The Quaternary Payún Matrú caldera, andean back-arc of the Southern Volcanic Zone: insights into its caldera-forming eruption deposits

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

Hamburgo

Congreso; 25th Latin-American Colloquium of Geosciences; 2019

Universidad de Hamburgo

TheQuaternaryPayúnMatrú caldera is locatedin the back-arc of the Southern Volcanic Zone of the Andes,western Argentina, andrepresents the main volcanic edifice of the Payún Matrú VolcanicField (PMVF).ThePMVFis located within Payenia, avolcanicprovincecharacterizedby alkalicbasalticvolcanism.ThePMVF presents the homonym caldera along with the Payún Liso volcanoand 220 basaltic monogenetic cones. PayúnMatrú hasa long-lived pre-caldera stage wherea shield-like edifice has been built. Thesyn-caldera stage is represented by the PleistocenePortezueloIgnimbrite and the resulting caldera depression of 8.5 km indiameter. Afterwards the caldera event, volcanism continued mostlyalong the caldera rim.Wepresentstratigraphic sections, facies analysis and mineralogical studies ofthe Portezuelo Ignimbrite, inorder to determine pre-eruptive conditions, onset of collapse anderuptive history of the caldera-formingeruption deposits.The extra-calderaPortezueloIgnimbrite shows differentfacies,being most of them massive or eutaxitic and massive lapilli tufffacies, deposited by dense pyroclastic density currents.Fall deposits are found only restrained to the caldera margin,suggestingthe lack of a sustained eruptive column at the onset of eruption.Massive tuff facies arefound atthe topmostdeposits southof the caldera, and indicatesthe development of a co-ignimbritic plume as the pyroclastic densitycurrents wanned. Fourdistinct juvenile clasts were recognized on the basis of color,shape, size, vesicularity and crystallinity. These are: gray fiamme,light gray pumice, dark gray pumice and black juveniles. Whole-rockchemical analyses ofgray fiamme and black juveniles havea similar trachytic composition, whilethemineralogical composition of juvenile clasts reveals somedifferences, specially in feldspars.On the basis of discontinuity surfaces and the juvenile clastspopulation of the ignimbrite, severaleruptive units weredefined.Giventhe absence of lithic breccias, the onset of collapse is suggested bya change in the juvenile clasts population inthetopmosteruptive unit, whichpresents dark gray and light gray pumice togetherwith black juveniles. Thesequential appearance of different juvenile clasts suggests a zonedmagma chamber, in terms of crystal content and mineralogicalcomposition.p { margin-bottom: 0.25cm; line-height: 115%; }