Post-caldera pumice cones of the Payún Matrú caldera, Payenia, Argentina: morphology and deposits characteristics

HERNANDO, I. R.; PETRINOVIC, I. A.; D'ELIA, L.; GUZMÁN, S.; PÁEZ, G. N.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2019 vol. 90 p. 453 - 462

0895-9811

Theformation of pumice cones by deposition of near-vent pumice fragmentsis relatively rare in comparison with the occurrence of their maficequivalents, the scoria cones. The Quaternary Payún Matrú caldera(36° S, 69° W), within the back-arc of the Southern Volcanic Zoneof the Andes (Argentina) presents several trachytic pumice conesdeveloped during its post-caldera stage and distributed mostly alongthe southeastern to northwestern caldera rim. These cones arespatially and temporally related to trachytic domes and lava flows ofsimilar composition. Given the good preservation of pumice cones inPayún Matrú, exposures of their internal deposits are observedsporadically and are restricted to their outer deposits. Magmafragmentation was purely magmatic, as indicated by the clast sizes ofthe juvenile fragments and the low abundance of basement lithoclasts.Three distinct facies were defined: non-welded lapilli/agglomeratefacies, moderately welded lapilli/agglomerate facies, and denselywelded lapilli/agglomerate facies. The non-welded facies areinterpreted as ultraproximal fall deposits formed by accumulation ofballistic ejecta and by deposition of clasts derived from thejet/lower plume regions in low energy explosive events. Themoderately welded facies presents dense and relatively vesiculardomains, suggesting a formation by the agglutination of spatter uponlanding. The densely welded facies, with a banded lava-likeappearance, indicates their formation by a high deposition rate athigh temperatures in a fire fountain-like eruption of low viscositytrachytic magma. Lapilli sized fall deposits found outside thecaldera suggest the development of a more explosive eruption pulse/s(violent strombolian to subplinian). p { margin-bottom: 0.21cm; direction: ltr; color: rgb(0, 0, 0); text-align: left; }p.western { font-family: "Times New Roman", serif; font-size: 12pt; }p.cjk { font-family: "Droid Sans"; font-size: 12pt; }p.ctl { font-family: "FreeSans"; font-size: 12pt; }