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

CAFFE, PABLO J.; CABRERA, AGUSTÍN; PRESTA, JUAN; PERALTA, YÉSICA; MARO, GUADALUPE

Foz do Iguaçu

Encuentro; American Geophysical Union Meeting of the Americas 2010; 2010

The eruptive products of mafic monogenetic (olivine phyric basaltic andesite to plagioclase-pyroxene phyric andesite) volcanic centers from the northern Puna are much less abundant than their silicic (dacite to rhyolite) counterparts erupted from large calderas or composite volcanoes. Although scarce, understanding the eruption of monogenetic volcanoes is relevant for hazard studies, because they behave in a similar way as basaltic volcanic centers that are the most common eruptive structures on Earth. Monogenetic centers from the northern Puna are mainly distributed 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, isolated structures, or may be related to large composite volcanoes, but most frequently form fields of different size (15 km2 - 120 km2) and complexity. Main mafic volcanic fields are aligned in the NNE-SSW direction, coincident with the orientation of the principal Andean thrusts. Transverse, NW-SE and E-W fractures also seem to have 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-5 Ma) were studied in detail. While the Patahuasi field comprises only lava flows and dykes, the other two include the remnants of several scoria cones. Lavas have slabby or massive aspects, some of them showing sheath -like flow folding; blocky lavas are scarce. Stacking of flows and presence of rafted pyroclastic deposits dragged during eruption are common features. Rare pseudofiamme in a few lavas from Cerro Morado suggest origins by clastogenesis. In Patahuasi, peperites indicate brecciation during injection of magma in wet volcaniclastic sequences. Scoria cones exhibit typical facies of Strombolian edifices elsewhere. Recognized facies include: a) rare massive and cross bedded beds, interpreted as hydrovolcanic deposits formed during opening events; b) unwelded spindle-shaped bomb and scoria deposits, typical of the wall or external facies of the cone; c) interstratified beds of weakly welded scoria and moderately to strongly welded spatter in the internal facies of the cone, which commonly coincides with a change in the orientation of layers towards the interior of the edifice; d) dykes, interpreted as the representant of the complex plumbing system that cut different parts of the cones. The facies model here defined permit to infer that many pyroclastic and effusive eruptions were probably concurrent, and that they developed with Strombolian styles, although brief periods of fountaining were also possible. Estimated discharge rates for lavas (< 1 m3/s to 20 m3/s) and the absence of interruptions in the volcanic activity (evidenced by the lack of paleosoils or interstratification of other volcanic rocks) are consistent with a short eruptive life-span, as observed in monogenetic fields of other regions.