Structure and Evolution
Ollague is a composite cone located on the frontier between Bolivia and Chile (Figure 18.1;18.2). A conspicuous feature of its early history was a large debris avalanche (DA) extending westwards from the volcano (Francis & Wells, 1988). A K-Ar age of 0.8 +0.1 Ma was obtained from a lava overlying the avalanche deposit (Francis & Rundle, 1976). A younger scoria cone, La Porunita (LP), which was erupted through the avalanche deposit pre-dates glacial lake terraces formed ~10,000 yr BP and points to a long pre-Holocene evolution for Ollague. Post-collapse healing of the volcano appears to have been completed in the period between the last high stand of the lake and the ~600 yr BP Little Ice Age advance, as the present edifice contains a well defined girdle of moraine (M) around the southwestern flanks at about 4500m. The summit region contains an eroded crater (SC) with a diameter of about 1.2 km, which is extensively altered by fumarolic activity. Sulphur is mined at several locations in the summit region.
A persistent fumarolic steam plume more than one hundred meters high is conspicuous on the southern flanks of the volcano, and appears to emanate from a single vent located on a summit dacite dome (Figure 18.3). This dome was probably constructed within the collapse amphitheater left by the debris avalanche event. There is little evidence of any recent major eruptive events in the form of young lava flows or ash, and morphologically, the most youthful features appear to be some extrusive domes low on the northern, southern and eastern flanks (LD).
There is strong contemporary fumarolic activity.
Although no analyses are available, Ramirez and Huete (1980) report that the lavas of Ollague are dominantly grey, pyroxene andesites. An analysis of a sample from La Porunita reported by Thorpe et al. (1984) shows it to be a basaltic andesite (56.18% SiO2). Feeley and Davidson (1994) report a high-K, calc-alkaline range from basaltic andesite to dacite.
Casertano,L., 1963. Catalogue of the active volcanoes of the world; Part XV, Chilean continent I.A.V.C.E.I. Naples, Italy, 55pp.
Feeley, T.C. and Davidson, J.P., 1994. Petrology of Calc-Alkaline Lavas at Volc«n OllagŸe and the Origin of Compositional Diversity at Central Andean Stratovolcanoes. J. Petrology 35: 1295-1340.
Francis, P.W., & Rundle, C.C., 1976 Rates of production of the main magma types in the Central Andes. Geol. Soc. Am. Bull. 87:474-480
Francis, P.W., & Wells G.L., 1988 Landsat Thematic Mapper observations of débris avalanche deposits in the Central Andes. Bull Volcanol. 50:258-278.
Ramirez C.F., & Huete, C.L., 1980. Hoja Ollague. Carta Geologica de Chile No. 40. S.N.G.M. Santiago, Chile.
Thorpe, R.S., Francis, P.W., & O' Callaghan, L.J., 1984. Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks. Phil. Trans. Roy. Soc. Lond. A310:675-692
Vezzoli, L., Tibaldi, A., Renzulli, A., Menna, M., Flude, S., 2008. Faulting-assisted lateral collapses and influence on shallow magma feeding system at Ollague volcano (Central Volcanic Zone, Chile-Bolivia Andes). Journal of Volcanology and Geothermal Research, 171(1-2): 137-159.