Dacitic Lava Shields Of The Southern Central Andes: Morphology And Characteristics

PETRINOVIC; GROSSE; GUZMAN

Jujuy

Congreso; XVII CGA; 2008

AGA

The best known volcanic shields in the world are in Hawaii, Galapagos, Iceland and Kenya-Ethiopia and are all basaltic to basaltic-andesitic in composition. There isn?t much information about shields of compositions other than basaltic; with the exception of few well documented andesitic examples from Central Mexico (Hasenaka, 1994) and central Costa Rica (Grosse and Alvarado, 2004). Shields of dacitic composition appear to be very atypical, because of their high viscosity; only a few descriptions are available from Puracé volcano (Colombia), Asama volcano (Japan) and the ancient Mount Mazama (crater Lake: USA). Other examples of non-basaltic shields with variable characteristics are described from Io, Venus and Mars. In the southern Central Andes (22º-26º S) there are frequent volcanic structures which are dacitic in composition and show typical characteristics of shields (radial dikes, low slopes, polygenetic character, gently-dipping convex profiles, etc). They were often interpreted as lava domes (Petrinovic et al., 1999) or, in some cases, as stratovolcanoes (Arnosio et al., 2005). Notwithstanding, they have neither the typical lava dome slopes (Fink and Anderson, 2000) nor the characteristic deposits of the Andean stratovolcanoes. They are characterized by slopes of 5º to 7º, small overlapping lava flows and minor pyroclastic deposits, base/height relations from 0.03 to 0.05, and could be separated in two different groups according to their size and characteristics. The smallest ones are usually monogenetic and effusive in nature; the largest ones are typically polygenetic and start each eruptive cycle with explosive events followed by lava effusion. Frequently, the continuous emptying of the magma chamber favours a central collapse of the structure in the largest ones, producing a singular case of collapse calderas less than 10 km in diameter (i.e. Negra Muerta caldera: Petrinovic et al., 2005). These could be classified as ?andesite-dacite calderas?, according to the examples of Cole et al. (2005) and as a characteristic without a final resurgent stage, as is frequent in the well known central Andes collapse calderas related with extensive ignimbrite sheets. If collapse occurs, frequently is in a piecemeal pattern conditioned by the stress field (i.e. Negra Muerta caldera: Petrinovic et al., 2004; Ramelow, 2005). This succession of events is alike to the sequence described in Io (Schenk et al., 2004): steep-sided shields (3º-8º) evolve (collapse) to Pateras (Io Calderas) of lower slopes (1º) and high diameters without a resurgent stage. Other examples recognized with such features appear to be common in other regions of the Central Andes.