Re-evaluation of the Granada Ignimbrite (Northern Puna, Argentina): A pre-caldera volcanic event, or the first eruptive phase at the Vilama caldera?.

SOLER, M.M., CAFFE, P.J., COIRA, B. AND A.T. ONOE

Pucón, Chile

Congreso; General Assembly IAVCEI; 2004

IAVCEI

The central Andean plateau hosts one of the largest ignimbritic provinces in the world, erupted from several collapse calderas during 10-2 Ma. Particularly 10 m.y.a., the N Puna was the locus of numerous explosive volcanic centers, whose interdigitating products -covered by younger volcanic rocks cannot be easily recognized. Previous recognizance works assigned a large number of unwelded ignimbrites from this area to the Granada Ignimbrite, an apparently extense unit interpreted as the first eruptive phase of the Vilama caldera (9-10 Ma). Here, we redefine the Granada Ignimbrite based on new studies of its stratigraphy, age and geographic distribution. Our results indicate that Granada Ignimbrite has only moderate volumes (150-215 km3), and is distributed only to the northeast of the studied area (22.57ºS – 66.58ºW), covering a much smaller area (800 km2) than previously thought (2400 km2). Its distribution and interpreted eruptive dynamics suggest an origin from a compound volcano probably coincident with, and covered by, the Granada, Caucani and Solterío Volcanoes (<8 Ma), from which pyroclastic flows spread out. The outcrops to the southwest (23ºS) do not represent a single eruptive event either: at least nine unwelded, moderate- to small-volume (1-<20 km3) ignimbrites, could be distinguished. Distribution and stratigraphic characteristcs of those, suggest eruptions from vents located to the southwest: Pairique volcanic complex (11 Ma) and Torona stratovolcano (10 Ma). The Granada Ignimbrite was then misinterpreted as part of the Vilama caldera. Instead, it represents a group of pre-caldera units. Volumes and eruptive mechanisms of the Vilama caldera should be reviewed.