Cordilleran Section - 108th Annual Meeting (29–31 March 2012)

Paper No. 11
Presentation Time: 12:20


ROBERGE, Julie1, DE SILVA, Shanaka2, VIRAMONTE, Jose G.3, ARNOSIO, Marcelo3 and BECCIO, Raúl3, (1)Instituto de Geología, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, México, D.F, 04510, Mexico, (2)College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin. Bldg, Corvallis, OR 97331, (3)Instituto GEONORTE and CONICET, Universidad Nacional de Salta, Av. Bolivia 5150, Salta, 4400, Argentina,

Studies of ignimbrites provide valuable insights into the evolution processes occurring within silicic magma bodies. Whilst most attention has been directed towards “supereruptions” in the Central Volcanic Zone of the Andes, little is known about the smaller arc-related caldera forming systems that characterize the modern arc. In this work, we present a detailed study of the ~50 km3 Cerro Blanco volcanic complex (CBVC), Argentina, which stratigraphy consists of three main phases: 1) the ~73 ka Campo de la Piedra Pomez (CPP); 2) Purulla (22 ka) and Medano ignimbrites (15 ka) and 3) the post-caldera domes and associated block and ash flows (>12± ka). This work integrates knowledge of the eruptive history, with volatile content, geochemistry of quartz and sanidine-hosted melt inclusions, matrix glass and whole rock, and hornblende phase equilibria. The calc-alkaline rhyolites and rare dacites from CBVC can be divided into two groups. A high Sr group with high Cl, low Rb, and a weak Eu anomaly that corresponds to the early erupted CPP. The second group with low Sr has lower Cl, high Rb, and a strong Eu anomaly corresponding to the young ignimbrites and post caldera domes. Trace element modeling shows that all ignimbrites from CBVC are co-magmatic, related by ~54% fractional crystallization of 30% quartz, 30% sanidine, 20% plagioclase, 15% amphibole and 5% biotite. Dissolved water contents in the melt inclusions vary between 2.6 and 8.6 wt%, while dissolved CO2 ranges from 9 to 244 ppm. Saturation pressures of H2O and CO2 and mineral phase equilibria suggest that the high Sr magma equilibrated at high temperatures and depths of 15 to 23 km (328 to 511 MPa), deeper than the plagioclase stability field. The low Sr group however equilibrated at lower temperature and shallower depths of 5 to 9 km (114 to 198 MPa). Cl content in melt inclusions are systematic with ~700ppm in the low Sr group and >1000pm in the high Sr group. These volatile characteristics are in contrast with data from the “supervolcanic” ignimbrites from the Central Andes and elsewhere that show a wide range of CO2 for a much more restricted range of water. These data combine with physical volcanology modeling revealing for the first time, details of the magma dynamics and time-integrated architecture of an arc-related silicic magmatic system in the Central Andes.