The caldera-forming eruption of the quaternary Payún Matrú volcano, Andean back-arc of the southern volcanic zone

Abstract

The Payún Matrú caldera is a Quaternary Andean back-arc caldera (36° 25′ S, 69° 12′ 30? W), and represents the main volcanic edifice of the homonym Payún Matrú volcanic field, located within the Payenia Basaltic province. The Payún Matrú volcanic field is also characterized by the Payún Liso volcano, and 220 monogenetic basaltic cones and associated lava flows. The Pleistocene ? Holocene Payún Matrú caldera presents a long lived pre-caldera stage, which built a multi-vent and shield-like volcanic edifice, mainly trachytic in composition. The syn-caldera stage is represented by the trachytic Portezuelo Ignimbrite (the caldera-forming eruption deposits) 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 the caldera rim. Stratigraphic sections of the Portezuelo Ignimbrite and its correlations are presented, in addition to mineral compositions and previous whole-rock chemical analyses. Only the extra-caldera deposits can be observed, while the intra-caldera ignimbrite is hidden by post-caldera volcanic products and unconsolidated modern sediments. Different facies within the ignimbrite were recognized, being most of them massive or eutaxitic and massive lapilli tuff facies, deposited by dense pyroclastic density currents. Fall deposits are found only proximally and restrained to the caldera margin. The absence of a widespread fall deposit suggests the lack of a sustained eruptive column at the onset of eruption. Massive tuff facies at the topmost deposits indicates the development of a co-ignimbritic plume as the pyroclastic density currents wanned. On the basis of color, size and shape, vesicularity and crystallinity, four distinct juvenile clasts types were defined: gray fiamme, black juvenile, dark gray and light gray pumice. Available chemical analysis of gray fiamme and black juvenile clasts indicate a similar trachytic composition, although their mineralogical composition presents some differences. Several eruptive units for the northern and southern outer slopes of the caldera were defined, on the basis of the juvenile types present and discontinuity surfaces. The onset of caldera collapse is not marked by the presence of a lithic breccia, but it is suggested by the appearance of dark gray and light gray pumice along with black juveniles in the topmost eruptive units, together with an increase in lithic content relative to previous units. The sequential appearance of different juvenile clasts types suggests a zoned magma chamber, in terms of crystal content and composition.