Explosive eruptions of the Ayarza calderas, southeastern Guatemala
The Ayarza calderas, southeastern Guatemala, form a figure “8”-shaped depression that is now occupied by Laguna de Ayarza. The smaller eastern caldera is also the older. It formed upon eruption of a “mixed” plinian air fall that is locally overlain by a comagmatic ash-flow deposit. Called the “Mixta” unit, the air-fall portion contains rhyolitic (75.2 wt.% SiO2) pumice, basaltic (52 wt.% SiO2) blobs and compositional hybrids that are either spectacularly banded or homogeneous in color. Although of limited aerial extent and volume (∼ 0.1 km3, dense rock equivalent), the unit is of considerable petrologic interest because the basalt contains pristine phenocrysts and quench-crystallites of calcic hornblende. Virtually identical 87Sr/86Sr ratios (0.70373) of both rhyolite and basalt end members suggest a common magmatic source. A log from the Mixta ash-flow deposit has been C14-dated at 27,000 ± 1,600 y.B.P.
Eruption of the younger pyroclastic sequence at Ayarza culminated in formation of the western caldera. This rhyolitic sequence also consists of a plinian air fall overlain by a comagmatic ash-flow deposit. The ash-flow unit is quite fine-grained and probably phreatomagmatic in origin. A thin horizon of air-fall material is locally interbedded within the ash unit. Volume (DRE) of the younger sequence has been estimated at a minimum 2 km3. A log from the younger ash-flow unit has been C14-dated at 23,100 ± 500 y.B.P., the age of formation of the western caldera and the last recognized activity at Ayarza.
Although major-element geochemistry of the Ayarzan rhyolites is very similar to that of other silicic units of northern Central America, minor- and trace-element contents permit all the rhyolites to be distinguished from each other. Sr-isotopic ratios of the Mixta rhyolite reaffirm that older radiogenic crustal rocks are not involved in the petrogenesis of rhyolites in northern Central America.
Journal of Volcanology and Geothermal Research
Peterson, Paula S. and Rose, William I., "Explosive eruptions of the Ayarza calderas, southeastern Guatemala" (2003). Department of Geological and Mining Engineering and Sciences Publications. 132.