Magmatic controls on eruption dynamics of the 1950 yr B.P. eruption of San Antonio Volcano, Tacana Volcanic Complex, Mexico-Guatemala

San Antonio Volcano, in the Tacana Volcanic Complex, erupted similar to 1950 yr. B.P., with a Pelean type eruption that produced andesitic pyroclastic surges and block-and-ash flows destroying part of the volcano summit and producing a horse-shoe shaped crater open to the SW. Between 1950 and 800 yr B.P. the eruption continued with effusive andesites followed by a dacite lava flow and a summit dome, all from a single magma batch. All products consist of phenocrysts and microphenocysts of zoned plagioclase, amphibole, pyroxene, magnetite ilmenite, set in partially crystallized groundmass of glass and microlites of the same mineral phases, except for the lack of amphibole. Included in the andesitic blocks of the block-and-ash flow deposit are basaltic andesite enclaves with elongated and ellipsoidal forms and chilled margins. The enclaves have intersertal textures with brown glass between microphenocrysts of plagioclase, hornblende, pyroxene, and olivine, and minor proportions of phenocysts of plagioclase, hornblende, and pyroxene. A compositional range obtained of blocks and enclaves resulted from mixing between andesite (866 degrees C +/- 22) and basaltic andesite (enclaves, 932 degrees C +/- 22), which may have triggered the explosive Pelean eruption. Vestiges of that mixing are preserved as complex compositional zones in plagioclase and dinopyroxene-rich reaction rims in amphibole in the andesite. Whole-rock chemistry, geothermometry, experimental petrology and modeling results suggest that after the mixing event the eruption tapped hybrid andesitic magma (<= 900 degrees C) and ended with effusive dacitic magma (similar to 825 degrees C), all of which were stored at similar to 200 MPa water pressure. A complex open-system evolution that involved crustal end-members best explains the generation of effusive dacite from the hybrid andesite. Amphibole in the dacite is rimmed by reaction products of plagioclase, orthopyroxene, and Fe-Ti oxides produced by decompression during ascent. Amphibole in the andesite, however, lacks such rims. Because the andesite was at 866 +/- 22 degrees C and the dacite was at similar to 825 degrees C, the reaction rims indicate that the andesitic magma ascended at 0.023 m s(-1) during the explosive phase of the eruption, whereas the dacitic magma rose more slowly at similar to 0.002-0.004 m s(-1). (C) 2013 Elsevier B.V. All rights reserved.