum of compositions from low-K to high-K rhyolite and the differences in p H2O 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.
GSA Special Papers
Rose, W. I.,
Grant, N. K.,
Geochemistry of the Los Chocoyos Ash, Quezaltenango Valley, Guatemala.
GSA Special Papers,
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