The caldera-forming eruption of the Quaternary Payún Matrú volcano, Andean back-arc of the Southern Volcanic Zone

HERNANDO, I. R.; PETRINOVIC, I. A.; GUTIÉRREZ, D.A.; BUCHER, J.; FUENTES, T.; ARAGÓN, E.

JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2019 vol. 384 p. 15 - 30

0377-0273

ThePayún Matrú caldera is a Quaternary andean back-arc caldera (36°25? S, 69° 12? 30? W), and represents the main volcanicedifice of the homonymPayún Matrú volcanic field, located within the Payenia Basalticprovince. The Payún Matrú volcanic field is also characterized bythe Payún Liso volcano, and 220 monogenetic basaltic cones andassociated lava flows. The Pleistocene ? Holocene Payún Matrúcalderapresents a long lived pre-caldera stage, which built a multi-vent andshield-like volcanic edifice,mainlytrachytic in composition. The syn-caldera stage is represented bythetrachytic Portezuelo Ignimbrite (the caldera-forming eruptiondeposits) constrained between 148 and 82 ka,and the resulting caldera depression of 8.5 km in diameter.Afterwards the caldera event, volcanism continued mostly along thecaldera rim.Stratigraphicsections of the Portezuelo Ignimbrite and its correlations arepresented, in addition to mineral compositions and previouswhole-rock chemical analyses. Only the extra-caldera deposits can beobserved, while the intra-caldera ignimbrite is hidden bypost-caldera volcanic products and unconsolidated modern sediments.Different facies within the ignimbrite were recognized, being most ofthem massive or eutaxitic and massive lapilli tuff facies, depositedby dense pyroclastic density currents. Fall deposits are found onlyproximally and restrained to the caldera margin. The absenceof a widespread fall deposit suggest thelack ofa sustained eruptive column at the onset of eruption. Massive tufffacies at the topmost deposits indicates the development of aco-ignimbritic plume as the pyroclastic density currents wanned.Onthe basis of color, size and shape, vesicularity and crystallinity,four distinct juvenile clasts types were defined: gray fiamme, blackjuvenile, dark gray and light gray pumice. Available chemicalanalysis of gray fiamme and black juvenile clasts indicate a similartrachytic composition, although their mineralogical compositionpresents some differences.Severaleruptive units for the northern and southern outer slopes of thecaldera were defined, on the basis of the juvenile types present anddiscontinuity surfaces. The onset of caldera collapse is not markedby the presence of a lithic breccia, but it is suggested by theappearance of dark gray and light gray pumice along with blackjuveniles in the topmost eruptive units, togetherwithan increase in lithic content relative to previous units. Thesequential appearance of different juvenile clasts types suggests azoned magma chamber, in terms of crystalcontent and composition.p { margin-bottom: 0.25cm; line-height: 115%; }p { margin-bottom: 0.25cm; direction: ltr; color: rgb(0, 0, 10); line-height: 120%; text-align: left; }p.western { font-family: "Liberation Serif", serif; font-size: 12pt; }p.cjk { font-family: "Droid Sans Fallback"; font-size: 12pt; }p.ctl { font-family: "FreeSans"; font-size: 12pt; }