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 (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.