Miocene mafic volcanic centers from the northern Puna, central Andes. Facies model and eruptive styles

CAFFE, P.J., CABRERA, A., PRESTA, J., PERALTA, Y., MARO, G.

Foz do Iguaçu

Congreso; AGU Meeting of the Americas; 2010

The eruptive products of mafic monogenetic (olivine phyric basaltic andesite toplagioclase-pyroxene phyric andesite) volcanic centers from the northern Puna aremuch less abundant than their silicic (dacite to rhyolite) counterparts erupted fromlarge calderas or composite volcanoes. Although scarce, understanding the eruptionof monogenetic volcanoes is relevant for hazard studies, because they behave in asimilar way as basaltic volcanic centers that are the most common eruptivestructures on Earth. Monogenetic centers from the northern Puna are mainlydistributed across a narrow (50 km) area near the boundaries between Argentina,Chile and Bolivia (~80 km east of the current arc). They are either small, isolatedstructures, or may be related to large composite volcanoes, but most frequentlyform fields of different size (15 km2 - 120 km2) and complexity. Main maficvolcanic fields are aligned in the NNE-SSW direction, coincident with the orientationof the principal Andean thrusts. Transverse, NW-SE and E-W fractures also seem tohave participated in the eruptions. Three of these fields, the Patahuasi Andesites(11-10 Ma), the Cerro Morado Andesites (6.7 Ma) and the El Toro Andesites (pre-5Ma) were studied in detail. While the Patahuasi field comprises only lava flows anddykes, the other two include the remnants of several scoria cones. Lavas have slabbyor massive aspects, some of them showing sheath -like flow folding; blocky lavasare scarce. Stacking of flows and presence of rafted pyroclastic deposits draggedduring eruption are common features. Rare pseudofiamme in a few lavas from CerroMorado suggest origins by clastogenesis. In Patahuasi, peperites indicate brecciationduring injection of magma in wet volcaniclastic sequences. Scoria cones exhibittypical facies of Strombolian edifices elsewhere. Recognized facies include: a) raremassive and cross bedded beds, interpreted as hydrovolcanic deposits formedduring opening events; b) unwelded spindle-shaped bomb and scoria deposits,typical of the wall or external facies of the cone; c) interstratified beds of weaklywelded scoria and moderately to strongly welded spatter in the internal facies of thecone, which commonly coincides with a change in the orientation of layers towardsthe interior of the edifice; d) dykes, interpreted as the representant of the complexplumbing system that cut different parts of the cones. The facies model here definedpermit to infer that many pyroclastic and effusive eruptions were probablyconcurrent, and that they developed with Strombolian styles, although brief periodsof fountaining were also possible. Estimated discharge rates for lavas (< 1 m3/s to20 m3/s) and the absence of interruptions in the volcanic activity (evidenced by thelack of paleosoils or interstratification of other volcanic rocks) are consistent with a short eruptive life-span, as observed in monogenetic fields of other regions.