Explosive eruptions of the Ayarza calderas, southeastern Guatemala

Abstract

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.